The the capacitor​ on the left is to simulate your battery bank. The 5 volt voltage source is used as a reference. The current source on the right would be your solar array or your wind turbine.
The max output current for the 555 is 200 ma so it should drive a small relay no problem. That relay could drive an automotive relay capable of handling 40 amps.
When the battery bank reaches 14.1 V the relay dumps the voltage to an alternate load like a fan or 12 volt water heating element. When the voltage drops below 11.9 it switches back to charging the battery bank.
The alternate load would be disconnected so the battery bank could have all of the available current.
Adjust the pots to change the on and dump voltages.
