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PWM: Pulse-Width Modulation & MPPT: Maximum Power Point Tracking
PWM and MPPT are the two different types of charging methods solar charge controllers can use to charge batteries from a solar array/panel. PWM is more complex, but does not have any mechanical connections to break. The most recent and best type of solar charge controller is called Maximum Power Point Tracking or MPPT. MPPT controllers are basically able to

convert excess voltage into amperage. This has advantages in a couple of different areas.Most solar power systems use 12 volt batteries, like the ones you find in cars.Solar panels can deliver far more voltage than is required to charge the batteries. By, in essence, converting the excess voltage into amps, the charge voltage can be kept at an optimal level while the time required to fully charge the batteries is reduced. This allows the solar power system to operate optimally at all times.Another area that is enhanced by an MPPT charge controller is power loss. Lower voltage in the wires running from the solar panels to the charge controller results in higher energy loss in the wires than higher voltage.

With a PWM charge controller used with 12v batteries, the voltage from the solar panel to the charge controller typically has to be 18v. Using an MPPT controller allows much higher voltages in the cables from the panels to the solar charge controller. The MPPT controller then converts the excess voltage into additional amps. By running higher voltage in the cables from the solar panels to the charge controller, power loss in the cable is reduced significantly.When using high voltage “Grid Connect” panels with VOC voltages above 35v to charge a 12v battery bank, the only controller option is an MPPT charge controller.
The final function of modern solar charge controllers is preventing reverse-current flow. At night, when solar panels are not generating electricity, electricity can actually flow backwards from the batteries through the solar panels, draining the batteries.