I have a very simple question about batteries in parallel and I think I know the answer, but I'm having a terrible time finding clear confirmation. My question is this: What happens if two batteries of different capacities, but same voltage, are placed in parallel? For example, if I put a 2 Ah 1.5v AA battery in parallel with a 20 Ah 1.5v D battery. I think that the larger battery would simply supply more current than the smaller one, so that they both lose voltage at the same rate. So in my example, since the D is 10x the capacity of the AA, it would supply 10x the current. So if I have a circuit that draws 11A, the AA would supply 1A and the D would supply 10A. Right? Like I said, I'm just having trouble confirming, or denying, this, and I would love it if someone could give me a clear answer.
Thanks
~Josh Blackburn

 

Series and/or parallel combinations of batteries should be matched.

Current supplied by each battery will depend on it's voltage and internal resistance. The AA will get depleted sooner than the D. When that happens, the D will try to charge the AA and also supply more current.

http://batteryuniversity.com/learn/article/serial_and_parallel_battery_configurations

 

If the two batteries are the same brand and chemistry but different capacities, then they will both discharge at a rate more or less proportional to their capacity (the larger delivering more current since it has a lower internal impedance and these impedances increase as the batteries discharge).
This continues until they both end up discharged at the same time.
The two in parallel act more or less like one larger battery.

That's okay for discharge of batteries but different batteries should never be paralleled for charging.

 

Josh, do you have any other news on this? I am wondering the same thing. I want to add a battery to a circuit that has a 3 year old battery. Both the same chemistry and exact same cells, just aged differently. After reading threads all day long everyone always says not to do it and that my house is going to burn down.

It sounds like you guys have it figured out and its not that bad.

 

You must first connect the different batteries through the current-limiting resistance. Leave this combination of batteries and resistance for a day or more. Then remove the resistance and connect the batteries.