Solar Panel Shunt Regulator

A solar electric power system needs panels for generation, batteries for storage, a regulator to keep the batteries within a safe operating range, and in some cases a power converter for AC output. For those who need to set up a few panels for a summer cottage, a boat, a remote mountaintop installation, or whatever, I'm herewith providing a version of the regulator circuit that I have used in a lot of such installations.

Such a solar panel regulator should perform at least two operations: The obvious one is protecting the battery from overcharge at times of strong sun and little consumption, and the other is protecting it from excessive discharge in bad weather conditions. Both overcharge and deep discharge are harmful to a battery.

For regulating a solar panel's output, there are several possible ways. A linear series regulator can be used, but has the disadvantage of causing some voltage drop and having some internal power consumption at times when the sun is weak and the load is heavy. It's much better to use a shunt regulator, which is inactive at such times, and springs to life only when there is excess energy. For this reason, most solar panel regulators use the shunt scheme, the one presented here being no exception.

But such shunt regulators come in two flavors: Most commercial units are ON-OFF regulators. That means, they have a simple switch device, most often a transistor or MOSFET, sometimes even just a relay, that stays off until the battery reaches overvoltage, and then switches in, shorting out the panel until the battery voltage has dropped off. Then the full panel current is switched on again. The only advantage of this method is that's cheap. The power switch operates with very low power dissipation, allowing a small, low cost construction. But the disadvantages of this system are major: The voltage output is all the time fluctuating between about 13 and 14.5V. The battery is cycling between getting overcharge and having to deliver all the load current, which severely reduces the battery's lifetime. And in the event of battery disconnection or failure, the regulator cycles quickly, applying pulses of full panel voltage to the output, which can destroy sensitive equipment powered by the system!

The circuit presented here uses linear shunt regulation. Simply spoken, it burns off all excess energy from the panel, keeping output voltage constant. At times when the solar panel output is equal or greater than the load, and the battery is fully charged, the load gets its power from the panel, while the battery rests at full charge. Five years battery lifetime are entirely normal with this system, while the same batteries last only two to three years when used with pulsing regulators!

Here is the schematic diagram of my regulator. You may want to print it out, and then go on reading. This regulator is designed for 12V systems employing panels of up to 7A total current, and loads of not over 20A. It can be easily modified for greater currents.