How Charge Controllers Work

Low Voltage Disconnect (LVD)

The deep-cycle batteries used in renewable energy systems are designed to be discharged by about 80 percent. If they are discharged 100 percent, they are immediately damaged. Imagine a pot of water boiling on your kitchen stove. The moment it runs dry, the pot overheats. If you wait until the steaming stops, it is already too late!

Similarly, if you wait until your lights look dim, some battery damage will have already occurred. Every time this happens, both the capacity and the life of the battery will be reduced by a small amount. If the battery sits in this overdischarged state for days or weeks at a time, it can be ruined quickly.

The only way to prevent overdischarge when all else fails, is to disconnect loads (appliances, lights, etc.), and then to reconnect them only when the voltage has recovered due to some substantial charging. When overdischarge is approaching, a 12 volt battery drops below 11 volts (a 24 V battery drops below 22 V).

A low voltage disconnect circuit will disconnect loads at that set point. It will reconnect the loads only when the battery voltage has substantially recovered due to the accumulation of some charge. A typical LVD reset point is 13 volts (26 V on a 24 V system).

All modern inverters have LVD built in, even cheap pocket-sized ones. The inverter will turn off to protect itself and your loads as well as your battery. Normally, an inverter is connected directly to the batteries, not through the charge controller, because its current draw can be very high, and because it does not require external LVD.

If you have any DC loads, you should have an LVD. Some charge controllers have one built in. You can also obtain a separate LVD device. Some LVD systems have a "mercy switch" to let you draw a minimal amount of energy, at least long enough to find the candles and matches! DC refrigerators have LVD built in.

If you purchase a charge controller with built-in LVD, make sure that it has enough capacity to handle your DC loads. For example, let's say you need a charge controller to handle less than 10 amps of charge current, but you have a DC water pressurizing pump that draws 20 amps (for short periods) plus a 6 amp DC lighting load. A charge controller with a 30 amp LVD would be appropriate. Don't buy a 10 amp charge controller that has only a 10 or 15 amp load capacity!