"old" carbon-zinc batteries were 1.5 volts per cell. The newer ones ( NiMH, NiCd, etc) run 1.2-1.25 per cell. Even fully chrged, they may not provide enough voltage for something not designed for them. I remember a number of radios ( 2 way ) that had 8 cell packs (12V) for regular batteries, and 10 cell packs (12V) for rechargeable NiCds.