16 June 2018
Well, first test because sometime last night (8.34 pm, to be exact) the Sensor Tile stopped working. I know this because that's the last entry in the SPI flash data log. I don't know why. The battery had nearly a full charge (91% at 4.11 V) at the time of stoppage so I assume it was a glitch or that the battery somehow became loose (the JST connector is just pressed into the JTS PTHs on the SensorTile, not soldered) or just one of those things. But this is somewhat rare in my testing to date, but usually I have the devices mounted in some kind of box, not a glass jar as in this case.
First the record of the ambient light in lux measured from the VEML6040. I am using the smallest (40 ms) integration time since the sensor is in direct sunlight and the light pegs the sensor at its maximum output of 16384 lux for several hours per day. I estimate the incident sunlight is ~20 klux for at least four hours (x-axis grid spacing) a day. According to the data sheet, at 50 klux sun light the solar cell should be producing 16.9 mA at 2.2 V, so I am probably getting ~20 mA/50 mA x 16.9 mA ~7 mA per hour as I estimated in my previous experiments.
The battery voltage responds as one would expect, reaching a minimum of 3.76 V at around 8 AM and peak battery voltage of 4.14 V at 4 PM 6/15.
I will restart the experiment, maybe with the JST connector soldered to the SensorTile this time and see if I can get several days of data.
Overall, I would say the BQ25504 Solar Cell LiPo charger works as advertised, at least with this particular solar cell. For some projects, 50 mAH per day of charging might not be enough, and the other variables including hours of sunlight, cloud cover, container, system device, etc will determine whether this solution will work for any given application.
One thing I am curious about is whether this solution will work for some other method of power harvesting, not a solar cell but maybe thermoelectric or piezoelectric device. The only requirement is a DC voltage > 0.35 V. If I can find a suitable energy transducer, I will give this a try...