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 KXOB25-05X3F-TR solar cells..
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Power (Watts) - Max 24.5 mW
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Current @ Pmpp 5.5 mA
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Voltage @ Pmpp 4.46 V
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22 x 7 x 1.8 [mm]
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Measured at 1 sun (= 100 mW/cm²)
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8 Cells in series?
Do we need energy harvesting / MPPT IC?
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To charge fully we need 4.2v open circuit (which is not peak effeciency)
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4.2/8= 0.525v per cell
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Works down to 10.5 W/m
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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.