I thought it wouldn’t matter much, but apparently there is an order for disconnecting too. My question refers to jump starting one car with the help of another.
In: Engineering
There is a small but significant chance of hydrogen gas (or other flammable fumes) to build up near/in the battery. Because of this, you do not want any igniting sparks to appear near there. Because connecting the final circuit will often produce sparks, the final connection will be done far away from any battery.
The other connection orders is to reduce the chance of a loose cable from accidentally connecting somewhere while hanging: disconnect the ones that are completing the circuit while you have both hands available, and *then* drop the completely unconnected ends that won’t matter if they bump into something metallic in the car.
The battery might release explosive gases when charging, and disconnecting the cables under load might spark, which could ignite the gases.
So you want to disconnect negative first on the dead car, as that’s the end that should be connected to a ground point, and not directly to the battery.
Also, since electricity flows from the negative (black) to the positive (red), this way you also reduce the risk of shorting anything or doing other damage if you’re clumsy with the cables.
Two reasons.
1) All the steel parts of both cars are typically connected to the negative of their batteries so if you are not careful you cause a short if you let a positive connected jumper cable to hit the anything metal on the car.
2) When you make the last connection there often will be a small spark produced. Ideally you don’t want to make that connection at the battery as there is a very small chance you could cause a battery explosion from the hydrogen released by charging batteries.
I have experienced a lead acid battery explosion from a different cause and I do not recommend. I was fine, a coworker got battery acid all over him and some shards of plastic stuck slightly into his skin.
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