A 12V solar panel is designed to output atleast enough voltage to charge a 12V battery under worst case conditions (lowlight level, high temperature etc). A 12V battery needs at least 13.6 volts tocharge, therefore under worst case conditions a solar panel needs to output atleast 13.6 volts. This means that in perfect conditions a 12V solar panel mayoutput around 17V or more.

        If you plug a solar panel, which is generating 17V,straight into your battery it is easy to understand how this can cause damage.Solar regulators are designed to accept the voltage from the solar panel, andoutput a voltage that is safe and usable to charge a battery. A good regulator will incorporate a 3 stage charging output.