1.5 V batteries are typically Alkaline-manganese and/or carbon zinc which have a native open circuit voltage of 1.5 volts. But during discharge they will drop to lower voltage levels due to becoming depleted and internal resistance to current flow. Knowing this, virtually all battery powered devices are designed to be able to work with depleting batteries down to 1.1, 1.0 or even 0.9 volts per cell.
I don't have the book so I can't give a line of evidence but Atticus was all about teaching Jem and Scout those little life lessons without them even noticing so this instance was one of those little lessons he gives them. Hope this helps :)
