I thought this one deserved a topic of its own, so it contains some info from previous posting (which I removed)
Due to lack of sunny days in mid winter, the tempest discharged to a point where it sampled only once every 5 minutes and disabled the rain and lightning sensor (power save mode 3). In the beginning I used a single 15W Led source up close to the tempest. I managed to charge it to mode 2, but charging went very slowly. I decided to use two 15W Led sources instead, one against the east solar panels and one against the west panels. The actual distance of the Leds was about 5 cm. Note that the amount of light captured decreases with the square of the distance, so putting the Leds at double the distance, would decrease the charging rate by a factor four (you can’t/shouldn’t put incandescent or halogen lights at 5 cm from the unit because of the heat they generate).
Here is the curve:
It takes only 2 days to charge from 2.37V to 2.80V, at which point the charging stops.
The power consumption of the unit increase when the sampling rate goes up. If anything, you might expect charging to go slower. But the slope in the red and orange part is less then the slope in the beginning of the blue part. There is a sharp increase in charging rate above 2.62V. Something I learned today: this increase in voltage when a battery gets close to 100% capacity is the normal behavior for the LTO batteries used in the Tempest (https://www.researchgate.net/figure/The-OCV-SOC-characteristic-of-the-LTO-based-battery-measured-at-different-temperatures_fig4_324117029).
Interesting enough, due to the fact that only 32 data points are seen in weatherflow’s curves ( https://tempestwx.com/station/33745/graph/105013/battery/3) it is pretty hard to see the increased charging rate in those curves. (I’m still hoping at some point we will get a better graphing system ). I created this graph by using the Rest api at https://weatherflow.github.io/Tempest/api/swagger/ and did a couple of requests for the last few days in csv format. Saved the output containing 2900 measurements, and imported it in Excel.
In general Leds are 10 times more efficient compared to incandescent lights. What’s more, many incandescent lights emit light in all directions, but Leds are always directional. I got mine from Amazon (https://www.amazon.de/gp/product/B07WD8Y92W), but I’m sure you could find even more powerful led lights)
For more fun, I’ll keep this setup running in order to see it discharge a bit and pick up charging again.
Note that a battery power booster is in development by Weatherflow, so hopefully this artificial charging isn’t needed anymore next winter.