How long can I expect my 12 volt battery to last? My 2004 is now a little over 6 years. Does anyone know the correct charging voltage for the 12 volt battery. My 2004 charges at 13.9 volts. Conventional cars charge at 14.4 to 14.6 or so. Even after driving 240 miles the car charged the battery to about 60% of full charge. An external charger will charge the battery to 100%.