History of versions

We have tried 5 permutations of solar-powering nodes:

Version 1.0 — RAKBox-B2, 2000 mAh LiPo

Using a RAK RAKBox-B2 enclosure with a solar panel. The panel was connected directly to the RAK Wireless WisBlock Meshtastic Starter Kit US915, with a 2000 mAh lithium-polymer battery wired directly to the RAK board. We drilled a hole in the top and attached a fiberglass antenna via an iPEX to N-Type connector. Mounted on top of a UT building, it died within a month — the 0.4 watt panel is way too small.

Version 2.0 — USB Solar

Same as 1.0, but we added a second 5 watt solar panel connected to the RAK board via USB. Also died within a month.

Version 3.0 — Internal Solar, Power Module, 18650s

To minimize components outside the box we used a much larger box with a clear lid and placed the solar panel inside. We added a Texas Instruments green power module between the panel and the RAK board to regulate power, switched to four 18650 batteries, and added an over-discharge protection circuit from Voltaic Enclosures (not Voltaic Systems) for a clean shutoff below 2.5 V. Mounted on a UT building, it died in 3 months.

Version 4.0 — External Solar

Same as Version 3.0, but the solar panel was mounted outside the box (using a smaller box) on a pole. It died within 3 months.

Version 5.0 — Currently running

We stopped using the RAK's solar and battery connectors entirely. Instead we connected a large 12 watt solar panel to a Voltaic Systems battery via USB, then connected the battery to the RAK board via USB. The Voltaic Systems battery acts as the solar charge controller and the over-discharge protection circuit: it charges across a range of panel voltages (even when shaded) and shuts off if voltage drops too low, automatically rebooting once it has recharged.

Lessons learned

1. Don't trust the RAK board to handle varying solar or battery voltage — it is very unhappy outside a narrow range. There is a known issue where the Nordic nRF52840 inside the RAK 4631 enters a "Super Deep Sleep" fault state at 3.3 V, requiring a manual reboot.
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 handle wind, rain, and UV better than rubber-ducky antennas.
5. Heat can kill the battery — shade the box containing it.
6. Don't add modules. A GPS module increases battery usage significantly and is unnecessary for a fixed repeater.
7. Seal everything properly and give water a way out. Use o-rings for the antenna mount, wrap connectors in Proxicast, add a drain plug for condensation, and spray electronics with conformal coating in high-humidity environments.

These lessons come from hot Texas summers, where the main stressors are heat and thunderstorms — long stretches of 100°F+ (38°C) days and spells above 105°F (40.5°C), during which two radios died. It can be hot and humid at once, swinging from around 80°F (27°C) / 85% humidity in the morning to 105°F (38°C) / 25% humidity in the afternoon. These learnings apply well to North Texas and other southern and desert climates.

Considerations for other climates

1. Northern climates: Higher latitudes get much less winter sun. Use a larger panel and angle it lower toward the horizon — the rule of thumb is to match your latitude. In North Texas, panels should sit at about 33°, while Calgary's optimal angle would be 50°. Batteries should also be bigger in colder climates, as low temperatures reduce performance.
2. Coastal climates: Salt spray and fog condensation can kill electronics quickly. In coastal climates, spray the electronics with conformal coating.