Seems like you’d want to be careful with the source of the electrons so the negative terminal would be covered/protected and the positive would be wired to the frame. But it is the opposite. I presume there is a good reason that I do not understand.
In: Engineering
It’s mostly for liability purposes.
Batteries generate hydrogen gas when the are charging. This can sometimes explode when connecting the battery cable as there are usually sparks. Ideally, connect the positive first, then connect the ground further away from the battery.
By jumping directly to the positive and negative terminals, you increase the risk of igniting potentially flammable gasses coming from the battery via the spark that often happens during the connection. These gasses can occur in certain scenarios, though is much more rare in this day and age.
If they tell you to connect both the positive and negative, and you cause an explosion, they could potentially be held liable.
If you find a CLEAN ground, you can successfully jump a car in the manner described, and it IS safer. But it is often hard to find a clean ground inside an engine chassis on a vehicle that needs a jump.
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