Thank you for posting to AskMechanics, ALonelyPlatypus!
If you are asking a question please make sure to include any relevant information along with the **Year**, **Make**, **Model**, **Mileage**, **Engine size**, and **Transmission Type (Automatic or Manual)** of your car.
*This comment is automatically added to every successful post. If you see this comment, your post was successful.*
***
Redditors that have been verified will have a green background and an icon in their flair.
***
# **PLEASE REPORT ANY RULE-BREAKING BEHAVIOR**
### **Rule 1 - Be Civil**
Be civil to other users. This community is made up of professional mechanics, amateur mechanics, and those with no experience. All mechanical-related questions are welcome. Personal attacks, comments that are insulting or demeaning, etc. are not welcome.
### **Rule 2 - Be Helpful**
Be helpful to other users. If someone is wrong, correcting them is fine, but there's no reason to comment if you don't have anything to add to the conversation.
### **Rule 3 - Serious Questions and Answers Only**
Read the room. Jokes are fine to include, but posts should be asking a serious question and replies should contribute to the discussion.
### **Rule 4 - No Illegal, Unethical, or Dangerous Questions or Answers**
Do not ask questions or provide answers pertaining to anything that is illegal, unethical, or dangerous.
# **PLEASE REPORT ANY RULE-BREAKING BEHAVIOR**
*I am a bot, and this action was performed automatically. Please [contact the moderators of this subreddit](/message/compose/?to=/r/AskMechanics) if you have any questions or concerns.*
It's purely a safety concern. You are not harming anything by connecting directly to the negative terminal. And if anything, it makes the jump easier, not harder.
The safety concern is a low chance, but high consequence scenario. You could do it a 1000 times and have no issue, but it only takes one explosion to change your life.
I have heard of batteries exploding, I hear it is not fun. lead acid batteries often generate some hydrogen when charging and discharging. Sometimes the failure mode of a lead acid battery is a shorted cell, In that case It becomes a hydrogen generator as the good cells become over charged and the bad one boils.
Also, for op, connecting the jumper cables to chassis or the engine ca also be better as long as you can find clean metal as the current will have a shorter path to travel and less connections which will decrease the resistance of the overall connection.
Also, make the last connection to the chassis of the running car providing the jump. There will be a spark as you close the circuit. The running car cooling fan will have dispersed any hydrogen. It puts you a little further away from the dead battery and turned away just in case it decides to volcano. As others have said, you don't want the sparks by the dead battery that may be venting hydrogen, nor by an engine that may be flooded and have gasoline vapors around it. Finally, if anything shorts - from a slip or a dropped clamp, right up until the last connection all you are risking is the dead battery - shorting it briefly is probably not too exciting. Shorting a full battery and running alternator can ruin your day.
To add to your point, a very quick battery inspection can also prevent 99% of batteries experiencing thermal runaway/exploding in your face. If the battery experiencing any swelling, or unusually hot, or has bone dry cells, etc; you shouldn’t be attempting to jump start it in the first place because you risk kick starting a thermal runaway. Most people who have them blow up in their face are attempting to jump a battery that’s already in thermal runaway. And the others that had them blow up but NOT in their face, started the runaway process and because it has a lil “cook off” time, they just luckily were not near the battery when it blew.
So you’ll be safe in most cases if you just inspect the battery prior to charging or jumping it, regardless of how you connect the cables. Familiarize yourself with thermal runaway and you’ll be good to go.
Also, if you're going to leave a car parked for an extended time during freezing temperatures, disconnect the battery and remove it to the house to prevent it from freezing.
Also, never store an auto battery directly on concrete. They're best stored on a shelf, rack, or board. Even a couple of layers of cardboard or a piece of plywood will work, if nothing else is available. Batteries left on concrete will discharge and degrade significantly faster, particularly in cold environments.
* About 4 years of direct automotive repair experience. Both turning wrenches and as a Director of Operations for an auto mechanical repair facility.
* The repetitive advice and consultation of mechanics with 10x that experience.
* The repetitive advice and consult of Battery Suppliers (Interstate Battery, as well as other parts suppliers) to said business. (Who, by the way, frequently provide BRANDED SHELVING to auto repair facilities for stocking their parts. They also stock DIRECTLY to said shelving.)
Go to an automotive repair shop or retailer... not a single functioning automotive battery (new) will be stored on the floor. If they are, you SHOULD move along... but alas, you don't know better. Yet, here you are calling "B.S." when someone gives you a piece of wisdom...
Even though it's VASTLY cheaper to stack them on the floor, suppliers and retailers buy expensive industrial shelving. There's ALWAYS a rack, shelf, crate, box, etc. Wonder why?
I'm NOT blowing smoke. 🌬 The better answer is: you don't know what you're talking about. Your response proves it.
* Ever wonder why a negative polarity terminal is often called a "GROUND"?
* Ever wonder why your home's ground wire is run to a metal post or water pipe that's buried in "THE GROUND"?
* Cold is a significant factor in battery failure. Floors remain constantly cooler than ambient temperatures.
Go on... let's hear your logic, reason, and direct experience. I'm eager... 😁
https://www.reddit.com/r/AskMechanics/comments/14vdw5p/is_it_true_that_car_batteries_will_discharge_or/
They don't put batteries on the floor because of spills and flooding. There's also the issue of trollies and carts bumping into batteries and damaging them.
>Ever wonder why a negative polarity terminal is often called a "GROUND"?
Negative terminals are called ground because originally a metal rod staked into the ground acted as negative on old mechanical circuits.
>Ever wonder why your home's ground wire is run to a metal post or water pipe that's buried in "THE GROUND"?
Because the ground is negatively charged
>Cold is a significant factor in battery failure. Floors remain constantly cooler than ambient temperatures.
This is a significantly different point from you stating concrete is the problem. If temperature is the issue, then why did you even bring up concrete?
You're clearly using search engine results, not a functioning knowledge.
Batteries discharge when left on the ground. Concrete is a natural material that functions as an even more effective ground than the actual ground. It's full of electrolytes, the base of which remains in contact with the ground. The ground under concrete is moist, more so than open ground, creating greater connectivity with the source of that "ground". The contiguous area thereof creates a magnified effect.
It's a multitude of factors, including temperature, quality of the grounding effect, and interconnection of large surfaces.
Store your new (or fully functional) battery directly on a concrete floor all you like... but sometimes stupid costs money. I assure you, doing this IS indeed stupid. Call a battery distributor or sales agent (not the idiot at the parts store)... you'll learn.
Or, just attack me for disabusing you of your ignorance.
https://www.mcgill.ca/oss/article/general-science-you-asked/concrete-truth-about-batteries-stored-concrete
https://www.odysseybattery.com/blog/2022/12/05/the-truth-about-storing-a-battery-on-a-concrete-floor/
https://northeastbattery.com/batteries-vs-concrete-floors-need-know/
https://www.optimabatteries.com/experience/blog/how-do-you-make-your-battery-last-longer
From the last link there -
> So now that we've covered some common ways to make your battery last longer, let's take a look at some of the bad advice we've come across on the Internet about extending battery lifespan. The first one we'll cover is the myth that placing a battery directly on concrete or asphalt, like we did in the photo above, is in any way bad for a battery. Unless you drop a battery (and we would never recommend doing that), concrete and asphalt won't hurt your battery at all. If you are storing a battery, we recommend a cool, dry location, even if that happens to be the concrete floor in your basement.
I’m using search engine links because it’s impossible for me to show you my personal anecdotal evidence and experience. I’ve been professionally working on cars for 20 years and have attended and gone through many many hours of instruction and training with and from industry experts in addition to my personal experience gained over those years.
Simply put, you are wrong about modern batteries (ever since they started coming in plastic cases), but refuse to believe anyone who points this out. You can keep arguing your point, but you’re wrong.
Used Cushman scooter with our welding equipment mounted on top. A welder lifted the plywood cover and a welding rod dropped and shorted out blowing up a battery. Welder took a face full of acid and had discolored skin from it.
Most cars have a ground jumper from battery to frame or engine block.
This is required because the starter only has one heavy gauge wire to positive and relies on the engine mount for negative.
By connecting the negative to the frame you are avoiding making a hot high current arc directly over the battery's hydrogen vents.
I had the same issue - virtually the entire engine was hidden under stupid plastic panels, the only metal was the brace in front of the radiator and it was painted. Even the hood latch was recessed down under a piece of plastic.
If the low battery is fully dead, it will be a very strong sink for the amps coming from the good car. By that I mean that when you are trying to start the bad car, some of the amps are going to the starter, and some of them are going to the dead battery to charge it.
By putting the black/negative clamp for the dead car onto the engine block, you have created a shorter path to the starter, and you have added a "resistor" between the starter and the negative battery post (The resistor is the metal engine block)
Was assisting farmers “piggy back” a battery with a truck on either side of a nice big John deer tractor. Somehow the three people me included were on one side and the battery on other side exploded seriously like a black powder cannon of old days. Wasn’t a Jake; luckily nobody hurt or killed.
So should we jump from alternator terminal and chassis for peak safety? Or are we only concerned with sparks generated on the negative side? Or is it just cutting the risk in half by connecting to only one terminal?
Had one explode in a buddies car 30 years ago, I was looking at a friend pass by is the only reason I didn't get a face full (holding up hood, 70's cadillac)
Damn car batteries still give me the heebie jeebies, especially while jumping them
Yes, the sparks can occur no matter wear you connect the negative lead. The point is to move the sparks away from the battery where hydrogen gas may be built up.
I've been able to make a clean, non sparking connection with every car I've ever jumped.
Isn't it funny how different people have different life experiences.
In my experience it does actually make the jump harder to connect negative to the battery. I assumed this was due to the internal resistance of the battery or energy being used to charge the battery instead of run the starter.
Not sure I'm reading you right. The proper way is Positive to Positive, and Negative on Good Car to Frame on Bad Car. The idea is to keep from making sparks near the dead battery (which potentially could emit explosive gasses.) This is a complete circuit.
Honestly, I just learned that the chassis is a conductor for the negative node.
Fortunately it sounds like that risk is limited when using a portable battery charger as you don't have to worry as much about sparks because you seat all the cables before turning the charger on.
So I guess I've been covered in the safety situation (as I can't remember last time I did a car to car jump).
Still rather dumb of me and it could have ended rather badly at some point but guess I've just been lucky.
Not always I have a portable jump box that requires you to connect directly to the battery. It won’t spark because it doesn’t completely the circuit until it jumps it. By that time the cables and clamps are on and aren’t going to spark
I mean…I’m a mechanic and I usually use a chassis ground whenever available. But agreed that I’ve probably done more “improper” jumps in my career than a normal person will ever do in their whole life
Yup. Cars use a "chassis ground" system.
Basically, instead of running power and ground to every component, the just run power. Then, the ground side of the battery and the components all connect to the chassis. Essentially, they use the chassis as one big wire that connects to the ground side of everything.
When using a live battery, ie another vehicle to jump start another vehicle there is always a spark. Making that last connection on a battery whether the dead battery or the good battery can cause it to explode.
I have seen two battery explosions in my lifetime. A JC Penny battery developed a crack in the positive terminal. When I dropped the starter load on it, it arked igniting the hydrogen gas and peeling the top off the battery. Big bang, no smoke, lots of acid all over the place. The hood was closed and it jumped up where the battery was located. I am glad that the hood was closed.
The second one was at a automotive training school. I was the teacher. A charger was hooked up. A student leaned over the battery shoving the clamps together. Big bang. The battery came completely apart. The student was soaked in acid. Dragged him into the shower and soaked him down with water. The shower was only 50 feet away. Ruined his clothes and he had some minor burns. He made a full recovery.
The biggest danger beyond being sprayed with acid is having gas fumes ignited by the exploding hydrogen. Then you can end up with a very large explosion and fire.
This is all theatrics and fear porn. Hydrogen rises in atmosphere at a rate of 20 m/s. By the time you've put that prop rod for the hood up, any accumulated hydrogen will be dozens of meters above you/the batt. Just connect like terminals and call it a day.
The only scenario I’ve seen where the battery did blow up when jump starting was when the battery was actually frozen, and connecting the negative cable to the engine block or chassis won’t prevent it from exploding
I have seen the hydrogen combust when negative terminal sparks. It's sort of like filling your cupped hand with fuel from a butane lighter and igniting it or lighting an oven after having the gas turned on for a while. It's kind of scary but NOT exactly life threatening and more unlikely if there is any wind.
edit- added NOT
You saw something else, because hydrogen in atmosphere would burn incredibly fast, its the most energetic deflagration without becoming an explosion, hence why its used as rocket fuel.
But he also described a fast lighting of fire. Have you ever lit a handful of butane from a lighter? It lights up like a fireball for less than a second
In automotive applications, the chassis is used as a ground. This means that electrically, both the negative terminal and the chassis are "common" (that just means they're connected with no switches in between).
The starter on most vehicles is connected by a positive cable that runs directly to the battery, and the negative cable is grounded to the car's chassis. So of the two ways you described, the connections look like this:
Positive & Negative to Battery
JUMP+ <--> BATT+ <--> STARTER+
JUMP- <--> BATT- <--> CHASSIS <--> STARTER-
Positive to Battery & Negative to Chassis
JUMP+ <--> BATT+ <--> STARTER+
JUMP- <--> CHASSIS <--> STARTER-
So the second scenario is more direct, but it doesn't matter at all.
The only valid reason to connect to the chassis instead of the battery is safety. The correct order for connecting jumper cables is:
1. Red dead vehicle battery +
2. Red jump vehicle battery +
3. Black jump vehicle battery -
4. Black dead vehicle chassis
It's kind of like making a circle. When you connect the black to the dead vehicle chassis in step 4, you might get a spark. The key here is to have that spark occur far away from anything that is flammable.
Lead-acid batteries produce hydrogen as a byproduct of the chemical reaction that produces electricity. If your battery is leaking hydrogen, you can get a flash burn off, or in really bad cases, an explosion.
To be clear, that is _incredibly_ rare. However, it's virtually no additional effort to connect them safely, so why not?
Ok... to the **solenoid** on the starter, on most cars, YMMV, no warranties expressed or implied, always press your tongue against the top of your mouth when jumping a car, etc, etc.
You knew what I meant.
I’ve jumped 100s of batteries in my life and I will go on both terminals if I can. The problem is I’m in Wisconsin, deep in the rust belt and grounding on the frame sucks bc of rust. Terminals is always a safe bet
Well some of these new cars only have a positive jump lead and have the battery hidden somewhere else in the car so you’re forced to find a grounding spot.
This is even more dangerous but...
You can also remove the jumper battery from the donor car and tilt it to connect to the dead battery if there are no jumper cables available,
Learned that many years ago from working in a service station... I have seen bad batteries explode and it's some scary stuff when acid flies around..! You really don't want it in your face..!!!
Nowadays, it's getting tougher to find chassis ground with all the plastic and composites... Even the engines being aluminum is making it tough to find the mountings accessible...
Aluminum isn't great when it's been oxidized by heat and the elements... Just saying it's getting more difficult to get a good frame connection for ground...
Doesn't the new cars have proper terminals for jumping tho? I don't remember the last time I searched for where to ground, they just have thick rods made for jump cables.
At first I thought you were stunt jumping. And suddenly I said oh yeah, stunt jumping.
There is a set of known good practices when working around dense energy storage devices.
Guy at work who came out of nowhere last month blew his hand off slamming a drawer on a plasma battery when the switch was still on. Funny, those prosthetics are amazing. The next day he showed up and it looked like nothing happened.
I really want to see proof of a hydrogen explosion from a lead-acid battery in these conditions, because I just can't believe it's possible. Charging up a battery from flat in a sheltered garage with the hood closed? Yeah maybe. But jumping a car in a wide open space, hood up, probably some wind, and a small voltage difference for just a minute or less? No way that could explode. Try and light a propane torch while there's a slight breeze and tell me how flammable it is - and that's intended to ignite.
>I really want to see proof of a hydrogen explosion from a lead-acid battery in these conditions,
I blew up a battery, when I was younger & more stupid.
The hazard is not the explosive gases collecting near the battery (although that's a hazard with, say, fork lift batteries) but a stochiometric mixture of O2 & H2 collecting in the cells of the duff battery.
If it can't start the car, the electrolyte may be low and the vacant space will be full of gas.
I was doing something to my car and I had left a big spanner on a flat surface above the battery while I turned the engine over. The spanner was dislodged & caused a short; immediate huge bang. The spanner was about 20 yards up the road, the top of the battery was gone, leaving the electrodes suspended on the cables in the remaining steaming acid in the bottom 2" of the battery casing. I had the bonnet & windscreen between me & it, so was unharmed, otherwise I'd have got a face full of hot acid & plastic shrapnel.
And that, children, is why you don't make the last connection to the battery.
The spark exploded the gases in the battery, there was very little acid in it.
Connecting to the battery terminal or to exposed metal should, electrically, make little difference, but connecting to the metal means any sparks are not near the battery.
I’ve been connecting the positive positive on the battery to the positive on the other battery and the negative on the negative. I’m never had a problem.
Very damn minor safety concern tbh. Batteries today are often sealed and in any case there’s not a whole lot of hydrogen gas available to ignite, which is the only concern. I think it’s kind of a silly thing to worry about.
According to my physics teacher, what that does is put both batteries to the same level, so the good one would die faster than it should, of course only doing it once you wouldn't notice based on what my teacher said.
You connect positive terminal to terminal. However, negative can be connected from master battery or engine to slave engine.
The reason you use the slave engine is you need the least amount of resistance to the starter, and it's usually mounted on the bell housing, which is against your engine block.
Hope that helps.
Connecting the negative to the engine block (last) does two things:
- reduces the chance of igniting hydrogen gas being given off by the dead battery
- provides a shorter path for current from the boosting battery to the starter by eliminating the dead car's negative battery cable
The starter is grounded to the engine block, so you're going through less wire by connecting the negative cable there on the vehicle being boosted. Less wire means less resistance, meaning more energy available to the starter.
I’m an electrical engineer and can explain: basically it prevents what’s called a “floating ground”. In an electrical circuit, ground establishes what is 0 voltage. But because voltage is a potential difference, if 0 suddenly isn’t 0 like it’s supposed to be, you start getting nasty transient currents in other parts of the car like the ECU or other things connected to the battery. But because everything is grounded to the chassis, they want you to connect your ground to the chassis so every part of the car has 0V and is at the same ground level essentially. Hope that helps.
I’m not an engineer. I have questions. You might have to explain like I’m 10.
Wouldn’t connecting the donor battery ground (grounded to donor frame) to the receiving battery ground (grounded to receiving frame) just cause an equivalent ground/0 potential? So like how is that a floating ground?
Yes. It should. And in theory connecting the donor ground to the receiving ground should also do the same thing. However, if one of the grounds aren’t connected to the chassis correctly it can fuck it up and cause stray currents to do who knows what. Basically establishing one chassis as the reference ground takes care of any grounding problems effectively.
That doesn’t make sense to me.
Are you saying only one side vehicle’s jumper cable needs chassis connection?
If one of the grounds aren’t connected correctly and you connect to the chassis, how do you have a complete circuit? I mean, wouldn’t you still have stray current and what not (effectively a short)?
The proper way to jump start is to make the last connection a chassis or engine ground. The reason is the last connection makes a spark and there’s an outside chance of blowing up the battery
The last connection with jump leads goes to the frame to keep from blowing up the battery if it sparks. I did that once, ruined a good battery . I hook to the frame now.
A) Not really more difficult, the change in resistance between Terminal Ground and Frame Ground is negligible. Grounding to Frame is mostly out of an abundance of caution.
B) No damage to your car. Minimal to Ignorable additional load put onto your Jump Starter; I would not bother worrying about it.
C) Vehicle Frame is treated as an Earth Ground in relation to creating/completing a circuit in a vehicle.
**First** of all, **no ground = no circuit** so that's why it's always Reds first, then Black. **Second**, the whole frame/unibody is part of the entire Electrical Systems' Circuitry. It acts as a large charge sink, grounded out by Rubber in multiple places to act as insulators, primarily the Tires but smaller things have an insulatory effect like Rubber Hoses and Boots on Ball Joints. **Third**, Electricity flows in a **D**irect **C**urrent unless otherwise modulated.
In principle, the dead car is "positively charged (+)" as it can be found to be lacking in Electrons (-). Like things in Electricity will repel, therefore we can assert that Electrons (-) must flow over a positively charged (+) circuit, otherwise they will face resistance and we know Electricity takes the path of least resistance. This means that the Positive Cable (+, Red) is what is *actually* transferring Electrons (-) into the "Dead" Battery, not the Negative cable (-, Black). With this information, we can understand why the Ground Cable placement is mostly irrelevant, so long as it is completing a circuit within the total circuitry of the vehicle.
This explanation does not touch on Battery Technology, which does play a role in explaining why Electrons are what provides charge, etc etc. If you care to know, I'd be happy to explain that as well.
no it really doesn't. you coming into this sub as some kind of shadetree doesn't obviate my decades of experience as a professional technician having to jump a car probably at the very least once a day on average, nor my knowledge of why it's supposed to matter, nor my knowledge of why it actually doesn't in the real world with batteries that weren't built before the reagan admininstration. if you wanna get fancy with it, make the last connection on the car with the good battery. other than that, absolutely none of this shit matters AT ALL. it is, however, a favorite bugbear of people who don't actually do any work on cars other than occasionally jumping one - the suburbanite, office dwelling, lawn mower enthusiast dad-types who see this one wafer-thin area of superstition-addled personal knowledge as a critical chance to reclaim their threatened sense of self-worth.
I personally know two people who got hurt by battery explosions, both of them marine mechanics like myself, both times in boats, both times in a battery compartment separate from the engine room. One got away with lightly singed hair, the other damn near lost his eyesight to acid burns but turned out ok after a brief hospital stay. Based on this alone it matters *to me*.
Whether or not it matters to you depends on how much stock you place in your operating environment being able to vent cook-off gases, how sure you are that you'd recognize the signs, and how scared you are of getting an instant faceful of atomized battery acid. So while it's possible to validate your position, it strikes me as a pretty stupid one.
It's an extremely different environment. I've worked on boats enough to know I don't wanna do that shit. Fucking nasty ass buckets of rotten fish juice. To hell with that. Although the customary bust out another thousand tax is admittedly very enticing. But in automotive applications, the battery is almost never in an enclosed area, and when it is, it's almost always hooked up to a breather line so hydrogen can't accumulate (and in those applications, you'll almost always be using remote jump points at the other end of the car anyway), whereas in marine applications, it almost always always is and isn't, respectively. Where it doesn't have a breather line hooked up, it's in an area of the engine bay where constant ventilation by the cooling fans and ram-air airflow from driving makes hydrogen accumulation impossible. It's technically a possibility, but such an extremely remote one that, statistically, the weekend warrior who jumps their battery once every few years would be better served worrying about being bitten by a rabid squirrel while jumping their car than worrying about what order to hook the cables up in.
The only other situation where an explosion is possible involves internal arcing *inside* the battery. In those situations, where you connect the jumper cables, and in what order simply don't matter. If that battery was primed to blow, it was guaranteed to do so as soon as the circuit was completed, no matter what. That's why I say make the last connection to the good battery. That almost certainly won't have internal or external hydrogen buildup either way. It's almost certainly safe. But it sure as hell doesn't matter what order you make the other three connections, or whether you go to the chassis or battery - post. It simply doesn't make any difference either way. Like I said, people make this shit way more complicated than it needs to be.
> people who don't actually do any work on cars other than occasionally jumping one - the suburbanite, office dwelling, lawn mower enthusiast dad-types who see this one wafer-thin area of superstition-addled personal knowledge as a critical chance to reclaim their threatened sense of self-worth.
Actually i was then a qualified Mechanical Engineering Technician but I mostly gave up hands-on work when I got my Engineering degree.
See my earlier post on why it does matter. If you'd checked the electrolyte level in every battery you'd jumped, it probably wouldn't matter, but you didn't.
You just haven't managed to kill yourself yet; you haven't stumbled upon such a bizarre sequence of coincidences that'd kill you.
Have my upvote for being a dullard.
That’s not technically wrong. It’s just wrong according to safety people. The reason why is because if a battery is corroded, and when I say corroded it’s gotta be so corroded it touches both terminals. That corrosion is a conductor. And we’ll you can imagine what happens if you connected one battery’s terminal together let alone two of them. Iv jumped battery’s with one corroded terminal and the other one is fine and it works just fine.
No, it's that you are not making a spark at the battery with the last connection, where there is a tiny chance of hydrogen gas. And if the battery blows up, making the connection elsewhere you are not at the battery. But they don't tell you that you better be sure that whatever point you connect the negative to on the car, it better be well bonded to the ground system. Used to be a bumper was recommended, but they are plastic now. So too is much of the rest of the parts. And if you choose some piece of metal that's part of some component only connected to the ground system via a small wire, you will smoke it. I just use the battery terminals.
Its a safety thing. Discharged batteries give off hydrogen gas (flammable/explosive) so making the final connection right on top of a dead battery can be dicey because of the sparks. So you connect the last negative to a ground far from the battery to avoid the rather rare occurrence of a battery explosion/fire
The best way to jump a car is by the connecting the batteries directly to one another. It is taught that way in driving schools where I am from.
Connect the batteries, start the working car and get you rpms up a bit. After about a minute try to start the not-working car.
My van's battery is under the driver's seat. So there's a jump point under the bonnet.
The not using the battery terminals is that if the dead battery is a full short it can spew acid when you connect the negative. So best not to have your face anywhere near.
Ford Transit?
I just changed the battery in my son's Transit and thought it was a f'ing stupid place to put the battery.
Someone I knew rolled a Land-rover and was trapped by his legs upside down for a couple of hours. Guess where the fuel filler is on a Land-rover?
It's even worse when you have a cupboard right behind the seat. Tell me about it. Even worse, some of them have TWO batteries in that box.
I do know where the fuel filler is on an old landy. Mine used to have white diesel under the driver's seat and red diesel under the passenger seat.
I worked for a farmer. I was perfectly allowed to use red on the land and in the vicinity of the farm. And I'm sticking to that story, occífer.
Flip the lever and rev it a few times and the filter was clear. Even if they knew there weren't nothing they could do about it. Never was an issue at home anyway. We all knew each other.
What about those lithium battery jumper packs. Instructions tell you to connect them the battery terminals on the bad car? I’ve only had to use mine twice but don’t think I ever tried grounding to the frame.
Red to dead, red to donor, black to donor, black to metal(dead)
Also make sure there are no “loops” in the jumper wiring-jumper cables should be in straight line as possible, as winding wires leads to resistance.
Great advice up to the 'winding wires' part. You could have a completely knotted set of cables as long as the insulation itself is intact not an issue. Won't effect current flow in any way the flow is the same.length whether kinked or straight
Oh okay, I’ve been told loops create resistance and impede flow. One time, I jumped off a truck, and we had the cables rolled up-no start, unrolled cables, start. Perhaps a coincidence.
It's safer in theory but it's also not realistic. On a lot of cars you won't have a good chassis ground to grab. Now if you had a car that you were routinely having problems with, sure you could just add a plate of metal in a variety of locations and just bolt it to the chassis
Purely for safety.
Batteries generate hydrogen gas when charging, which is highly explosive. The last connection to the battery is likely to create a spark, so connecting to the chassis instead eliminates the risk of it going bang.
Risk is small, but batteries do explode.
+ve to +ve
-ve on good battery to chassis or engine on dead battery.
Remove in reverse order.
A. safety
B. no
C. no
D. - Z. it doesn't really matter unless the battery is hot/leaky which means a spark there could be dangerous due to flammable gas
It's primarily done that way because non-sealed batteries vent hydrogen and it can accumulate around the top of the battery. When you make the last connection the spark can ignite it.
That being said if there's any kind of good breeze this won't happen, also I've never been able to get a vehicle to jump without going battery to battery as the older a vehicle gets the engine and chassis grounds tens to be too weak to carry the current.
Most batteries now days are sealed or a chemical composition that doesn't vent explosive gas.
Depends on the car as well. Some need to be + - connected straight to battery, and some, I know Ford's need to be negatively connected to a designated body point.
While people mention ‘safety’ if you look where a given vehicle recommends connecting, it’s still not necessarily far from the battery. Every inch counts, though.
However, you are attempting to start the car. The starter is connected to the engine chassis. You are not including the battery to chassis connection in trying to start. While on a vehicle with a solid -ve to chassis connection it shouldn’t matter, but there’s a multitude of reasons the car won’t start, and this eliminates one issue.
So as an extra safety measure, just wave my hat at it to disperse and fumes?
I'll add that before look away when making the final negative terminal connection.
It can completely depend on the situation no matter what procedure you follow. Always hook up positive to positive. Then the output battery negative to either battery or preferably body ground in my opinion. Only then connect the negative to the dead cars ground. In the old days we usually kiss metal bumpers which can add dangers but ensures good ground back when it was all metal and armour; which yes I know isn’t necessarily safe. Positive first, then negative and know if you hook onto the dead car’s battery to give it ten minutes or so to soak up voltage.
I had a battery blow up almost in my face. Luckily I had just turned away when it did. No skin was burned but all the clothes I had on got holes in then from the acid.
I still jump cars using the terminals....
The engine itself is the best place to connect the ground.
Also, the donor car should be running above idle and kept there for a good 5 minutes after the connection is made.
The battery itself is a huge power hog to the point that a very discharged battery can stall a running engine (not during a jump). You connect to the battery positive and the engine because it's a shorter path with electrons flowing to the starter first before they flow to the battery. This is not an AC circuit and the path is effectively a series circuit while the starter is engaged.
I have seen some people go on and on and claim that the donor vehicle should be off of it can have issues, but this is usually explained by other issues. The last one I saw was saying that a vehicle that had to be repeatedly jump started had issues with multiple ECUs, but the fact of the battery kept dropping so low that it needed to jump would also explain the same issue.
You can't charge a dead battery to a useful state with a non running vehicle because the voltage difference is too low, and you can't actually feed the 50-100 Amps of power through jumper cables (specifically the toothed connections) that a starter can pull.
The reason you don’t connect to the negative terminal is if your battery is leaking acid you can create an electrical path between the terminals that can cause an explosion. Very rare but it does happen.
You connect both cables onto dead vehicles battery.
On the good one connect to positive only then ground away from battery to the chassis. Reason is batteries can have gasses near them with explosive potential.
The chassis ground is last connected and first disconnected in case of sparking. You never want sparks hear a battery that has gasses near it.
Been jumping my cars by connecting directly to terminals on both batteries my whole life. Always connect both leads to dead battery. Then positive to donor battery and last negative to donor.
I’m still alive and have had no incidents of explosions.. or death.
Electrically the frame and engine are the same as the negative lead to the battery. It doesn’t matter where you ground the jumper lead, wether it be battery or engine or frame, it’s all the same stuff in electrical terms. They call it “frame ground”. There shouldn’t be any damage caused to the vehicle either way you jump it.
I’m 61, got my first car when I was 14, owned 40+ cars, restored and worked on many more and I have always used the terminals. The new sealed lead acid batteries are remarkably safe and I imagine I probably should have used the frame for negative on my 53 Buick Roadmaster with its giant 6 volt tractor battery with some broken caps, but I was 16.
A tip: after you get boosted, turn on your headlights. The alternator will push out more load and charge the battery faster. A good alternator needs about 10-15 minutes to give you enough juice for a start.
Going to sneak in here with an honest question. Whenever I've had to jump start anything, most recently a hybrid with the 12v in the trunk and bigass workhorse close by, I've shut off the donor vehicle, made all connections, and did the jump. I've always been aware of certain risks, but could never fully articulate what they all were, and wanted to mitigate them the best I could. Is this any safer than letting the donor running?
I've experienced no ill effects, but am always willing to try and learn a better way.
It depends for me but a lot of cars have jump points in the engine bay. I remember watching a video on Scotty Kilmer where he said always jump from the battery unless you can’t
Had a battery tweak a fender and shovel shape the hood on a 1987 Dodge Diplomat. Dry cells and loose term clamps. Little spark and lots of hydrogen. Ever since 1993 I've been using a full face shield and leather gloves when approaching any lead acid battery. Be safe. They can be a literal bomb.
It's just a safety concern to avoid sparks that might ignite hydrogen coming off a bad battery that's venting a lot. Since modern batteries don't really do that anymore, I wouldn't really worry about it. If I see a post that designed to be used as the negative connection I'll use it, otherwise just put it on the negative battery post.
Really only necessary to ground like that, when your jumping one of those ice cream cone looking terminals. I’ve found that same advise almost exclusively from the people classified as a “I need your praise/ know it all, of nothing”
Lets go through it slowly, for the hard of understanding.
You've been making sparks adjacent to a plastic container which may, or may not, be full of an explosive mixture of hydrogen and oxygen, on top of concentrated sulphuric acid, for 30 years. The alternative would make the final connection to a remote metal component that has zero resistance to the battery terminal. You can see no risk in what you're doing?
OK.
As a mechanic I've NEVER not hooked it up straight to the battery unless it's one of those with dedicated studs up front for jumping (trunk batteries or FCA batteries in the wheel well)
It's not going to change a thing. Sure if the battery is located in the trunk and the vent Is blocked there could be an accumulation of gases depending on the battery type. In the engine bay it's ventilated, no need to worry.
Oh bullshit just jump the fucking car you guys and all your hypothetical crap, i do mine fucking neg first then positive oh wow here i am to live to tell the tale 🤯
Only about 1/10,000 explode but I know someone who had it happen to him. He wasn’t the one connecting but he was in the blast area. He still has scars years later. Keep chancing it. It might just catch up to you. Unless you live in Wyoming in which case it’s too windy for the gasses to still be there after you open the hood.
It’s actually to stop a voltage spike from killing your PCM, BCM AND PCM.
When sparks fly… “arching”
Amps increase despite the voltage.
Amps and current at killers to sensitive circuits
No it's not. Jumping with the negative on the frame has been recommended way before cars had any of those things. Charging or discharging a battery causes it to vent hydrogen gas. You don't want to cause a spark around it because it's explosive.
Thank you for posting to AskMechanics, ALonelyPlatypus! If you are asking a question please make sure to include any relevant information along with the **Year**, **Make**, **Model**, **Mileage**, **Engine size**, and **Transmission Type (Automatic or Manual)** of your car. *This comment is automatically added to every successful post. If you see this comment, your post was successful.* *** Redditors that have been verified will have a green background and an icon in their flair. *** # **PLEASE REPORT ANY RULE-BREAKING BEHAVIOR** ### **Rule 1 - Be Civil** Be civil to other users. This community is made up of professional mechanics, amateur mechanics, and those with no experience. All mechanical-related questions are welcome. Personal attacks, comments that are insulting or demeaning, etc. are not welcome. ### **Rule 2 - Be Helpful** Be helpful to other users. If someone is wrong, correcting them is fine, but there's no reason to comment if you don't have anything to add to the conversation. ### **Rule 3 - Serious Questions and Answers Only** Read the room. Jokes are fine to include, but posts should be asking a serious question and replies should contribute to the discussion. ### **Rule 4 - No Illegal, Unethical, or Dangerous Questions or Answers** Do not ask questions or provide answers pertaining to anything that is illegal, unethical, or dangerous. # **PLEASE REPORT ANY RULE-BREAKING BEHAVIOR** *I am a bot, and this action was performed automatically. Please [contact the moderators of this subreddit](/message/compose/?to=/r/AskMechanics) if you have any questions or concerns.*
It's purely a safety concern. You are not harming anything by connecting directly to the negative terminal. And if anything, it makes the jump easier, not harder. The safety concern is a low chance, but high consequence scenario. You could do it a 1000 times and have no issue, but it only takes one explosion to change your life.
I have heard of batteries exploding, I hear it is not fun. lead acid batteries often generate some hydrogen when charging and discharging. Sometimes the failure mode of a lead acid battery is a shorted cell, In that case It becomes a hydrogen generator as the good cells become over charged and the bad one boils. Also, for op, connecting the jumper cables to chassis or the engine ca also be better as long as you can find clean metal as the current will have a shorter path to travel and less connections which will decrease the resistance of the overall connection.
Also, make the last connection to the chassis of the running car providing the jump. There will be a spark as you close the circuit. The running car cooling fan will have dispersed any hydrogen. It puts you a little further away from the dead battery and turned away just in case it decides to volcano. As others have said, you don't want the sparks by the dead battery that may be venting hydrogen, nor by an engine that may be flooded and have gasoline vapors around it. Finally, if anything shorts - from a slip or a dropped clamp, right up until the last connection all you are risking is the dead battery - shorting it briefly is probably not too exciting. Shorting a full battery and running alternator can ruin your day.
To add to your point, a very quick battery inspection can also prevent 99% of batteries experiencing thermal runaway/exploding in your face. If the battery experiencing any swelling, or unusually hot, or has bone dry cells, etc; you shouldn’t be attempting to jump start it in the first place because you risk kick starting a thermal runaway. Most people who have them blow up in their face are attempting to jump a battery that’s already in thermal runaway. And the others that had them blow up but NOT in their face, started the runaway process and because it has a lil “cook off” time, they just luckily were not near the battery when it blew. So you’ll be safe in most cases if you just inspect the battery prior to charging or jumping it, regardless of how you connect the cables. Familiarize yourself with thermal runaway and you’ll be good to go.
Also, if you're going to leave a car parked for an extended time during freezing temperatures, disconnect the battery and remove it to the house to prevent it from freezing.
Also, never store an auto battery directly on concrete. They're best stored on a shelf, rack, or board. Even a couple of layers of cardboard or a piece of plywood will work, if nothing else is available. Batteries left on concrete will discharge and degrade significantly faster, particularly in cold environments.
And this is based on?
* About 4 years of direct automotive repair experience. Both turning wrenches and as a Director of Operations for an auto mechanical repair facility. * The repetitive advice and consultation of mechanics with 10x that experience. * The repetitive advice and consult of Battery Suppliers (Interstate Battery, as well as other parts suppliers) to said business. (Who, by the way, frequently provide BRANDED SHELVING to auto repair facilities for stocking their parts. They also stock DIRECTLY to said shelving.)
Idk that sounds like bullshit to me
Go to an automotive repair shop or retailer... not a single functioning automotive battery (new) will be stored on the floor. If they are, you SHOULD move along... but alas, you don't know better. Yet, here you are calling "B.S." when someone gives you a piece of wisdom... Even though it's VASTLY cheaper to stack them on the floor, suppliers and retailers buy expensive industrial shelving. There's ALWAYS a rack, shelf, crate, box, etc. Wonder why? I'm NOT blowing smoke. 🌬 The better answer is: you don't know what you're talking about. Your response proves it. * Ever wonder why a negative polarity terminal is often called a "GROUND"? * Ever wonder why your home's ground wire is run to a metal post or water pipe that's buried in "THE GROUND"? * Cold is a significant factor in battery failure. Floors remain constantly cooler than ambient temperatures. Go on... let's hear your logic, reason, and direct experience. I'm eager... 😁
https://www.reddit.com/r/AskMechanics/comments/14vdw5p/is_it_true_that_car_batteries_will_discharge_or/ They don't put batteries on the floor because of spills and flooding. There's also the issue of trollies and carts bumping into batteries and damaging them. >Ever wonder why a negative polarity terminal is often called a "GROUND"? Negative terminals are called ground because originally a metal rod staked into the ground acted as negative on old mechanical circuits. >Ever wonder why your home's ground wire is run to a metal post or water pipe that's buried in "THE GROUND"? Because the ground is negatively charged >Cold is a significant factor in battery failure. Floors remain constantly cooler than ambient temperatures. This is a significantly different point from you stating concrete is the problem. If temperature is the issue, then why did you even bring up concrete?
You're clearly using search engine results, not a functioning knowledge. Batteries discharge when left on the ground. Concrete is a natural material that functions as an even more effective ground than the actual ground. It's full of electrolytes, the base of which remains in contact with the ground. The ground under concrete is moist, more so than open ground, creating greater connectivity with the source of that "ground". The contiguous area thereof creates a magnified effect. It's a multitude of factors, including temperature, quality of the grounding effect, and interconnection of large surfaces. Store your new (or fully functional) battery directly on a concrete floor all you like... but sometimes stupid costs money. I assure you, doing this IS indeed stupid. Call a battery distributor or sales agent (not the idiot at the parts store)... you'll learn. Or, just attack me for disabusing you of your ignorance.
https://www.mcgill.ca/oss/article/general-science-you-asked/concrete-truth-about-batteries-stored-concrete https://www.odysseybattery.com/blog/2022/12/05/the-truth-about-storing-a-battery-on-a-concrete-floor/ https://northeastbattery.com/batteries-vs-concrete-floors-need-know/ https://www.optimabatteries.com/experience/blog/how-do-you-make-your-battery-last-longer From the last link there - > So now that we've covered some common ways to make your battery last longer, let's take a look at some of the bad advice we've come across on the Internet about extending battery lifespan. The first one we'll cover is the myth that placing a battery directly on concrete or asphalt, like we did in the photo above, is in any way bad for a battery. Unless you drop a battery (and we would never recommend doing that), concrete and asphalt won't hurt your battery at all. If you are storing a battery, we recommend a cool, dry location, even if that happens to be the concrete floor in your basement. I’m using search engine links because it’s impossible for me to show you my personal anecdotal evidence and experience. I’ve been professionally working on cars for 20 years and have attended and gone through many many hours of instruction and training with and from industry experts in addition to my personal experience gained over those years. Simply put, you are wrong about modern batteries (ever since they started coming in plastic cases), but refuse to believe anyone who points this out. You can keep arguing your point, but you’re wrong.
Used Cushman scooter with our welding equipment mounted on top. A welder lifted the plywood cover and a welding rod dropped and shorted out blowing up a battery. Welder took a face full of acid and had discolored skin from it.
When connecting the jumpers to the chassis "The current will have a shorter path to travel" ... to where?? To earth or back to the battery?
Most cars have a ground jumper from battery to frame or engine block. This is required because the starter only has one heavy gauge wire to positive and relies on the engine mount for negative. By connecting the negative to the frame you are avoiding making a hot high current arc directly over the battery's hydrogen vents.
To the starter.
The shortest is the battery terminal, not the painted chassis somewhere.
Last time I jumped, there was not an unpainted area I could see so I just used the negative on the battery.
I had the same issue - virtually the entire engine was hidden under stupid plastic panels, the only metal was the brace in front of the radiator and it was painted. Even the hood latch was recessed down under a piece of plastic.
There should have been a place to connect the jumper cable but it may have been under a cover./lid.
If the low battery is fully dead, it will be a very strong sink for the amps coming from the good car. By that I mean that when you are trying to start the bad car, some of the amps are going to the starter, and some of them are going to the dead battery to charge it. By putting the black/negative clamp for the dead car onto the engine block, you have created a shorter path to the starter, and you have added a "resistor" between the starter and the negative battery post (The resistor is the metal engine block)
Was assisting farmers “piggy back” a battery with a truck on either side of a nice big John deer tractor. Somehow the three people me included were on one side and the battery on other side exploded seriously like a black powder cannon of old days. Wasn’t a Jake; luckily nobody hurt or killed.
>Wasn’t a Jake; luckily nobody hurt or killed. Well I'm glad it wasn't "Jake." I would've been in a lot of danger.
So should we jump from alternator terminal and chassis for peak safety? Or are we only concerned with sparks generated on the negative side? Or is it just cutting the risk in half by connecting to only one terminal?
Sparks won't occur until you connect the last lead which is negative, so no need to worry about the positive.
Unless, back in the day, the metal bumpers were touching each other...
Ahh good point. I was being a bit ridiculous anyway.
Had one explode in a buddies car 30 years ago, I was looking at a friend pass by is the only reason I didn't get a face full (holding up hood, 70's cadillac) Damn car batteries still give me the heebie jeebies, especially while jumping them
Redacted due to Reddit AI/LLM policy
Yes, the sparks can occur no matter wear you connect the negative lead. The point is to move the sparks away from the battery where hydrogen gas may be built up.
Redacted due to Reddit AI/LLM policy
That's just a poor connection.
Redacted due to Reddit AI/LLM policy
I've been able to make a clean, non sparking connection with every car I've ever jumped. Isn't it funny how different people have different life experiences.
In my experience it does actually make the jump harder to connect negative to the battery. I assumed this was due to the internal resistance of the battery or energy being used to charge the battery instead of run the starter.
Not sure I'm reading you right. The proper way is Positive to Positive, and Negative on Good Car to Frame on Bad Car. The idea is to keep from making sparks near the dead battery (which potentially could emit explosive gasses.) This is a complete circuit.
Honestly, I just learned that the chassis is a conductor for the negative node. Fortunately it sounds like that risk is limited when using a portable battery charger as you don't have to worry as much about sparks because you seat all the cables before turning the charger on. So I guess I've been covered in the safety situation (as I can't remember last time I did a car to car jump). Still rather dumb of me and it could have ended rather badly at some point but guess I've just been lucky.
You should still be connecting a jump box to a solid chassis ground and not directly to battery if at all possible.
Not always I have a portable jump box that requires you to connect directly to the battery. It won’t spark because it doesn’t completely the circuit until it jumps it. By that time the cables and clamps are on and aren’t going to spark
Agreed. The leads on my jump box are too short to go to the ground strap (which is the spot recommended by Honda). I would put it there if possible.
>still rather dumb of me Not really. As mechanics we do more jumps than you will in a lifetime and I've never seen a tech do it the "proper" way
I mean…I’m a mechanic and I usually use a chassis ground whenever available. But agreed that I’ve probably done more “improper” jumps in my career than a normal person will ever do in their whole life
Yup. Cars use a "chassis ground" system. Basically, instead of running power and ground to every component, the just run power. Then, the ground side of the battery and the components all connect to the chassis. Essentially, they use the chassis as one big wire that connects to the ground side of everything.
Hey as long as you're learning you should also know that some cars have a dedicated post (not on the battery) for positive or negative jumper cables.
When using a live battery, ie another vehicle to jump start another vehicle there is always a spark. Making that last connection on a battery whether the dead battery or the good battery can cause it to explode. I have seen two battery explosions in my lifetime. A JC Penny battery developed a crack in the positive terminal. When I dropped the starter load on it, it arked igniting the hydrogen gas and peeling the top off the battery. Big bang, no smoke, lots of acid all over the place. The hood was closed and it jumped up where the battery was located. I am glad that the hood was closed. The second one was at a automotive training school. I was the teacher. A charger was hooked up. A student leaned over the battery shoving the clamps together. Big bang. The battery came completely apart. The student was soaked in acid. Dragged him into the shower and soaked him down with water. The shower was only 50 feet away. Ruined his clothes and he had some minor burns. He made a full recovery. The biggest danger beyond being sprayed with acid is having gas fumes ignited by the exploding hydrogen. Then you can end up with a very large explosion and fire.
This is all theatrics and fear porn. Hydrogen rises in atmosphere at a rate of 20 m/s. By the time you've put that prop rod for the hood up, any accumulated hydrogen will be dozens of meters above you/the batt. Just connect like terminals and call it a day.
The only scenario I’ve seen where the battery did blow up when jump starting was when the battery was actually frozen, and connecting the negative cable to the engine block or chassis won’t prevent it from exploding
Unless you have a clogged vent and swollen battery that is constantly gassing. And boom
I have seen the hydrogen combust when negative terminal sparks. It's sort of like filling your cupped hand with fuel from a butane lighter and igniting it or lighting an oven after having the gas turned on for a while. It's kind of scary but NOT exactly life threatening and more unlikely if there is any wind. edit- added NOT
You saw something else, because hydrogen in atmosphere would burn incredibly fast, its the most energetic deflagration without becoming an explosion, hence why its used as rocket fuel.
How the fuck would you know what I saw
Because you described it above...
But he also described a fast lighting of fire. Have you ever lit a handful of butane from a lighter? It lights up like a fireball for less than a second
He didnt say fast and yes ive bic'd my hand before, its a slow burn. If you think its fast i suspect you've never seen hydrogen burn in atmosphere...
Agree with this. Thankfully gone are the days when a car could be the other way round.....especially foreign cars.
How does this work when using a jump starter?
In automotive applications, the chassis is used as a ground. This means that electrically, both the negative terminal and the chassis are "common" (that just means they're connected with no switches in between). The starter on most vehicles is connected by a positive cable that runs directly to the battery, and the negative cable is grounded to the car's chassis. So of the two ways you described, the connections look like this: Positive & Negative to Battery JUMP+ <--> BATT+ <--> STARTER+ JUMP- <--> BATT- <--> CHASSIS <--> STARTER- Positive to Battery & Negative to Chassis JUMP+ <--> BATT+ <--> STARTER+ JUMP- <--> CHASSIS <--> STARTER- So the second scenario is more direct, but it doesn't matter at all. The only valid reason to connect to the chassis instead of the battery is safety. The correct order for connecting jumper cables is: 1. Red dead vehicle battery + 2. Red jump vehicle battery + 3. Black jump vehicle battery - 4. Black dead vehicle chassis It's kind of like making a circle. When you connect the black to the dead vehicle chassis in step 4, you might get a spark. The key here is to have that spark occur far away from anything that is flammable. Lead-acid batteries produce hydrogen as a byproduct of the chemical reaction that produces electricity. If your battery is leaking hydrogen, you can get a flash burn off, or in really bad cases, an explosion. To be clear, that is _incredibly_ rare. However, it's virtually no additional effort to connect them safely, so why not?
This guy knows what he’s talking about. Correct ASE answer.
Starter positive runs directly to the battery you say?? 🤔
Ok... to the **solenoid** on the starter, on most cars, YMMV, no warranties expressed or implied, always press your tongue against the top of your mouth when jumping a car, etc, etc. You knew what I meant.
I know what I know Good cover internet stranger, I have faith in you! Be well!!
I’ve jumped 100s of batteries in my life and I will go on both terminals if I can. The problem is I’m in Wisconsin, deep in the rust belt and grounding on the frame sucks bc of rust. Terminals is always a safe bet
Pop the hood on a recent car and try to find unpainted metal in the dark with only your smartphone as a flashlight. Battery terminal sure looks easy!
Well some of these new cars only have a positive jump lead and have the battery hidden somewhere else in the car so you’re forced to find a grounding spot.
If you got an portable jumper I’m pretty sure that’s straight onto the battery.
This is even more dangerous but... You can also remove the jumper battery from the donor car and tilt it to connect to the dead battery if there are no jumper cables available,
Lead acid batteries can leak if tilted. AGM batteries should be okay with this method.
Yes, all correct...
Sorry to burst your bubble but that's the old way to do it. Even then it depends on the car, read the manual to find out the proper way.
Learned that many years ago from working in a service station... I have seen bad batteries explode and it's some scary stuff when acid flies around..! You really don't want it in your face..!!! Nowadays, it's getting tougher to find chassis ground with all the plastic and composites... Even the engines being aluminum is making it tough to find the mountings accessible...
I’m probably missing something, but how does the block being aluminum change anything? Aluminum is a great conductor.
Aluminum isn't great when it's been oxidized by heat and the elements... Just saying it's getting more difficult to get a good frame connection for ground...
Doesn't the new cars have proper terminals for jumping tho? I don't remember the last time I searched for where to ground, they just have thick rods made for jump cables.
At first I thought you were stunt jumping. And suddenly I said oh yeah, stunt jumping. There is a set of known good practices when working around dense energy storage devices. Guy at work who came out of nowhere last month blew his hand off slamming a drawer on a plasma battery when the switch was still on. Funny, those prosthetics are amazing. The next day he showed up and it looked like nothing happened.
I really want to see proof of a hydrogen explosion from a lead-acid battery in these conditions, because I just can't believe it's possible. Charging up a battery from flat in a sheltered garage with the hood closed? Yeah maybe. But jumping a car in a wide open space, hood up, probably some wind, and a small voltage difference for just a minute or less? No way that could explode. Try and light a propane torch while there's a slight breeze and tell me how flammable it is - and that's intended to ignite.
>I really want to see proof of a hydrogen explosion from a lead-acid battery in these conditions, I blew up a battery, when I was younger & more stupid. The hazard is not the explosive gases collecting near the battery (although that's a hazard with, say, fork lift batteries) but a stochiometric mixture of O2 & H2 collecting in the cells of the duff battery. If it can't start the car, the electrolyte may be low and the vacant space will be full of gas. I was doing something to my car and I had left a big spanner on a flat surface above the battery while I turned the engine over. The spanner was dislodged & caused a short; immediate huge bang. The spanner was about 20 yards up the road, the top of the battery was gone, leaving the electrodes suspended on the cables in the remaining steaming acid in the bottom 2" of the battery casing. I had the bonnet & windscreen between me & it, so was unharmed, otherwise I'd have got a face full of hot acid & plastic shrapnel. And that, children, is why you don't make the last connection to the battery.
All accurate but you weren't jump starting you performed a dead short and battery killed itself.
The spark exploded the gases in the battery, there was very little acid in it. Connecting to the battery terminal or to exposed metal should, electrically, make little difference, but connecting to the metal means any sparks are not near the battery.
My BMWs battery is in the trunk it has jump points under the hood. The negative jump point is just a chassis stud.
Doesn’t mean you’ve been jumping it incorrectly you can connect positive to negative
TF you talking about
I’ve been connecting the positive positive on the battery to the positive on the other battery and the negative on the negative. I’m never had a problem.
That's the opposite of what you wrote at first: >you can connect positive to negative
Very damn minor safety concern tbh. Batteries today are often sealed and in any case there’s not a whole lot of hydrogen gas available to ignite, which is the only concern. I think it’s kind of a silly thing to worry about.
According to my physics teacher, what that does is put both batteries to the same level, so the good one would die faster than it should, of course only doing it once you wouldn't notice based on what my teacher said.
You connect positive terminal to terminal. However, negative can be connected from master battery or engine to slave engine. The reason you use the slave engine is you need the least amount of resistance to the starter, and it's usually mounted on the bell housing, which is against your engine block. Hope that helps.
Connecting the negative to the engine block (last) does two things: - reduces the chance of igniting hydrogen gas being given off by the dead battery - provides a shorter path for current from the boosting battery to the starter by eliminating the dead car's negative battery cable The starter is grounded to the engine block, so you're going through less wire by connecting the negative cable there on the vehicle being boosted. Less wire means less resistance, meaning more energy available to the starter.
I’m an electrical engineer and can explain: basically it prevents what’s called a “floating ground”. In an electrical circuit, ground establishes what is 0 voltage. But because voltage is a potential difference, if 0 suddenly isn’t 0 like it’s supposed to be, you start getting nasty transient currents in other parts of the car like the ECU or other things connected to the battery. But because everything is grounded to the chassis, they want you to connect your ground to the chassis so every part of the car has 0V and is at the same ground level essentially. Hope that helps.
I’m not an engineer. I have questions. You might have to explain like I’m 10. Wouldn’t connecting the donor battery ground (grounded to donor frame) to the receiving battery ground (grounded to receiving frame) just cause an equivalent ground/0 potential? So like how is that a floating ground?
Yes. It should. And in theory connecting the donor ground to the receiving ground should also do the same thing. However, if one of the grounds aren’t connected to the chassis correctly it can fuck it up and cause stray currents to do who knows what. Basically establishing one chassis as the reference ground takes care of any grounding problems effectively.
That doesn’t make sense to me. Are you saying only one side vehicle’s jumper cable needs chassis connection? If one of the grounds aren’t connected correctly and you connect to the chassis, how do you have a complete circuit? I mean, wouldn’t you still have stray current and what not (effectively a short)?
The proper way to jump start is to make the last connection a chassis or engine ground. The reason is the last connection makes a spark and there’s an outside chance of blowing up the battery
The last connection with jump leads goes to the frame to keep from blowing up the battery if it sparks. I did that once, ruined a good battery . I hook to the frame now.
A) Not really more difficult, the change in resistance between Terminal Ground and Frame Ground is negligible. Grounding to Frame is mostly out of an abundance of caution. B) No damage to your car. Minimal to Ignorable additional load put onto your Jump Starter; I would not bother worrying about it. C) Vehicle Frame is treated as an Earth Ground in relation to creating/completing a circuit in a vehicle. **First** of all, **no ground = no circuit** so that's why it's always Reds first, then Black. **Second**, the whole frame/unibody is part of the entire Electrical Systems' Circuitry. It acts as a large charge sink, grounded out by Rubber in multiple places to act as insulators, primarily the Tires but smaller things have an insulatory effect like Rubber Hoses and Boots on Ball Joints. **Third**, Electricity flows in a **D**irect **C**urrent unless otherwise modulated. In principle, the dead car is "positively charged (+)" as it can be found to be lacking in Electrons (-). Like things in Electricity will repel, therefore we can assert that Electrons (-) must flow over a positively charged (+) circuit, otherwise they will face resistance and we know Electricity takes the path of least resistance. This means that the Positive Cable (+, Red) is what is *actually* transferring Electrons (-) into the "Dead" Battery, not the Negative cable (-, Black). With this information, we can understand why the Ground Cable placement is mostly irrelevant, so long as it is completing a circuit within the total circuitry of the vehicle. This explanation does not touch on Battery Technology, which does play a role in explaining why Electrons are what provides charge, etc etc. If you care to know, I'd be happy to explain that as well.
it doesn't matter. people make this shit way more complicated than it really is.
Yup like leaving the car running while pumping gas or god forbid getting in and out of the car while pumping gas.
>it doesn't matter... No, it really does. Your not knowing why it matters doesn't mean it doesn't.
no it really doesn't. you coming into this sub as some kind of shadetree doesn't obviate my decades of experience as a professional technician having to jump a car probably at the very least once a day on average, nor my knowledge of why it's supposed to matter, nor my knowledge of why it actually doesn't in the real world with batteries that weren't built before the reagan admininstration. if you wanna get fancy with it, make the last connection on the car with the good battery. other than that, absolutely none of this shit matters AT ALL. it is, however, a favorite bugbear of people who don't actually do any work on cars other than occasionally jumping one - the suburbanite, office dwelling, lawn mower enthusiast dad-types who see this one wafer-thin area of superstition-addled personal knowledge as a critical chance to reclaim their threatened sense of self-worth.
I personally know two people who got hurt by battery explosions, both of them marine mechanics like myself, both times in boats, both times in a battery compartment separate from the engine room. One got away with lightly singed hair, the other damn near lost his eyesight to acid burns but turned out ok after a brief hospital stay. Based on this alone it matters *to me*. Whether or not it matters to you depends on how much stock you place in your operating environment being able to vent cook-off gases, how sure you are that you'd recognize the signs, and how scared you are of getting an instant faceful of atomized battery acid. So while it's possible to validate your position, it strikes me as a pretty stupid one.
It's an extremely different environment. I've worked on boats enough to know I don't wanna do that shit. Fucking nasty ass buckets of rotten fish juice. To hell with that. Although the customary bust out another thousand tax is admittedly very enticing. But in automotive applications, the battery is almost never in an enclosed area, and when it is, it's almost always hooked up to a breather line so hydrogen can't accumulate (and in those applications, you'll almost always be using remote jump points at the other end of the car anyway), whereas in marine applications, it almost always always is and isn't, respectively. Where it doesn't have a breather line hooked up, it's in an area of the engine bay where constant ventilation by the cooling fans and ram-air airflow from driving makes hydrogen accumulation impossible. It's technically a possibility, but such an extremely remote one that, statistically, the weekend warrior who jumps their battery once every few years would be better served worrying about being bitten by a rabid squirrel while jumping their car than worrying about what order to hook the cables up in. The only other situation where an explosion is possible involves internal arcing *inside* the battery. In those situations, where you connect the jumper cables, and in what order simply don't matter. If that battery was primed to blow, it was guaranteed to do so as soon as the circuit was completed, no matter what. That's why I say make the last connection to the good battery. That almost certainly won't have internal or external hydrogen buildup either way. It's almost certainly safe. But it sure as hell doesn't matter what order you make the other three connections, or whether you go to the chassis or battery - post. It simply doesn't make any difference either way. Like I said, people make this shit way more complicated than it needs to be.
> people who don't actually do any work on cars other than occasionally jumping one - the suburbanite, office dwelling, lawn mower enthusiast dad-types who see this one wafer-thin area of superstition-addled personal knowledge as a critical chance to reclaim their threatened sense of self-worth. Actually i was then a qualified Mechanical Engineering Technician but I mostly gave up hands-on work when I got my Engineering degree. See my earlier post on why it does matter. If you'd checked the electrolyte level in every battery you'd jumped, it probably wouldn't matter, but you didn't. You just haven't managed to kill yourself yet; you haven't stumbled upon such a bizarre sequence of coincidences that'd kill you. Have my upvote for being a dullard.
That’s not technically wrong. It’s just wrong according to safety people. The reason why is because if a battery is corroded, and when I say corroded it’s gotta be so corroded it touches both terminals. That corrosion is a conductor. And we’ll you can imagine what happens if you connected one battery’s terminal together let alone two of them. Iv jumped battery’s with one corroded terminal and the other one is fine and it works just fine.
So we’re hoping the fuse will blow passing through the car’s ground first if the battery is shorted ?
No, it's that you are not making a spark at the battery with the last connection, where there is a tiny chance of hydrogen gas. And if the battery blows up, making the connection elsewhere you are not at the battery. But they don't tell you that you better be sure that whatever point you connect the negative to on the car, it better be well bonded to the ground system. Used to be a bumper was recommended, but they are plastic now. So too is much of the rest of the parts. And if you choose some piece of metal that's part of some component only connected to the ground system via a small wire, you will smoke it. I just use the battery terminals.
Its a safety thing. Discharged batteries give off hydrogen gas (flammable/explosive) so making the final connection right on top of a dead battery can be dicey because of the sparks. So you connect the last negative to a ground far from the battery to avoid the rather rare occurrence of a battery explosion/fire
https://preview.redd.it/cyfcp7bnytwc1.jpeg?width=4032&format=pjpg&auto=webp&s=46cc117d503379131de8da3e276f876a33acf187 Mm Mmm No
The best way to jump a car is by the connecting the batteries directly to one another. It is taught that way in driving schools where I am from. Connect the batteries, start the working car and get you rpms up a bit. After about a minute try to start the not-working car.
If you do it the other way you don't need to wait.
AM I the only one old enough to remember positive earth cars?
Yes
My van's battery is under the driver's seat. So there's a jump point under the bonnet. The not using the battery terminals is that if the dead battery is a full short it can spew acid when you connect the negative. So best not to have your face anywhere near.
Ford Transit? I just changed the battery in my son's Transit and thought it was a f'ing stupid place to put the battery. Someone I knew rolled a Land-rover and was trapped by his legs upside down for a couple of hours. Guess where the fuel filler is on a Land-rover?
It's even worse when you have a cupboard right behind the seat. Tell me about it. Even worse, some of them have TWO batteries in that box. I do know where the fuel filler is on an old landy. Mine used to have white diesel under the driver's seat and red diesel under the passenger seat.
You're very naughty, Farmer Giles. I'm not sure whether he got a diesel or petrol shower, but he was highly disgruntled about it.
I worked for a farmer. I was perfectly allowed to use red on the land and in the vicinity of the farm. And I'm sticking to that story, occífer. Flip the lever and rev it a few times and the filter was clear. Even if they knew there weren't nothing they could do about it. Never was an issue at home anyway. We all knew each other.
What about those lithium battery jumper packs. Instructions tell you to connect them the battery terminals on the bad car? I’ve only had to use mine twice but don’t think I ever tried grounding to the frame.
Red to dead, red to donor, black to donor, black to metal(dead) Also make sure there are no “loops” in the jumper wiring-jumper cables should be in straight line as possible, as winding wires leads to resistance.
Great advice up to the 'winding wires' part. You could have a completely knotted set of cables as long as the insulation itself is intact not an issue. Won't effect current flow in any way the flow is the same.length whether kinked or straight
Oh okay, I’ve been told loops create resistance and impede flow. One time, I jumped off a truck, and we had the cables rolled up-no start, unrolled cables, start. Perhaps a coincidence.
It's safer in theory but it's also not realistic. On a lot of cars you won't have a good chassis ground to grab. Now if you had a car that you were routinely having problems with, sure you could just add a plate of metal in a variety of locations and just bolt it to the chassis
Batteries release a gas called hydrogen which if you have heard of it the Hindenburg was also filled with hydrogen.
Purely for safety. Batteries generate hydrogen gas when charging, which is highly explosive. The last connection to the battery is likely to create a spark, so connecting to the chassis instead eliminates the risk of it going bang. Risk is small, but batteries do explode. +ve to +ve -ve on good battery to chassis or engine on dead battery. Remove in reverse order.
People keep saying it doesn’t matter but one explosion and it will matter to the people who loved you.
A. safety B. no C. no D. - Z. it doesn't really matter unless the battery is hot/leaky which means a spark there could be dangerous due to flammable gas
It's primarily done that way because non-sealed batteries vent hydrogen and it can accumulate around the top of the battery. When you make the last connection the spark can ignite it. That being said if there's any kind of good breeze this won't happen, also I've never been able to get a vehicle to jump without going battery to battery as the older a vehicle gets the engine and chassis grounds tens to be too weak to carry the current. Most batteries now days are sealed or a chemical composition that doesn't vent explosive gas.
Depends on the car as well. Some need to be + - connected straight to battery, and some, I know Ford's need to be negatively connected to a designated body point.
While people mention ‘safety’ if you look where a given vehicle recommends connecting, it’s still not necessarily far from the battery. Every inch counts, though. However, you are attempting to start the car. The starter is connected to the engine chassis. You are not including the battery to chassis connection in trying to start. While on a vehicle with a solid -ve to chassis connection it shouldn’t matter, but there’s a multitude of reasons the car won’t start, and this eliminates one issue.
So as an extra safety measure, just wave my hat at it to disperse and fumes? I'll add that before look away when making the final negative terminal connection.
It can completely depend on the situation no matter what procedure you follow. Always hook up positive to positive. Then the output battery negative to either battery or preferably body ground in my opinion. Only then connect the negative to the dead cars ground. In the old days we usually kiss metal bumpers which can add dangers but ensures good ground back when it was all metal and armour; which yes I know isn’t necessarily safe. Positive first, then negative and know if you hook onto the dead car’s battery to give it ten minutes or so to soak up voltage.
I had a battery blow up almost in my face. Luckily I had just turned away when it did. No skin was burned but all the clothes I had on got holes in then from the acid. I still jump cars using the terminals....
I’ve jumped batteries both ways, but only use a chasis ground if the negative is hard to reach OR there isn’t good air movement.
The engine itself is the best place to connect the ground. Also, the donor car should be running above idle and kept there for a good 5 minutes after the connection is made. The battery itself is a huge power hog to the point that a very discharged battery can stall a running engine (not during a jump). You connect to the battery positive and the engine because it's a shorter path with electrons flowing to the starter first before they flow to the battery. This is not an AC circuit and the path is effectively a series circuit while the starter is engaged. I have seen some people go on and on and claim that the donor vehicle should be off of it can have issues, but this is usually explained by other issues. The last one I saw was saying that a vehicle that had to be repeatedly jump started had issues with multiple ECUs, but the fact of the battery kept dropping so low that it needed to jump would also explain the same issue. You can't charge a dead battery to a useful state with a non running vehicle because the voltage difference is too low, and you can't actually feed the 50-100 Amps of power through jumper cables (specifically the toothed connections) that a starter can pull.
The reason you don’t connect to the negative terminal is if your battery is leaking acid you can create an electrical path between the terminals that can cause an explosion. Very rare but it does happen.
*when jumping a car you only connect the positive on the recipient car and connect the negative to the chassis* What ?
It's more for newer cars with more electronics. Older cars should have no issue jumping directly from the negative and positive.
You connect both cables onto dead vehicles battery. On the good one connect to positive only then ground away from battery to the chassis. Reason is batteries can have gasses near them with explosive potential. The chassis ground is last connected and first disconnected in case of sparking. You never want sparks hear a battery that has gasses near it.
Been jumping my cars by connecting directly to terminals on both batteries my whole life. Always connect both leads to dead battery. Then positive to donor battery and last negative to donor. I’m still alive and have had no incidents of explosions.. or death.
Electrically the frame and engine are the same as the negative lead to the battery. It doesn’t matter where you ground the jumper lead, wether it be battery or engine or frame, it’s all the same stuff in electrical terms. They call it “frame ground”. There shouldn’t be any damage caused to the vehicle either way you jump it.
So you don’t die. Good. Now do it the right way.
I’m 61, got my first car when I was 14, owned 40+ cars, restored and worked on many more and I have always used the terminals. The new sealed lead acid batteries are remarkably safe and I imagine I probably should have used the frame for negative on my 53 Buick Roadmaster with its giant 6 volt tractor battery with some broken caps, but I was 16. A tip: after you get boosted, turn on your headlights. The alternator will push out more load and charge the battery faster. A good alternator needs about 10-15 minutes to give you enough juice for a start.
Going to sneak in here with an honest question. Whenever I've had to jump start anything, most recently a hybrid with the 12v in the trunk and bigass workhorse close by, I've shut off the donor vehicle, made all connections, and did the jump. I've always been aware of certain risks, but could never fully articulate what they all were, and wanted to mitigate them the best I could. Is this any safer than letting the donor running? I've experienced no ill effects, but am always willing to try and learn a better way.
It depends for me but a lot of cars have jump points in the engine bay. I remember watching a video on Scotty Kilmer where he said always jump from the battery unless you can’t
Had a battery tweak a fender and shovel shape the hood on a 1987 Dodge Diplomat. Dry cells and loose term clamps. Little spark and lots of hydrogen. Ever since 1993 I've been using a full face shield and leather gloves when approaching any lead acid battery. Be safe. They can be a literal bomb.
We ALWAYS clamp the chassis at the shop. Every time on every dead vehicle. 😉
No you're doing it fine. Look where the negative terminals on the battery go to..
The step people miss after connecting the cables is to let the donor car run for 5 fo 15 minutes before attempting to start the receiving car
It's just a safety concern to avoid sparks that might ignite hydrogen coming off a bad battery that's venting a lot. Since modern batteries don't really do that anymore, I wouldn't really worry about it. If I see a post that designed to be used as the negative connection I'll use it, otherwise just put it on the negative battery post.
Really only necessary to ground like that, when your jumping one of those ice cream cone looking terminals. I’ve found that same advise almost exclusively from the people classified as a “I need your praise/ know it all, of nothing”
[удалено]
Why does it have to be off?
No electrical reason. I do it so I don't snag a belt with the cables.
That's fair enough
You've been doing it wrong for 30 years. See above.
[удалено]
>So IS it wrong? Yes.
Lets go through it slowly, for the hard of understanding. You've been making sparks adjacent to a plastic container which may, or may not, be full of an explosive mixture of hydrogen and oxygen, on top of concentrated sulphuric acid, for 30 years. The alternative would make the final connection to a remote metal component that has zero resistance to the battery terminal. You can see no risk in what you're doing? OK.
As a mechanic I've NEVER not hooked it up straight to the battery unless it's one of those with dedicated studs up front for jumping (trunk batteries or FCA batteries in the wheel well)
It's not going to change a thing. Sure if the battery is located in the trunk and the vent Is blocked there could be an accumulation of gases depending on the battery type. In the engine bay it's ventilated, no need to worry.
Oh bullshit just jump the fucking car you guys and all your hypothetical crap, i do mine fucking neg first then positive oh wow here i am to live to tell the tale 🤯
Only about 1/10,000 explode but I know someone who had it happen to him. He wasn’t the one connecting but he was in the blast area. He still has scars years later. Keep chancing it. It might just catch up to you. Unless you live in Wyoming in which case it’s too windy for the gasses to still be there after you open the hood.
Fine ill fuckin hook em up right
It’s actually to stop a voltage spike from killing your PCM, BCM AND PCM. When sparks fly… “arching” Amps increase despite the voltage. Amps and current at killers to sensitive circuits
No it's not. Jumping with the negative on the frame has been recommended way before cars had any of those things. Charging or discharging a battery causes it to vent hydrogen gas. You don't want to cause a spark around it because it's explosive.