- protect the battery from being damaged, and to
- ensure optimal current to charge the battery.
A wide variety of types are available, from very cheap to quite expensive. Prices also vary widely between 10 Amp to 80 Amp controllers.
In most cases, the more expensive Maximum Power Point Tracker (MPPT) type is more desirable. It delivers an optimum DC current from the PV panels to the battery, even if conditions vary widely.