Actually what I found was that they have to be identically charged- the weaker one will pull from the stronger one-
If you connect batteries in parallel you must be very careful that they are exactly the same type, same state of charge, and the same age, much more so than in the serial connection above. Batteries have an internal resistance, which goes up as they discharge. Imagine the following connection where one battery
is fully charged and the other is discharged:
The discharged battery
on the right has a terminal voltage of 10.5V and an internal resistance of 8 milliohms. The fully charged battery
on the left has a terminal voltage of 13.2V and an internal resistance of 4 milliohms. The internal resistances are shown in the circuit for clarity. When the two batteries are connected together in parallel, the fully charged one will start charging the discharged one. A current will flow around the loop between the two batteries of
This current is very high and may damage some batteries.
If the batteries are at an identical state of charge, their terminal voltages will be the same, and so no appreciable current will flow when they are connected in parallel. Any imparity will be evened up as the one with greater charge and hence terminal voltage charges up the other.