3A Discharging Solar Charge Controller for 12/ 24V 40/ 80Watt PWM Charging Boat A charge controller is a device that regulates the voltage and current being sent to the battery from the solar panels Charge controllers are necessary in order to prevent the batteries from becoming overcharged or discharged. The main advantage of Pulse Width Modulation (PWM) is that power loss in the switching devices is very low. When a switch is off, there is practically no current. When the device is on, there is almost no voltage drop across the switch. Power loss, being the product of voltage and current, is thus in both cases close to zero. PWM also works well with digital controls, which, because of their on/off nature, can easily set to the needed duty cycle.