Just to clarify, 6 volt batteries won't hold a charge any longer than 12 volt batteries...assuming of course they are of like design ie; wet cell! Depending on the design, they will handle more cycles because the plates are thicker. A cycle is a discharge and charge..provided the discharge is not more than 50%..or about 12.3 volts. If you discharge more than 50% too often, you will shorten battery life considerably, regardless of the voltage, type or design.