Solar charge controllers

Solar charge controllers are used mainly in off-grid PV systems, but also in other systems with batteries. Their main function is to protect the batteries from overdischarging and overcharging, and to ensure efficient charging of the batteries. Battery voltage decreases as the battery discharges. The charge controller protects the batteries by disconnecting the DC loads if the battery’s state of charge gets too low, and switching on the DC loads again when the batteries have recharged.

Charge controller wiring in an off-grid PV system powering DC lights.

Charge controller from Steca, note the very clear battery state-of-charge (SoC) display. Source: Steca.

Most charge controllers currently on the market are designed for use with lead-acid batteries. However, charge controllers are also available for lithium-ion batteries, and some charge controllers can be used with both types.

Charge controllers range in size from very small in simple lighting systems to several thousand watts in larger systems, and there are many different types.

If there is a battery inverter in the system, the charge controller will not disconnect the AC loads being powered by the inverter, which can lead to the batteries being overdischarged as they are usually connected directly to the battery inverter. The system either has to be carefully managed by the system owner or an automatic solution implemented (some have integrated low voltage disconnect or overdischarging detection) to protect the batteries. Having DC lights in an off-grid system with an inverter can act as a warning mechanism – the charge controller will disconnect the DC lights before the battery discharges to such an extent that it can be damaged.

Wiring diagram of a system containing a 24 V battery inverter.

Having DC loads (lights) in the system can act as a warning mechanism –  if the battery state of charge is too low, the charge controller will disconnect the batteries.