Yes, that's true. We model that as output resistance. In this, a thevenin model, each battery is modeled as an ideal voltage source in series with an output resistor, Ro.



In the top pic, the load current is the result of the voltage divided by the sum of the output resistance and load resistance.

In the bottom pic, with two batteries in parallel, the output resistances are also parallel. Thus, the net resistance seen is lower, resulting in higher current to the load.

HTH.

Doug