Solar Panel for Ardurf1s

[Tomega3]

Hi,
any updates on your testing of a solar panel to run/charge an ArduRF1s?

Thanks

[StreamingMeeMee]

I've tried a small 9V panel from RS (no joy) and am now testing a larger 12V panel.

I have the panel inside a south-facing window. It took a bit to get the batt. up to operating voltage before the board would stay powered up but it appears to be working. It seems that the panel does not provide enough current to power the board by itself if the radio is active.

I have a simple DHT11 sensor wired up and it is reporting the temps every 2min. It is also sending the batt. and 'charge' voltage (A7 & A6) values. The batt. voltage varies betwen 320 and 340 but the charge voltage is all over the place; from 0 - 300+ over the course of 15min. I'm a bit perplexed by that one. I'll have to check my code to see if I did something wonky.

[StreamingMeeMee]

BTW, I should mention that I'm using a LiFePO4 cell rather than a LiPo.

[Daniel]

The trick with solar panels is to use the highest voltage that you can get away with. As you have discovered when the lighting is not optimal the voltage drops a lot and the panel becomes useless. I tested with a '12V', 5W panel that I bought on ebay http://www.ebay.com/itm/150Watt-100W-12 ... 35d63faffe It develops about 19V in bright winter sunshine and with the step down converter on the ArduRF boards it charges the cell very nicely. The fact that you use a LiFePO4 cell means that the cell will be overcharged slightly but it will not damage. LiFePO4 are very robust, much more so than LiPo cells.

The panel I use is specified as Polycrystalline, 5W, 17.9V and 0.28A at peak power, 22.41V open circuit voltage, 0.3A short circuit current with dimensions 255 x 194 x 17 mm. At full charge current the LiPo is being charged with about 1.5W so the panel is 'over sized' by a large margin. In practice the panel is about right because as soon as the sun is slightly obscured the panel output drops to about 1W and that is still enough to charge the cell.

If you are behind glass then the panel will be a lot less efficient than in direct sunlight. There are two main causes for the drop in output; Absorption of the energy by the window and reflection of the light by the window. Ordinary household glass is designed for cost first and foremost but it is pretty good at keeping a lot of the energy in sunlight out of the home. Combine that with the reflection from the surface and a solar panel behind a window will perform very poorly.

I hope this goes some way towards explaining why it doesn't work very well. For comparison try a test in direct sunlight with the 12V panel and see how it works.

[StreamingMeeMee]

I've put it out on the deck for the last week or so, no problem w/ keeping the batt. charged. This is with the panel laying flat on the deck so it is far from an optimal orientation.

I have been monitoring the charge voltage and it is still all over the place. Is this to be expected with the battery is mostly charged and just being 'topped up' by the solar panel?