Now lets say that I connect this pack in series with a single additional identical NiMH battery. Under load (a load that is close to or at the maximum discharge rate of the batteries), I assume that the voltage will drop as current flows through the batteries. Then I additionally assume that the voltage drop across the parallel pair will be less than the voltage drop across the added third battery due to the differences in their internal resistances. This would lead to a voltage imbalance between the parallel set and the third 'in series" battery which would, I assume, cause additional current to flow from the parallel pair to the third battery.