Normally it's 12.6 at full charge but I like me some 13 too.Craig, a battery should never drop below it's rated voltage. If it does likely it has a bad cell and it's time for a new one. Typical lead acid battery is ~2.2 volts per cell, 12volt lead acid battery at full charge will ready ~13 volts, if it drops below 12 volts it's time to replace. Cheers Ray