Think of a solar charge controller as a regulator. It delivers power from the PV array to system loads and the battery bank. When the battery bank is nearly full, the controller will taper off the charging current to maintain the required voltage to fully charge the battery and keep it topped off. By being able to regulate the voltage, the solar controller protects the battery. The key word is “protects.” Batteries can be the most expensive part of a system, and a solar charge controller protects them from both overcharging and undercharging.

The second role can be more difficult to understand, but running batteries in a “partial state-of-charge” can shorten their life tremendously. Extended periods with a partial state of charge will cause the plates of a lead-acid battery to become sulfated and greatly reduce life expectancy, and lithium battery chemistries are equally vulnerable to chronic undercharging. In fact, running batteries down to zero can kill them quickly. Therefore, load control for the connected DC electrical loads is very important. The low voltage disconnect (LVD) switching included with a charge controller protects batteries from over-discharging.

Overcharging all types of batteries can cause irreparable damage. Overcharging lead-acid batteries may cause excessive gassing that can actually “boil” the water away, damaging a battery’s plates by exposing them. In a worst-case scenario, overheating and high pressure can cause explosive results upon release.

Typically, smaller charge controllers include a load control circuit. On larger controllers , separate load control switches and relays can also be used for load control of DC loads up to 45 or 60 Amps. Alongside a charge controller, a relay driver is also commonly used to switch relays on and off for load control. The relay driver includes four separate channels to prioritize more critical loads to stay on longer than less critical loads. It’s also useful for automatic generator start control and alarm notifications.

More advanced solar charge controllers can also monitor temperature and adjust battery charging to optimize the charging accordingly. This is referred to as temperature compensation, which charges to a higher voltage in cold temperatures and a lower voltage when it is warm.