PWM Solar Charge Controllers

A PWM (pulse width modulation) controller can be thought of as an (electronic) switch between the solar panels and the battery:

• The switch is ON when the charger mode is in bulk charge mode
• The switch is “flicked” ON and OFF as needed (pulse width modulated) to hold the battery voltage at the absorption voltage
• The switch is OFF at the end of absorption while the battery voltage drops to the float voltage
• The switch is once again “flicked” ON and OFF as needed (pulse width modulated) to hold the battery voltage at the float voltage

Note that when the switch is OFF the panel voltage will be at the open circuit voltage (Voc) and when the switch is ON the panel voltage will be at the battery voltage + voltage drops between the panel and the controller.