# When jump-starting a car, won't there be a huge back current into the dead battery?

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

• As the battery discharges, its internal resistance increases. Also, the voltage of the discharged battery will quickly increase when charging off the good battery, causing less of a potential difference between the two batteries and therefor a decrease in current flow. Commented Dec 18, 2015 at 13:44

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.

• If you hook up two batteries as you described, they would still form a complete circuit. The stronger battery would push a current through the weaker battery and reach its own negative terminal. Commented Dec 15, 2015 at 23:39
• @JohnZheng - If you take two batteries, by themselves (out of the vehicle), and connect jumper cables to them (P2P & N2N), there is no circuit. You have just created a stronger battery which is just sitting there doing nothing. This is the way you jump start a vehicle. If you do it as you suggested, you create a flowing circuit which will fry your electronics (mainly the alternator). This is definitely the wrong way to try and jump start a vehicle. Commented Dec 16, 2015 at 1:31
• I'm not sure if that is correct. Please see this here: physics.stackexchange.com/questions/64509/… Again, I'm not sure myself but I don't want to destroy anything. Thanks. My dead car battery is only at 7 volts (12 v battery) and people tell me i should buy a new one :) Commented Dec 16, 2015 at 2:00
• @JohnZheng - I'm sorry if you don't believe me. I'm not a physicist, but I understand the principle. What I'm telling you is accurate. I've been dealing with cars for 30 years. You attach P2P and N2N. If you don't believe me, then have at it and blow your car up (could be quite literally). You can put your battery on a charger overnight (low 2A setting) and see if it will charge. If it won't, then you'll need to purchase a new battery. Give it a try on a charger, as you have nothing to lose. Commented Dec 16, 2015 at 2:06
• @JohnZheng You are in reverse land. You will let out the magic smoke from your vehicle and when you can't gather it all back up and jam it back into it you will regret living in reverse land. Once you let out the magic smoke, you can't put it back in. Just sayin. Commented Dec 18, 2015 at 0:12

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.

• To add to Ecnerwal's answer, the low voltage / high current circuit of the 12v car battery system requires a very good connection to the battery, so make sure the clamps are as tight as possible on the battery leads, to eliminate any resistance. The problem of hydrogen gas is an issue once the charging process is going, the battery will heat up also. Disconnect a charging battery at the G car end to avoid sparks near the charging battery.
– philbrooksjazz
Commented Dec 15, 2015 at 19:44
• Thanks for the answer. Just to clarify, the regulation circuit in the G car basically limits the amount of current flowing out the battery? I.e., hooking up cars this way is not the same as directly connecting + to + and - to - between the two batteries? I have never tried connecting the batteries directly but I assume one might explode. Commented Dec 15, 2015 at 23:36
• Luckily on my dead car, it is a Chevy and its actual exposed terminals are far away from the physical battery. Commented Dec 15, 2015 at 23:37
• Yes, the voltage regulator (actually on both cars) will protect your dead battery, and yes, there is current flow! (from comments above) how else would the dead battery charge? Also, jumper cables are excellent conductors - notice how big the wires are, probably 0 or 00 gauge. Commented Dec 16, 2015 at 20:14
• The voltage regulator on car G will only protect the battery on G from being overcharged. It will not prevent a huge current rush from G's battery do D's battery. Commented Jan 12, 2017 at 19:39

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.

• Actually, jumper cables are excellent conductors, they are usually 00 or 000 gauge pure copper. You won't find a better conductor unless it's an industrial application. Yes it's probably true that the jumper cable clips are not great, you need to clamp them on the battery terminals as hard as possible to reduce the resistance. Commented Jan 28, 2016 at 17:26
• @philbrooksjazz There are many more 10 and 12Ga jumper cables in the world than 2/0 or 3/0 ones. Some do have misleadingly thick insulation for marketing purposes. While a tow-truck might bother with 2/0 cables, few other people will pay for or pick up the weight of them (I have a set of 2/0 welding cables, so I'm rather familiar with the concept - and I have never seen jumper cables of that size...) - in most cases the smaller cables are actually perfectly adequate for the job (which is not directly starting the car, but recharging the victim battery enough that IT can.) Commented Jan 28, 2016 at 17:50

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:

1. The spark ignites hydrogen escaping from the battery. Rare, but apparently possible.
2. The current overheats the jumper cables. Don't buy cheap jumper cables that are too thin.

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.