I'm planning to use a 3S Li-Ion setup, which results in a 11.1V nominal voltage. I wanted to be close to 12V to be able to use stuff made for cars with it without having to deal with DC-DC conversion.

There are many cheap solar chargers out there, which have adjustable cut-off voltages, but unfortunately, none of the ones I tested so far allow to be adjusted enough to deal with 3S Li-Ion packs. So I'll have to build my own.

I decided to use a https://www.instructables.com/ARDUINO-SOLAR-CHARGE-CONTROLLER-PWM/ as a reference for the solar charger, but decided to make some changes to it. It's originally designed for 6V, so some of the protection circuitry needs to be adjusted, and I want to use a smaller OLED screen instead of the regular LCD. On top, I'll have to change the firmware quite a bit. I started drawing up my changes right onto a printout of the circuit.

Next step will be prototyping this. I thought I had all the parts around, but seems like I don't have any P-MOSFETs around, so I'll have to wait until the next big parts order, or find something in the bottom of my parts piles.