@NeverDie Well, looks like that is the space mission panel Won't be cheap anyway. On the other hand recently I seen a lot of news about so called nighttime photovoltaic power, who knows how fast it can be delivered to market.
In a meanwhile, I had a chance to play with the Harvester board. Please don't consider everything below as any serious test or a comprehensive review - I just rather tried to understand how the modules are working and what can I get from them.
Following our prior discussion, I've assembled one board in buck-boost mode (the original PCB), and have tweaked another one to the boost configuration. The only solar panels I have are:
- IXYS KX0B25-02X8F. Single panel is annoyingly weak so I connected two of them in series.
- The SCNE SC-1338 from the test above.
- SORBO SB-3077. A big (79x40) panel from solar torch. Didn't tested it like the other two, but noticed about 6V from it.
For the load there is 1500µF 4V tantalum capacitor installed. V_uvp threshold has been set to 2.2V, and V_eoc to 3.2V. Please also note, the MPPT resistor values were set to V_oc=12V (which is too high for all the tested panels) and no adjustments were made to them for any of the tests.
Most of the tests I made in the evening at home where LED lights are the most common source. I used light sensor on my phone to measure light illuminance. Looks like it gives more or less sensible values; such, 50 lux corresponds to a dim light, 250 lux is an office light or indoors illuminance in cloudy day, 1000 lux is outdoors light in cloud day - similar values I've seen in tables over the Internet. After checking illuminance I placed solar panels in the same area and measured most important voltages:
- V_pv - PV panel voltage when it is under load (but see my comments)
- V_store - voltage at the spv1050 store capacitors (2x47µF)
- V_out - voltage on the output capacitor (1500µF)
For low light conditions I waited several minutes for the capacitors to charge and read values only when the system became saturated (or at least look so). Results and my notes are represented in the following table:
The spv1050 datasheet provides quite detailed description on how it works. The only interesting question was - what happens in buck-boost mode if the harvester cannot maintain V_store voltage anymore due to low illuminance. I found that store and the load will be connected as long as V_out>2.2V. After that, spv1050 will shut down the DC/DC and connect PV to the store. I think DC/DC was on because panel voltage raises up from 0.25V to 0.52V when the store disconnects from the load. Why is it 2.2V and not 2.6V as specified in the datasheet, I don't know.
Another interesting test was how much time it will require to charge the 1500µF capacitor to 3.2V. For that I tried different panels with both buck-boost and boost boards. Please note, since the boost board has 4V limit (due to the tantalum capacitor) and pair of the IXYSes are capable to produce up to 8V, I've skipped this couple. The results are in the table below:
You was absolutely right when expressed the concern on the SolarBit panels - they work much better in the sunlight. The SCNE seem also prefers full spectrum to my LEDs.
Evaluate whether this is a good or bad result is possible when considering the load itself. Accordingly to the Nordic, the nRF52840 will consume about 32µC
per heavyweight BLE event, for example, when running at 2.2V (lowest possible value for the spv1050) and sending +8dBm advertisements. For tiny packets it may be as low as 5µC.
The tantalum capacitor stores 1500µF*3.2V = 4800µC - exactly this amount was delivered by the spv1050. This roughly equals to 150 advertisements, or 960 connection packets. If ignore other waste (like sleeping current) and assume 5 minutes as an average result on SCNE-alike panel (indoors, daylight), the system should be able to advertise once every two seconds or send small data packets once every 1/3 second without discharging a battery.
Of course, more light means more power, and there is the night too, but with proper panel and correct location I think it should be possible to maintain positive power balance.
So which configuration to prefer - boost or buck-boost? I still have no simple answer.
The boost is good when you're working in low-light conditions and can keep it low-voltage. I also have impression that the spv1050 is sensitive to the current it can get from a panel. Therefore a couple of single cell panels (i.e low voltage, high current) might be the right choice. On the other hand, it's risky to attach high voltage panels to the boost harvester - neither battery not capacitor will tolerate 6V.
The buck-boost configuration has no such drawback - any panel you may find on attic will very likely to work. Another advantage you might have noticed is that it delivers faster. When fast is fast enough - it depends. But in the low-light when it may took up to 30 minutes to charge a 1500µF capacitor the boost advantage is not so obvious.