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So imagine I have car D with the dead battery and car G with the good battery. Initially, both cars are turned off. I hook up car G to car D in the proper order. Before I start either car, won't there be a complete circuit going from battery G to battery D in the form of a back current? Because, AFAIK, the internal resistance of car batteries are so small, won't that mean the back current is rather large? So how is this procedure safe?
Thanks, John
I think you have an issue with the way you are thinking. You are describing the issue as if the batteries are hooked up in series. In that case, yes there'd be a huge back current going on. Both batteries would be made into a complete circuit and you'd have global thermal nuclear meltdown (or whatever the car battery equivalent would be). This is not the case when jumping a car as the batteries are hooked in parallel (positive-to-positive & negative-to-negative). There is no circuit made, just additional amperage to get the dead battery car started.
As for how you actually jump vehicles, you need to have your "good" car running at the time you attach the jumpers or you run the risk of damaging your vehicle's electrical system.
in practice the voltage on a resting depeleted battery while receiving a moderate charge and the voltage on a good battery at is about the same. so some current will flow into the flat battery, maybe 20A or so, but this is not much compared to the starter current.
Also if you're doing it right the leads on the broken car go to the top end of the started cable (battery positive terminal) and to the engine block, so when starting is attempted the resistance of the path through the starter will be lower than the path through the battery.
Charging the "dead" battery isn't actually a bad thing, The depelted battery does receive some charge, the smaller jumper cables (like 100A or 200A) won't carry enough current to start the engine, their use relies on the depleted battery receiving some charge anf giving some assistance before the engine can be started.
Yes, there will be some back current flowing into the dead battery, but the same regulation circuit in the G car will protect it from doing damage. You can connect the D car with the G car running to get more current. I have done this many times, and never had a problem - you'll notice a spark once you complete the circuit, but this is normal.
Jumper cables are not that great as conductors go, and jumper cable clamps are not that good as connectors go. If there's a hope in heck for the dead battery, the voltage difference is only a couple of volts. If the voltage difference is more than a couple of volts, the dead battery will also have a very high resistance due to expired cells. If you really follow the published proper procedures, you will be connecting to the metal body of the car rather than directly to the battery for even more resistance.
As for safe, numerous people manage to blow themselves up doing it wrong every year. But that's mostly from managing to combine a collection of hydrogen with sparks. Dead shorts are an interesting screw-up as well. It's reasonably safe if you know what you are doing and do it correctly.
I'm not sure I understand the question with regards to "How is this safe?"
I'd like to pose the alternate question; how is it unsafe?
The procedure is that you have the car with the good battery running. When you make the last connection you will sometimes see sparks. Jumper cables are typically well insulated so provided you've not got unburned fuel around the area that's sparking, what is the problem?
Also, you usually have to leave the good car hooked up and sometimes rev the engine for a while before there is enough charge in the flat battery to try to start the car.
Yes, there will be a large current going from G's battery do D's battery. You'll usually get a spark when you connect the last jumper cable.
There are two potential problems:
To mitigate the risk: be decisive when you attach the last jumper cable. Don't slowly approach the terminal and allow sparks to fly for too long, get it on quickly.
By the way, it's recommended to have G's engine running before you connect the jumper cables. If you leave the engine off until after connecting the cables, G's battery will have to start G's engine while there's a large current going to D, potentially not leaving enough current to start G.
The large current is also the reason that a jump start is seen as a last resort: the current inrush is not healthy for the battery. If at all possible, use a battery charger instead: this charges much slower, but puts less strain on the battery.
