Also, how does electricity flow from one car to the other if there isn’t a loop?
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Non-perfectly sealed car batteries will slowly leak flammable gas, and an active spark would light it on fire. So the proper procedure has you connecting the last time (which is most likely to produce sparks) far away from this potential fire source.
Cars are wired so that ‘ground’ connects to the negative terminal, and ‘ground’ is basically the metal chassis of the car. So connecting to the metal frame of the car *will* connect the batteries in a loop.
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.
Umm, where exactly do you get this advise. I have never heard it before, been using jump leads on car battery terminal to terminal since 1994.
I’m not sure this is fact, but when I learned about jumping cars it was explained to me that by grounding the negative on the target vehicle, the current reaches critical electrical devices more easily, thus the load requirement to start the target vehicle is reduced.
To answer the second question, you are putting the good battery and the dead battery in parallel using the jumper cables. This will cause 12 volts to appear across the terminals of the dead battery, and allow you to start the other car. (A charging current will also flow *through* the dead battery during this time, but the good battery can supply that no problem.)
Once the other car is running, it’s own alternator will put 12 volts across its dead battery, and hopefully charge it.
It’s more of a tradition now. The old timey batteries that had fill holes where you could top off the acid level would leak a lot of hydrogen gas.
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