When batteries are connected in parallel, the voltage across each battery remains the same. For instance, if two 6-volt batteries are connected in parallel, the total voltage across the batteries would still be 6 volts. Effects of Parallel Connections on Current
Yes, parallel batteries "can" supply twice the current when the load is less than the ESR of the battery. ( As shown above, for short circuit current, it is twice.) But otherwise, when the load is equal to battery ESR, the current is the same. With series cells it greater when the load R is higher than ESR, the higher V/R produces a higher current.
The voltage difference between A A and B B can be seen as the output voltage of the two batteries combined so that's why the voltage doesn't increase when you combine batteries in parallel. To see why every part of the wire is at the same voltage we can look at the water analogy. Connecting two wires together is like joining two canals together.
Although it is never advisable to connect two non identical batteries in parallel because it does not make any sense it is useless and may destroy the batteries. In short, when two non-identical batteries are connected in parallel, current will flow from higher voltage battery to lower voltage battery. Which is not good.
The parallel voltages are matched before putting in parallel. The series batteries are fresh and have same capacity in mAh before loading. Mismatch increases towards end of life so the weakest cell fails 1st. The short circuit test , Isc is momentary. simulate this circuit – Schematic created using CircuitLab
In a parallel connection, batteries are connected side by side, with their positive terminals connected together and their negative terminals connected together. This results in an increase in the total current, while the voltage across the batteries remains the same. Effects of Parallel Connections on Voltage