It would be a bit perverse to have the batteries lined up so their voltages subtracted. The lower-voltage one would then be charging up when the current flowed, taking in energy rather than putting it out. Batteries often cannot be overcharged very much. The ions that carry the current start to get depleted, and the resistance go up. Some explode when overcharged.
So let's say you're talking about the battery voltages adding. Some current will flow. If your circuit is ohmic, the current will just be the total voltage over the circuit resistance. Now that current flows through each battery. The power output of each battery is just its voltage times that current. You need for that to be less than the rated power for each of the batteries. Otherwise the voltage of the one exceeding its rated power will drop noticably.
Mike W.
(published on 10/22/2007)
