# If you connect 2 car batteries in parallel and one has a low charge, how much would the low charged one draw from the other battery?

I'm just wondering this for the sake of understanding.

If you have a low charged battery that is connected in parallel with another battery (that is fully charged), how many amps would the low charged one typically draw from the other while it's 'charging' or equalizing with it.

• You won't be seeing big currents from one lead acid battery to another, if they have equal amount of cells and no cells are shorted due to aging. 100% SOC battery may have 12.7 V and 0% 11 V (you shouldn't discharge most batteries that much). But when you put some load to that 12.7 V battery, the voltage drops very easily a few tenths. E.g. putting lights on on a car (~15 A) would drop 12.7 -> 12.1-12.4 V depending on the battery. Cranking at 100 A will drop to about 11 V and the voltage drops maybe to 10 V at 200 A. Dec 13 '20 at 20:51

It depends on so many variables! Let's try to make it simple.

For starters, I'm gonna assume that the two batteries are identical, standard deep cycle automotive lead-acid, 12V, 100Ah, brand new. One of them is fully charged and the other zeroed. The empty battery will request almost all available current on firsts seconds, which in this case, near 1100A. (assuming a zeroed battery have a standard internal resistance of 5mohm. It can vary, and you can look for the battery datasheet). And it will kill the battery due to overheating. Not a good thing.

But if you have a current regulator, you can plug it between the batteries and set the current to the standard maximum charge current (which is about 25% of the battery capacity), in this case, 25A, and let them equalize. Using this much current, the losses will be around 30% to 40%. Then, after a few hours, you gonna have the two batteries equalized with something around 40Ah of charge each.

• That’s odd, when I have connected a fully charged slave battery to a flat battery I have never seen a spark fromm 1100A, even when bouncing as making the connection. Dec 10 '20 at 19:44
• That makes sense - so best bet is to make sure that the batteries are fairly equalized before connecting them together. If we're dealing with an auxiliary battery, that would need to be topped up occasionally, I'm assuming a battery isolator would work best, as it can control the charge current - is this correct?
– xil3
Dec 10 '20 at 20:08
• @SolarMike is that true? I pretty much have the opposite experience: I'll get a spark any time I even wiggle one of the connections when trying to charge a low battery from a fully charged one. Enough of a spark to seriously concern me and make me take care. Dec 10 '20 at 20:24
• @RoryAlsop I always made sure all loads were off when connecting, minimised the chance of a spark. A friend did not and I watched the battery explode, covered 3 cars in acid that needed respraying and we had to drag him to the washroom to get the acid out of his eyes - lucky we were in time : no damage. Dec 10 '20 at 20:28
• @SolarMike Yeah, I got what you're saying!! I'm just showing the math of a perfect world here. Also, the AIR resistance to make ARC is bigger than the internal resistance of the battery! A simple Honda Civic engine START drains about 800A from the battery at the start, and as the voltage drops, the current increases to more than 1000A (The higher my amp meter can measure). I made these tests a lot. Dec 11 '20 at 12:46