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Proper jump start prcedure

I know that the correct way to jump start a car is to connect the positive terminals first, then the negative of the good battery, then the last connection goes to the frame of the car with the dead battery. This last part is confusing to me. I don’t understand the point of this. The frame is connected to the negative terminal of the dead battery, so connecting the cable to the frame should be equivalent of connecting it to the battery. I don’t understand why does this “reduce the risk of ignition”, as Wikipedia (and others) suggest.


Think of what the source of the vapors is,the battery. If the last connection (where a spark will occcur) is away from the flammable vapors, lower risk of ignition is the result.

A low/dead battery produces hydrogen gas. If there’s hydrogen gas coming out of the battery, and a spark is created when making that last negative connection to the bad battery, the battery can explode. Seen that one a few times.


Now I get it, thanks!