General Idea: Very small solar cell(s) in the tens of milliwatts range, MPPT IC, battery protection IC, 3.3v LDO (with additional 1.8v LDO depending on things tm)
I like the KXOB25-01X8F-TR solar cells..
- Power (Watts) - Max 24.5 mW
- Current @ Pmpp 5.5 mA
- Voltage @ Pmpp 4.46 V
- 22 x 7 x 1.8 [mm]
- Measured at 1 sun (= 100 mW/cm²)
- 8 Cells in series?
Do we need energy harvesting / MPPT IC?
- To charge fully we need 4.2v open circuit (which is not peak effeciency)
- 4.2/8= 0.525v per cell
- Works down to 10.5 W/m
- 0.55v also happens to be the MPP at 1000 W/m
Based on that graph, and the other stuff, I’m gonna make the simplification to only have overcharge protection because MPPT chips are like $8.