[Excaliber]

Virtually all battery "cells" are 2 volts. How many "cells" you have determines the end voltage (could be 6,12, 24, 36, 48). The difference with external and internal cells is that you can screw up more easily with external cell combinations (like two 6's for 12 volts). Putting a 12 volt charger on only HALF (6 volts) of the cells is bad news. But you could use a 6 volt charger on each half of the combination and that would be OK. Charging ONE of the two batteries at a time. I wouldn't recommend that though, use a 12 volt charger and get them both at the same time with a single charge.