The battery charger can be used to maintain the SL-35B battery during the winter. The charger is connected to the wall socket and has a connector that fits the battery connector. The battery must be fully charged when removed from the boat and then recharged every two months until the battery is placed in the boat again and connected to the solar panel. If you have the solar panel available during the winter, you can instead charge the battery with the help of the panel by pointing it at the sun.If you do not charge the battery in the winter, there is a risk that the battery will be drained completely and destroyed.