Why do you need a charge controller?
The charge controller sits between your solar panels and your batteries and acts as the "gate-keeper" in the flow of energy from the solar panels to your batteries. As the amount of sun reaching your panels changes during the day, so does the available voltage and current coming from your solar panels. In order to ensure the expensive batteries in your solar power system are not overcharged, the charge controller carefully monitors the state of your batteries and adjusts the charging current to the optimum level.
Important features of charge controllers are the maximum charging current, temperature compensation, and charger style (MPPT - Maximum Power Point Tracking or PWM - Pulse Width Modulation). The current rating of your charge controller must be higher than your panel's current rating in order to handle cases of high charge current, i.e. from sunlight reflecting off snow. In order to maximize the life of your expensive batteries, the charge voltages must be tightly controlled. This voltage changes with temperature hence temperature compensation is essential. Two types of charge controllers exist; PWM and MPPT. PWM controllers are simple and robust; however, the maximum current they can deliver to the battery is limited to the current output of the panel no matter what the potential voltage is. MPPT controllers are more complex; however, they have the advantage of maximizing the power that is delivered to your battery under all sunlight conditions and potential voltages of the panel. An MPPT charge controller can harvest up to 30% more power from your solar panels compared to a PWM charge controller.