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I know that when both connecting and disconnecting battery jumper cables between two car batteries there is a prescribed safe way to do it. I can never remember the correct way because I can’t understand the reasoning behind the prescribed sequence. Is there an easy explanation / (chemical?) principle that can make this procedure (the sequence of both connecting & disconnecting the four posts) more logical and therefore easier to remember?

My confusion is that at some point you must make a connection that causes sparks (which risks battery explosion) because it completes the circuit. Completing the circuit can’t be avoided, so why does it matter which post you are connecting to when you complete the circuit?

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    $\begingroup$ I'm voting to close this question as off-topic because if that's about chemistry then every experienced driver should get a diploma. $\endgroup$ – Mithoron Aug 10 at 18:09
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    $\begingroup$ Karl has the right answer. This is not about chemistry, it would be a better fit on mechanics.stackexchange where it has already been asked many times. mechanics.stackexchange.com/questions/22296/… $\endgroup$ – Level River St Aug 10 at 21:04
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    $\begingroup$ I think this is a valid chemistry question, wondering wether there can be a chemical reason behind the recommended procedure. $\endgroup$ – Karl Aug 11 at 6:43
  • $\begingroup$ My fault. I’m a new user and was attempting to answer a chem question that was protected and required 10 reputation. So I entered this question, but got stuck in chemistry and very confused about how to navigate this site. I’d be happy to move it if only I could figure out how to do that. Let me know. Thanks. $\endgroup$ – Drew Aug 11 at 12:11
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Many recommended procedures such as this one call for connecting (+) to (+), then (-) on the "good" battery to a metal component in the "dead" car engine. The negative terminal of the battery is grounded to the metal components, so we may think of the last connection as (-) on the dead car. If a spark were to form when the circuit is established on the last connection, it would occur at the metal component and not at the terminal where it could interact with electrolytically generated hydrogen or oxygen gas.

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  • $\begingroup$ Ive heard the same reasoning, but what hydrogen concentration could there be? Theres a good reason why lead batteries must be located in a well ventilated position. Also the dead battery specifically, being dead, is most unlikely to have recently produced any hydrogen. $\endgroup$ – Karl Aug 10 at 17:25
  • $\begingroup$ The "dead" battery could still have some voltage left, just not enough to start the car. Hydrogen could come from a small amount of water decomposition, which may not be complrtely avoided in an aqueous electrolytic cell. $\endgroup$ – Oscar Lanzi Aug 10 at 17:29
  • $\begingroup$ Surely, below ten volts, it cant start a car. But I thought hydrogen generation is (or was, rather, with modern batteries) usually occuring during recharging, isnt it? $\endgroup$ – Karl Aug 10 at 17:40
  • $\begingroup$ Recharging is basically what we're doing. And I agree the risk is less now but safety procedures should always be redundant. $\endgroup$ – Oscar Lanzi Aug 10 at 17:44
  • $\begingroup$ But it only starts when you have already connected the cables. One legitimate reason could be that an old, oldstyle battery with low electrolyte might contain quite an amount of H2+O2 mixture in its housing. That would be a bomb. $\endgroup$ – Karl Aug 10 at 17:51
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Done that dozens of times, have never seens sparks fly. The current flowing is not exactly huge, you are connecting (first) + to +, and (then) - to -, after all. The voltage difference is perhaps 3V, not 12.

I don't think there is an electrochemical reason to do it in this order. Afaik it's just so you dont short-circuit the good battery if you slip with the second cable and touch the chassis. That would give a huge spark, probably weld the connector to the metal, and definitely leave a hole.

From such a discharge, a spark might even find it's way into the battery, which, at least in the old days, might contain some highly explosive electrolysed $\ce{2 H2 : 1 O2}$ mixture. Especially if the battery was old, hadn't been refilled with dest. water for a while, and was therefore weak, which is why your car didn't start in the first place ...

Modern car batteries are usually sealed.

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  • $\begingroup$ Perhaps its also safer to first contact + on the empty battery. That way any mishap with the other end of that cable is less devastating. $\endgroup$ – Karl Aug 10 at 16:33
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    $\begingroup$ This is the right answer. The negative terminals are connected to the car bodies. So if you connect the negative lead first, then it is possible to accidentally connect the positive lead to one battery and some part of one of the cars' bodies, leading to a massive electrical discharge and fire. Get the positive lead securely fastened first, then you can connect the negative lead safely. $\endgroup$ – Level River St Aug 10 at 20:59
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    $\begingroup$ There is a chemical reason in that the battery might contain hydrogen gas which can cause the battery to explode. I'd guess that this is less of a problem now with "sealed" batteries, but in the olden days when the batteries weren't sealed and you had to monitor the fluid level in the batteries it was a very real problem. No just from the shrapnel, but also from the sulfuric acid that got spayed on everything. $\endgroup$ – MaxW Aug 10 at 21:38
  • $\begingroup$ @MaxW Not just hydrogen gas, but a nice stochiometric 2:1 mixture or H2 and O2. But it is inside. I guess together, these two arguments make sense: A mishap with the cables, a strong electric discharge, and a spark flying from there finding it's way into the battery. I'll update. $\endgroup$ – Karl Aug 11 at 6:23

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