It's have simple schematic, for save pins I used Charlieplexing(35 components in total):

https://en.wikipedia.org/wiki/Charlieplexing

In this project i use for charging 12 light sensor BPW34S, on clear sun one cell make 0.5-0.6 V and ~2 mA, connect its in serial and make simply charging schematic - use 2 components, ZD1 to drop voltage 6 -> 4.7 V and make protection to leakage of current in solar panels in to diode D1 name LL4148(because it have little reverse current):

The diode add 0.7 volts drop(4.7-0.7=4.0V) and battery don't overcharge up to 4 V.