For some reason I’ve found myself giving quite a few boosts (aka jump-starts) to folks lately with jumper cables. In reviewing best practices for this procedure, there appears to be some contention online as to the risks associated with connecting the negative (black) post on the live battery to the same post on the end battery. Some guides indicate there is a chance this could cause an explosion. I would like to know if this is highly unlikely or not. Here are some citations that illustrate the different views:
Globe and Mail (one of the largest and most reputable newspapers in Canada)
Make sure the connections are solid and don’t cross the terminals.
Attach the RED clamp to the positive post on the dead battery
Then put the other red clamp on the positive post on working battery
Black clamp to neg post on working battery Black clamp on negative post on the dead battery.
Whereas Wheels.ca another large car site says “Finally, connect the other black booster clamp to any bare metal part of the engine block, far away from the battery, on the car being boosted. Never make this last connection at the battery post. You don’t want to risk a spark and possible explosion from hydrogen gas emanating from the battery.”
And finally, WikiHow saysit’s preferable not to connect the two neg posts but it’s not a big deal. “As a last resort, you may connect to the negative (-) post of the dead battery, but this risks igniting hydrogen gas coming off the battery.”
So dopers, I ask you, how likely is this practice to actually result in an explosion with a modern car? Are there many documented cases? Whats the straight dope?
The issue is not blowing up the car, but blinding yourself with Sulphuric acid when the battery explodes. It’s probably a slim chance, but why risk it?
With all the rustproofing on a modern car it can be hard to find a suitable bare metal part. Just don’t lean over it while you make the final connection.
I have seen a battery pop when the final connection was made to the post. This was back in 1978 or so, so mileage may have varied somewhat since then. Still, I always attach the last clamp to the hood latch or something like that.
I personally have only seen one battery explode due to hydrogen gas, and this was back in the 1970s. It’s a pretty rare thing. On the other hand, if you do this sort of thing all the time, your chances of it happening to you increase proportionally.
The danger isn’t just from hydrogen gas. If one of the batteries has damaged cells for some reason and basically presents a short circuit then excessive current can flow from the voltage difference and cause a battery explosion due to the electrolyte boiling. Similarly, I’ve seen cars where the positive and negative wire colors were reversed, so if you don’t double-check with the plus sign on the battery it is very easy to connect positive to negative and vice-versa, again possibly causing an explosion due to the electrolyte boiling from the excessive current.
The idea of making the last connection to the frame is that it keeps your face away from the battery in the rare case where it does decide to go ka-blooey.
You might jump batteries frequently and never see a problem for the rest of your life. Or the next one you try could blow up in your face. Better safe than sorry.
I have personally been caught in a battery explosion.
When I was a teenager, I worked for the maintenance department at our city recreation facility. Our power backup system was a bank of large marine batteries, and one of the regular duties I was given was to check the charge levels of the batteries with a battery tester. During a routine test, I apparently caused a spark that ignited the hydrogen on one of the batteries, completely blowing out the front of it. It threw me across the room (not a huge room, mind), somehow blew the sole off of my right shoe, destroyed my watch, sprayed my face with something (didn’t seem particularly caustic) and was apparently quite loud, though I didn’t particularly notice. I just remember being upright one second, and picking myself up the next.
So if I understand correctly, you’re suggesting that connecting to the frame In no way lessens the chance of an explosion, it simply forces one to protect one’s face in case there is an explosion?
It does both. The last connection is the one that will generate a spark. It is the spark that can set off the hydrogen gas from the battery. Putting the spark further away from the source of the explosive gas will lessen the chance of explosion and put you further from ground zero should it happen anyway.
It will lessen (but not completely remove) the chance of a hydrogen gas explosion. It won’t lessen the chances of other types of explosions (built-up pressure from electrolyte boiling, for example).
But yeah, the main this is that if the battery decides to explode for whatever reason, it’s best not to have it go up right in your face.
Recognizing that IF there is to be a gas explosion (not a battery boiling over/out) the explosion will be ignited at the location of the spark = wherever you make that 4th connection. IOW, near your face again.
The hoped-for safety gain in that case vice the spark happening at the battery terminal (and also near your face) is twofold:
The odds of an explosive gas mixture well away from the battery is less than the odds of one close to the battery.
If there is an explosion of gas well away from the battery, there is some decent chance the battery itself won’t burst & spray acid; they’ll just be a brief flash of fire, a “pop” sound and that’ll be it. That’s much better than an explosion that also bursts the battery and sprays acid & plastic shrapnel too.
The ideal place to clamp on is the engine block or some other piece of metal on the engine, but that’s easier said than done with all the under hood plastic covers the car makers are so fond of these days.
The drawback to clamping onto the body of the car is that sometimes the body ground connection isn’t robust enough to actually jump start the car if the battery is really low. You just have to sit and wait for the battery to charge up a bit, which granted isn’t a bad idea for the health of the donor car’s alternator.
On over 40 years of doing automotive type stuff I have seen 2 battery explode and maybe a dozen or two batteries that exploded.
One I saw go up the owner was fucking around under the hood and shorted it. The other a technician used a flammable cleaner to clean the top of the battery during a service. Then when he disconnected removed the battery cable it sparked setting that off and some hydrogen also. Customer got a free battery. Tech got razzed.
The previously exploded ones were all the OE batteries on Puegot 504s which makes me think they were defective as the replacements didn’t have an issue.
I have never seen a battery pop when being jumped. Can it happen?
Sure it can, but I think you can file it under long odds.
This is not quite true. Boiling electrolyte does not explode/combust on its own. There has to be an ignition source.
Personally for this reason I prefer to make the last connection to the good car. There is less chance of it having a problem battery.
if you can’t make the last connection to metal on the engine then indeed it is best to connect to the good battery last.
also do look at the battery for the + and - markings. there are cars where the + is just two inches from body parts when you might think it would be better the opposite like most cars (that i’ve seen).
Wouldn’t putting a simple on/off switch in the middle of the cables solve the problem?
You connect all four ends of the cables with no current flowing through them. Then you flip the switch in the middle of the cable and the current starts flowing. If a spark is going to happen, it will happen in the middle of the cable, far away from either car and it prevents you from shorting the cables in the process of connecting them.
I’m sure I’m not the first person to think of this. Why isn’t it default option on jumper cables? Is there some obvious problem with it that I don’t see?
In searching to see if anyone sold such cables, all I could find were more sophisticatedcables designed to prevent cross-connecting the positive and negative terminals, but no simple on/off switches.
in operating the switch, and setting it down, you are moving the cables and could dislodge a clip.
also by clipping you are removing some corrosion and making some bare metal. the switch would be massive and hard to work to do the same. if the switch is small and easy to operate it will keep the corrosion, which is a bad thing.
It’s mostly just not that big of a deal. You might as well follow best practice when hooking up the jumper cables, but it’s not so much of a problem as to justify some sort of more expensive safety jumper cables. Especially since these days most risk-averse types just call AAA.
FWIW, when I need two sets of cables (which is probably also not best practice) I always hook up both sides and then clip them together in the middle.
The last couple times I’ve jumped a battery, it’s been outside, and there was a bit of a breeze, and I was wondering how there could possibly be enough build up of Hydrogen gas for it to ignite, let alone explode.
I dutifully tried to make the last connection to the frame, but there’s never a good connection point, so after a bit I end up connecting the the battery post anyway. Hopefully, if the battery is primed to pop, it would do it when I’m first trying to connect to the frame.
Does anyone make an aftermarket connector for jumping? Or any suggestions for making one? I’m thinking just a metal post with a tab with a hole, that you could undo one bolt, and replace it through the tab. Then I would have something I could connect that last cable to.
Bit of a hijack
A couple of time I’ve tried to jump a car with the last lead on the frame but it wouldn’t start. It would start if I put the last lead on the dead negative. Why is that?
Possibly related to the first. The lead on the frame isn’t really grounded is it? Aren’t tires a good insulator? If the opportunity presents itself, would it be better to ground it directly to earth like clamping it to a metal fence?
“Gounding” has nothing to do with it.
The negative terminal of the battery is supposed to make good contact with the frame and engine block of the car. If it doesn’t, it can lead to all kinds of problems, because the car body is used as a return path to the battery (instead of running two wires everywhere, only one is needed).
So, if you can’t start the car by connecting to the frame of the car, you either have picked a bad spot, or the negative lead from the battery is not making good contact with the frame (the battery is usually connected to the engine block with a very heavy wire, and with a smaller-gauge wire to the chassis of the car, so putting your jumper cable clamp on the engine block is the best place (I used to use the engine hoist point).