If I want to charge a battery from a solar panel without using a commercial charger, then the essential problem is to create two voltage-triggered switches. One switch turns current on when the voltage in the battery drops below a certain threshold and the other disconnects the panel when the voltage rises to the "full"/higher threshold.
One advantage of having a system like this is that I can select the threshold voltages, instead of the charger guessing what might be optimal.
What would be the right kind of circuit to solve this problem? Ideally, I would like a system that requires no power to operate, at least when it is charging, because I want the energy to be used to charge the battery, not run the circuit.