Almost any time batteries are used in a renewable energy system, a charge controller is needed to prolong battery life. The most basic function of a controller is to prevent battery overcharging. If batteries routinely are allowed to overcharge, their life expectancy will be reduced dramatically. A controller will sense the battery voltage, and reduce or stop the charging current when the voltage gets high enough. This is especially critical with sealed batteries that do not allow replacement of the water that is lost during overcharging.