A solar electric charge controller is needed
in virtually all solar power systems that utilize batteries

The job of the solar charge controller is to
regulate the power going from the solar panels to the batteries

The most basic charge controller simply monitors the battery voltage
and opens the circuit stopping the charging when the battery voltage rises to a certain level

More modern charge controllers use pulse width modulation (PWM) to slowly lower
the amount of power applied to the batteries
as the batteries get closer and closer to fully charged

Morningstar Solar Controllers

Specialty Concepts Solar Controllers

Steca Solar Controllers

Xantrex Solar Controllers

Samlex Solar Controllers

Enerwatt Solar Controllers