Got a break in the fog so I resumed my solar cell charging experiment with the same LiPo in the same state as when I stopped the last experiment. That is I didn't bother to charge it in the usual way seeing as how the solar cell is working pretty well.
I did orient the SensorTile2 more in line with the solar cell to catch the full sun and get a better measure of the incident lux. I got three days of data before the fog came back. These are the results:
You can see that the sunlight pegs the VEML6040 at 16500 lux so the actual light incident on the solar cell for most of six hours a day is greater than this. We can estimate how much greater in a second.
you can see that when the SensorTile2 is directly exposed to full sunlight, the humidity drops like a rock (the temperature soars too). This because these MEMS pressure/humidity/temperature sensors are notoriously sensitive to incident light, which is why most people cover them with porous foam when accurate ambient measurements are the goal. In this case, I sacrificed humidity and temperature accuracy during the day anyway to get a better measure of the incident sunlight. Can't have everything on a 0.67 sq. in. pcb!
After the direct light is removed the humidity (and temperature) return to accuracy and I had to end the experiment on the third evening as the fog rolled in and the humidity climbed toward 100%.
The most interesting thing about this slightly extended experiment is that the charge state drops to 70+/- 1 and rises to 90 +/- 1 for three days. I don't see much if any dimunition in peak charge and it looks like the SensorTile2 would just keep going indefinitely under these conditions. The next revision will have an 8 MByte SPI flash so I will be able to measure for three weeks instead of three days to confirm this.
If the SensorTile2 is at steady state, then the ~1.8 mA x 24 hours ~43 mAH of required power must be provided by ~43mAH/6 hours ~ 7.2 mA of average current from the solar cell. At 17 mA per 50,000 lux (per the data sheet) this must mean the sunlight peak is closer to (7.2/17) x 50,000 ~ 21k lux, which is quite plausible.
It is gratifying to be at or near steady state with this little 50 x 33 mm solar cell. This means my goal of a perpetual environmental sensor in a compact package is close at hand. I just need to find a way to protect it from the fog!