Charge Controllers

Charge controllers comes in all shapes, sizes, features and price ranges. They range from the smallest 4.5 amps
control, up to the 60amps. Often if current over 40 amps is required, two or more 20 to 40 amps units are wired in parallel.A charger control is an essential part of nearly all power systems that charge batteries. Whether the power source is PV, wind, hydro, fuel or utility grid. Its purpose is to keep your batteries properly charged and safe for the long run.
Its basic function is to block reverse current and prevent battery overcharge. Some control also prevents battery over discharge, electrical overloads and or display battery status and the flow of power.

The control regulates the voltage and current coming from the solar panels going to battery. Most panels out put 15.5 to 20 volts. So if there no regulation the batteries will be damaged from overcharging. Most batteries need around 14 - 14.5 volts to get fully charged.
Why 12 volts panels are 17 volts? The reason is that if you do that. The panel will provide power only under perfect conditions and full sun. This is not something you can count on in most places. the panel needs to provide some extra voltage so that when the sun is low in the sky, or you have haze, cloud cover, or high temperature, you will still get some output from the panel.
The charger controller regulates this 16 to 20 volts output of the panel down to what the battery needs at the time. This voltage will vary from 10.5 to 14.5 that the battery needs to get fully charged.