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Building a Solar-Powered Repeater for Meshtastic

A brief history

History of versions

We have tried 5 permutations of solar powering nodes:

Lessons learned:

1. Don’t trust the RAK board to handle varying solar voltage or varying battery voltage. It is very unhappy if either voltage goes outside of a narrow range. In fact there is a known issue where the Nordic nRF52840 module inside of the RAK 4631 chip causes it to enter a “Super Deep Sleep” fault state when it is supplied with 3.3V. This means when the battery voltage falls too low the RAK chip shuts down and can only be rebooted manually.
2. Solar panels: bigger is better. 10 watts or more is ideal.
3. Battery: bigger is better. 5,000 mAh or more is ideal.
4. Antenna: fiberglass antennas are the most robust and can handle wind, rain, and UV light better than rubber ducky antennas.
5. Heat can kill the battery – it is best to shade the box containing the battery.
6. Don’t add any modules. The GPS module increases battery usage significantly and is unnecessary if the repeater is in a fixed position.
7. Seal everything properly and make sure water has a way to get out of the case. Use o-rings for the antenna mount and wrap connectors in proxicast. Add a drain plug to the case so any condensation has a way to escape. For high humidity environments, spray the electronics with conformal coating.

These are lessons learned from Austin Texas, where the main environmental stressors are heat and thunderstorms. In the summer of 2023 we 80 days over 100° F (38°C) and we had a stretch of 11 days over 105° F (40.5°C), during which two radios died. We also can have both high humidity and high temperature at the same time in Austin; in the summer the weather would swing from 80° F (27°C) with 85% humidity in the morning to 105° F (38°C) with 25% humidity in the afternoon. Weather history here. Our learnings are probably applicable to other southern and desert climates.

Considerations for other climates

1. Northern Climates: At higher latitudes there is much less sunshine in the winter. A larger solar panel would be necessary and angling the panel lower to the horizon would be beneficial. The rule of thumb is to angle the solar panels at the same degree as your latitude. In Austin our panels should be angled at 30° but in Calgary the optimal angle would be 50°. Batteries should also be bigger in colder climates as low temperatures reduce their performance.
2. Coastal Climates: salt spray and condensation from fog can kill electronics quickly. In coastal climates it is best to spray the electronics with conformal coating.

Parts list

With all that said, here is the parts list for our most-preferred way to build a solar powered repeater. This design doesn’t require any soldering or complicated connectors. It also doesn’t require any battery management chips (which often have long shipping lead times and can be hard to get). For this design the solar panel is connected directly to the battery pack via USB. Then the battery pack is connected to the board via USB. The Voltaic Systems V25 battery is nice because it is optimized to charge from a solar panel and it is set standard to an “always on” mode which means the battery bank does not shut off after a set amount of time like other battery packs – this is useful as the RAK chip uses very little power and can trick other battery packs into shutting off. The other really nice thing about the Voltaic pack is than when it drains down completely it shuts down, but then once the solar panel has sufficiently charged it back up it will automatically turn itself back on again. The RAK radio uses between 100 and 1000 mAh per day, with about 400 mAh per day being average so theoretically the 6,400mAh Voltaic V25 could keep the radio running for 16 days without solar.

Regular parts

Birdhouse parts

To turn this repeater into a solar-powered meshtastic birdhouse you can just add the following components. Simply paint the top of the birdhouse black and glue on the solar panel, cut a hole in the back of the birdhouse big enough to put the box into it, attach the antenna mount L-Bracket to the side of the birdhouse, and connect the box to the antenna on the mount with the pigtail using the jam nut.