Solar and wind power are intermittent suppliers of power: they provide power when the sun is shining / the wind is blowing. But we usually want the supply to be “on demand”.

There are couple of ways of “buffering” between generation and usage. The simplest (conceptually, anyway) is to store the generated power in a battery (or, more likely, batteries).

As many people have discovered to their surprise, connecting (say) a wind turbine to a battery works fine while the wind is blowing. Then the wind stops and the turbine keeps spinning -  powered by the battery/batteries!

The solution comes in the form of a charge controller. This ensure that the wind/solar generated power is delivered to the batteries without overload. It also ensures that battery power cannot “leak back” to the generating device.