Solar Shunt Charger

Solar-Battery Shunt Charge Control Circuit

Shunt type charge controllers circuits were the first to be developed and are considered very basic. They are only used as solar charge controllers (not for water or wind power). They shunt current from a solar panel to ground when the battery is full. Understanding how these simple circuits work will give you a better understanding of batteries in general.

Charge controllers are needed to protect batteries from over charging which occurs when the battery is fully charged but the solar panels are still providing power to the charging circuit. If the solar panel is allowed to keep feeding the charging circuit and forcing the battery voltage higher the battery will be damaged.

Referring to the top circuit on the images to the right you see a solar panel and a battery connected by a shunt charging circuit. The left part of the circuit including the resister and part of the transistor make up the shunt path where current flows from the solar panel to ground. The shotkey diode in the center prevents the battery from discharging through the solar panel at night. The right part of the circuit including the zener diode and three rectifier diodes provides the switching to the other side of the transistor.

The second circuit shows a "charging" condition where the solar panel is feeding the charging circuit and the battery is accepting the charge. The shotkey diode is forward biased allowing the current to flow to the battery. The Zener diode is reversed biased blocking current and keeping the transistor switched off and the shunt path an open circuit.

The third circuit shows a "shunting" condition where the battery is fully charged but the solar panel is still feeding the charge circuit. The zener diode is forced into its reverse bias breakdown which causes the transistor to switch on. This closes the shunt path allowing current from the solar panel to flow through the resistor and transistor to ground.

The fourth circuit shows a non-charging condition where there is no current coming from the solar panel. The battery can be any voltage depending on its charge. The shotkey diode is reversed biased blocking current but it does allow a small amount of reverse leakage current. The battery will slowly discharge through the solar panel to ground although the discharge is slow and usually not a problem.