Estimate the battery impedance to be such that at short circuit, you will get 30 to 40 times the amp hours, in amps.
As such, this means that a 100 amp hour battery will dump 3000 to 4000 amps.
This means its impedance is 2 volts/4000 amps or one half milliohm per cell. or 3 milliohms for a 12v battery, 12 milliohms for a 48 volt battery.
When you connect batteries in parallel, if some batteries and their connection leads have lower resistance than others they will pass more current according to the difference in resistance.