A PWM solar controller is, in essence, like an electric switch between the solar panels andthe battery. They are effective in slowly lowering the amount of powergoing to your batteries as they get closer to fully charged, thus keeping themat a fully charged state while eliminating the stress on your batteries.
In addition to the above, an MPPT Controller is also capable of convertingexcess voltage to amperage. The benefit of this feature is it keeps an optimalcharge voltage while requiring less time to fully charge your batteries. Italso allows for reduced amounts of power loss as the conversion of excessvoltage to amps allows for greater voltage to travel through the panel to thecontroller.