Charging a deep-cycle battery is not difficult, but there are some essential things to keep in mind. First, it is best to charge the batteries with an automatic charger designed for this purpose, not a household charger. Secondly, the battery should be charged for at least eight hours before use to ensure it is fully charged and ready for service. The best way to charge a deep-cycle battery is with a constant current at the correct voltage. You should charge it until it reaches its peak voltage and trickle-charged for an extra two to three hours. A fully charged battery will have a voltage of about 13.6 volts and can be stored for up to three months at room temperature without losing its charge. A deep-cycle battery is designed to be discharged, so there is no need to charge it before storage fully. Overcharging a deep-cycle battery can permanently damage the battery's ability to accept and keep a full charge.