Auto charge controller..... Need help :/

[Mallison359]

Hi everyone, new to group and fairly new to electronics.

I have a few garden solar panels that I have in series (providing about 30 odd volts) that I use to charge a rechargeable 9v that I power my raspberry pi on. I'm trying to get my head around d how I can build a circuit whereby while the pi is on the batteries use to charge the 9v are being charged but have a cut off so the circuit never overcharged the battery pack.

I have a basic idea of what I want it to do but the actual building of it baffles me. I saw a video of a guy that did it with a 12v battery.

Can anyone give me some pointers?