Add Inhero MR2 board variant (nRF52840/RAK4630 solar repeater)

[liekmarflow]

Closes #3130. Adds the Inhero MR2, a solar repeater platform in production and maintained by its hardware manufacturer.

https://shop.inhero.de/en/products/inhero-mr-2-solar-mesh-repeater-board-rak4630-sx1262-mppt-red-ce-gepruft

Implementation scope

The board implementation is under variants/inhero_mr2/, with its definition in boards/inhero_mr2.json and MR2-specific host tests under test/. Periodic work uses the merged MainBoard::loop() hook; the board.* CLI uses MainBoard::handleCommand(). The MR2 sensor manager appends battery and solar telemetry using the existing CayenneLPP encoding and permission mask.

RTC initialization includes an INHERO_MR2-guarded configuration for this board's supply arrangement. Other boards retain the existing initialization path. The full EN/DE documentation is maintained in the manufacturer fork and linked from the variant README.

Hardware and features

• RAK4630 (nRF52840 + SX1262), BQ25798 charger with 3.6-24 V solar input and MPPT, INA228 coulomb counter, RV-3028 RTC and BME280; 45 x 40 mm, CE-certified under RED 2014/53/EU.
• Li-ion, LiFePO4, LTO and Na-ion profiles with per-chemistry charging and low-voltage thresholds. Li-ion and LiFePO4 use JEITA; LTO and Na-ion retain their chemistry-specific operation without JEITA supervision. The cell manufacturer's charging limits apply.
• SOC tracking, seven-day energy statistics, estimated battery runtime, and battery/solar telemetry.
• Power-management optimizations: averaged voltage monitoring, checked RTC scheduling, a one-hour low-voltage recovery interval, and persistent RTC configuration for operation without a separate backup battery. Periodic low-voltage wakes also maintain the configured charging and solar-input state.
• Solar-input qualification and MPPT housekeeping, a common 2.50 V minimum system voltage, and precharge current aligned with the configured charge-current limit. get board.mpptdiag reports requested and actual charger settings through checked register reads.
• Optional BME280 sea-level pressure (QNH): set board.altitude <m> stores installation altitude (-500 to 9000 m). set board.altitude clear selects station pressure and pressure-derived altitude. MR2 builds preserve the required floating-point checks under the nRF52 optimization settings.
• Repeater, RS232 bridge and sensor environments: Inhero_MR2_repeater, Inhero_MR2_repeater_bridge_rs232, Inhero_MR2_sensor.

Configuration behavior

An actual battery-chemistry change starts with charging defaults: 200 mA maximum current, MPPT off, no manually configured capacity, no user JEITA override, and the default frost setting. Selecting the same chemistry preserves settings and SOC.

The optional Li-ion/LiFePO4 JEITA override is off by default. It requires explicitly configured capacity and maximum charge current strictly below 0.05C. Incompatible requests are declined; accepted settings persist. While enabled, incompatible current/capacity changes are declined. Enabling the override or switching it off resets fmax to 0%. The override bypasses both cold and hot JEITA cutoffs.

Validation

All three MR2 environments and RAK_4631_repeater build successfully for this update. Host checks cover charger configuration and diagnostics, averaged alerts, RTC register handling and the low-voltage control flow. The port retains the upstream board-loop, CLI and sensor-manager integration.

The latest power-management hardware tests were completed on the manufacturer's MeshCore v1.17.1-based MR2 repeater build using a shortened test interval, including repeated sleep/wake cycles and a live battery-chemistry change. Production firmware uses one hour. The corresponding dev-based PR changes are validated by builds and host tests; this update of the PR branch has not been flashed for a separate hardware run. Earlier MR2 hardware checks covered the sensor-manager integration and pressure/altitude telemetry.

Maintenance

I manufacture the board and operate it in the field, including solar repeater sites in Saxony with links from 70 km to over 100 km. I can validate changes on hardware and am happy to be tagged on anything affecting this variant.

Background: https://shop.inhero.de/en/blogs/news/warum-es-das-mr2-gibt

This variant was developed with AI assistance under my continuous engineering direction and review, followed by pre-submission review and hardware testing. Stated upfront in line with the project's position on code provenance.