Charge Controllers

A charge controller, or charge regulator is basically a voltage and/or  current regulator to keep batteries from overcharging. It regulates the voltage and current coming from the solar panels going to the battery.  Most "12 volt" panels put out about 16 to 20 volts, so if there is no  regulation the batteries will be damaged from overcharging. Most  batteries need around 14 to 14.5 volts to get fully charged.

 

  

 
Generally, there is no need for a charge controller with the small maintenance, or trickle charge panels, such as the 1 to 5 watt panels. A rough rule is that if the panel puts out about 2 watts or less for each 50 battery amp-hours, then you don't need one. 

For example, a standard flooded golf car battery is around 210 amp-hours. So to keep up a series pair of them (12 volts) just for maintenance or storage, you would want a panel that is around 4.2 watts. The popular 5 watt panels are close enough, and will not need a controller. If you are maintaining AGM deep cycle batteries then you can use a smaller 2 watt panel.