Charge controllers or voltage regulators protect the batteries from overcharging, subsequent gassing, loss of electrolyte and possible plate damage. By monitoring battery voltage they regulate the charging current from the PV array enabling batteries to be charged but not over-charged. They also contain suitable protection to prevent any current drain from battery to PV array eg. at night.
This is usually best done by gradually reducing (or tapering) the flow of energy into the battery as a full charge is reached so achieving a more complete charge. Very fast solid state switching technology is used for this known as pulse width modulated (PWM) control.
Some controllers also include a low voltage warning or disconnect function to switch off loads if the batteries become very low preventing excessive discharge. Optional displays indicate charging currents, load currents and battery voltage, or separate meters and system monitoring can be used.
In choosing the the proper charge controller the main consideration is the peak charging current
you need to regulate. It is worthwhile selecting one that will handle extra power if needs change in the future.
Controllers should not be used with more modules than they are rated for. However, a PV array can be split into sub-arrays each with it's own charge controller, in order to accommodate increasing numbers of modules. For the smaller output panels, or where a relatively large battery store is used, regulators may not be necessary.
All controllers listed include blocking diodes or other protection to prevent any current drain from battery to PV panel at night, so no additional blocking diodes are required.