Charge controllers are typically used in off-grid photovoltaic power systems. The primary function of a charge controller (or regulator) is to maintain the battery at the highest possible State Of Charge (SOC) and provide the user with the required quantity of electricity, while protecting the battery from deep discharge (by the loads) or extended overcharge (by the PV array). Most charge controllers operate via voltage regulation set points. However, as voltage is not representative of the true SOC, new algorithms are being developed to evaluate the state of the battery, based on Ah or combined VAh monitoring. Additional features such as battery temperature or wire compensation, meters and alarms can enhance the ability of the charge controller to meet the load demand and extend battery lifetime. Other functions such as anti-theft protection, load management, data logging can also now be built into the charge controller.

Criteria for selecting solar charge controller:
- operating voltage (depends on the battery voltage)
- solar panel’s rated circuit (regulator – should be able to handle the rated current of all panels combined)
- LED charging indicator or meter (displays battery voltage and charging current)
- conformal coating (protects the electronics from humidity)

There are two basic types of charge controllers:
- Series – disconnect the charging source from the batteries when they are fully charged
- Shunt or Load – divert excess energy to secondary, non-priority appliances or “dump loads” such as electric water heater or air heaters, once the batteries are full

Many modern solar charge controllers include maximum power point tracking (MPPT) circuitry that also optimizes the amount of energy that goes into the battery.
Most modern controllers maintain system voltage regulation electronically by varying the width of DC pulses they send to batteries. This is called pulse width modulation or PWM.

Controllers are usually rated by their current handling capacity. Best choice to select one with a current rating that exceeds 1.25 times the maximum current that will be produced by your solar array.