A pwm solar charge controller or charge regulator is basically a voltage and/or current regulator to prevent overcharging the battery. It regulates the voltage and current from the solar panel to the battery. Most "12 volt" panels put out about 16 to 20 volts, so if not specified, the battery will be damaged by overcharging. Most batteries need about 14 to 14.5 volts to fully charge.

Always need a pwm solar charge controller? Not always, usually, there is no need for a charge controller with little maintenance or a trickle charge panel such as a 1 to 5 watt panel. A rough rule is that if the panel puts out about 2 watts or less per 50 battery amp hours, then you don't need one.

For example, a standard flooded golf cart battery is about 210 amp hours. So in order to keep a string of pairs (12 volts) just for maintenance or storage, you'll need a panel of about 4.2 watts. The popular 5-watt panels are close enough to not require a controller. If you are maintaining an AGM deep cycle battery, such as the Concorde Sun Xtender, then you can use a smaller 2 to 2 watt panel.