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Guides to jumping a car with a dead battery commonly advise that the cable from the negative terminal of the good battery be connected to the chassis of the stalled car rather than to the negative terminal of the dead battery. One of the reasons for this is that a spark or electricity arc can occur when the clamp is in close proximity to the terminal or chassis, and this spark can ignite hydrogen if it happens too close to the battery.

Is this similarly a risk when removing or installing a battery? I.E. can a spark cross the small space between the cable and the negative terminal as you are removing that cable, igniting hydrogen? Why or why not?

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3 Answers 3

The reason for connecting the earth lead to earth on the engine or the chassis and not the battery is for protection of the electronic equipment of the vehicle. With the battery connected it will act as the necessary resistance to prevent any voltage surge. A flat battery will not give off hydrogen gas. A hot/just charged battery will give off the gas which has a distinctive acrid smell which a spark can ignite. Your car has a diagnostic plug. If you look at a scanner plug that plugs into it, you will see that two of the central pins are longer than the others. Prevents a surge.

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Hydrogen is odorless, i.e. no acrid smell. Also, the reason for making the final connection to a chassis ground rather than the battery is simply to decrease the chance of igniting any hydrogen around the battery. The chassis ground and the battery negative terminal are at the exact same electrical potential. There is essentially no resistance between chassis ground and the battery negative terminal--they're connected by a big fat ground cable. –  mac Nov 18 '13 at 14:57
    
www.chapmanbmw.com › BMW Service Center › BMW Ultimate Service. (Should clear things up a bit) –  Allan Osborne Nov 19 '13 at 11:24

Batteries give off the most hydrogen while charging or discharging so an idle disconnected battery probably isn't in danger of exploding. If you are connecting a battery to a vehicle the risk is pretty low because the battery shouldn't be producing hydrogen. If you are connecting jumpers to another vehicle's battery there is a higher risk because there could be some draw on the battery and also if you had just been cranking the vehicle then you just placed a large load on the battery which could have caused a decent amount of hydrogen to be produced. In this case I would say give the battery a few minutes to air out with the hood open then connect jumpers.

Here is a video of how easy a battery can explode under the right circumstances (while charging).

Also AGM batteries are completely safe from this.

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I'd like to preface my answer by saying that I think the OP has asked an interesting theoretical question that deserves a theoretical/hypothetical discussion. In no way should the following be taken as an indication that removing/installing batteries carries low risk of explosion. Take precautions when working around vehicle batteries.

The risk of hydrogen gas explosion during battery removal/installation can be similar to the risk during jump starting, though as a thought experiment (read 'i have no proof'), I believe there are some important differences: the potential for/severity of arcing, and the presence/concentration of hydrogen gas.

Regarding the arc: when removing/installing a battery, presumably the vehicle has most electric systems switched off, so there is very little current draw on the battery, and thus lower potential for arcing (it's nearly an open circuit, which of course would not arc). Compare this with jump-starting. You have one vehicle with a good battery at 12-14V, and another vehicle with a discharged battery (maybe < 11V) that is prepared to suck up a whole bunch of current while trying to recharge itself. I would think this creates a higher potential for arcing.

Regarding the hydrogen gas: this stuff is lighter than air. It would tend to dissipate. when installing a new battery, you're making the connection in an airspace that should be largely free of hydrogen--When you removed the old battery, you removed the only source of hydrogen, and the new battery was just recently introduced to the space, and so would not tend to have had a chance to emit much hydrogen in the area.

That said, be careful whenever working around batteries. One source says 2300 people a year in the US are injured by lead-acid batteries, many of these are acid burns to the face. Ouch.

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Well put. Every where I read and everything I was taught never mentioned any concern for arcing when changing a battery. Castrol: How to Replace a Car Battery. In theory, however, it is entirely possible, though unlikely, that an arc could occur and ignite gases left by the battery. I imagine it would require an odd set of circumstances. –  Seminecis Aug 29 '13 at 4:49

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