I assume this is a PB based battery, i.e. gell cell or such. If so, they can easily be ruined by discharging to deeply, always recharge as soon as the voltage falls below 12 volts. They are also easily ruined by to fast a charge, best to limit to a charge rate of 0.1C and then only use a current limited constant voltage charger. By doing that you won't over charge even if you forget and leave the charger on to long. Check out www.rcbatteryclinic.com for some good advice on the care of your batteries, all types.