Yesterday I received components and soldered test PCBs, then bad news comes out.

I haven't noticed that the chosen voltage comparator has open-drain outputs, so my schematic is incorrect, though I managed to fix it with some wires. In general, under-voltage protection works but consumes from 20uA to 75uA, that's not acceptable. The next try is to use a voltage monitor IC, e.g. NCP300LSN185T1G or something similar.

The overvoltage protection circuit is crap. It kinda works, but the overall approach is completely wrong. The circuit monitors a solar panel's voltage and if it < 3.84V (240 mV voltage drop on a schottky diode) it charges a supercap, so we could easily have a case when the voltage on the supercapacitor is 2V, 5V on the solar panel, and charging is disabled :) It can be improved to monitor voltage on supercap, but in this case, it'll drain it (due to voltage divider). The good news here, that the alternative circuit works well (I'll test it more during the next days). It uses an adjustable precision shunt regulator that is capable to output up to 100mA, so basically one IC, a few resistors, and that's all.

(resistor values aren't final)

With this approach, I was able to charge a supercapacitor to 3.46V on a dark and cloudy day using a 53x30mm, 5.5V solar panel.

So my next steps are:

1. Select and order an appropriate voltage monitor for undervoltage protection.
2. Design and order new PCBs.
3. Test everything again.

Supercapacitor Discharge Calculator

Recently, I created one more online supercapacitor discharge calculator. All that I've seen don't support different power consumption modes (run, idle, sleep) and some other functionality. It doesn't validate all fields properly, so valid values are required.