What happens when we connect two identical batteries in parallel with a resistance R?
This is ohms law and a simple as it gets. Identical batteries will have the same voltage at the terminals. If the batteries are not identical then the current from the higher voltage battery will flow into the lower voltage and the current will be limited by the combined internal resistance of the batteries with the terminal  voltage determined by the resistance of the voltage divider comprising the internal resistance . The current thru the resistor is E/r