Can someone explain to me why they don’t recommend putting car jumper cables on the working battery black terminal to the dead battery black terminal but rather to an unpainted metal surface on the donor car?

Question

Also, how does electricity flow from one car to the other if there isn’t a loop?

In: 26

Answer 1

When charging, those batteries leak hydrogen gas. Which is flammable.

The final connection will generate a spark. None of the previous connections will.

If possible, you want the spark to NOT be near the battery, which will start charging as soon as the connection is made too. Thus you make the final connection to the chassis (which is connected to the negative terminal, so it’s effectively the same thing electrically), not the battery post.

It’s one of those rare things, and with many modern batteries it’s not an issue as they can absorb the gas rather than vent it (but not all!), but it literally doesn’t hurt anything to do it like that and get into that habit, so why not just learn that way?

And there is a loop. The positive of the “charger” car is sending electricity to the positive of the “dead battery” car to charge it, which is going through that battery, charging the plates inside it, returning down the negative cable, back into the negative of the “charger” car.

That’s why the battery charges up, that’s why it generates gas (because it’s charging from the other car) and it’s also why the “charger” car should be running when you do so, otherwise you’re just sharing battery power between two cars, rather than giving the excess generated by the engine of the first to the second.