New answers tagged jump-start
I don't know the actual justification, but to me it makes sense to always attach positive first because it is easy to accidentally touch the lead to something else while installing. If the negative lead were already attached, then touching the positive lead to just about anything on the car that is metal would short out the battery.
Conventional lead acid batteries produce hydrogen gas as a byproduct of the charging process. This gas tends to collect in and around the battery. As you are aware making the final connection can generate a substantial spark. By making the frame connection the point where the spark occurs it is far enough from the hydrogen gas to avoid an explosion.
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