Telemetry Channels¶

The Inhero MR2 transmits telemetry data in CayenneLPP format across four channels. The MeshCore app displays these as Channel 1–4.

Channel 1 — Device Status¶

Base data from the node.

Field	Unit	Source	Description
Battery Level	% / V	INA228	See note below on SOC workaround
Temperature	°C / °F	nRF52840 die	Die temperature of the processor, read via `getMCUTemperature()` and added by the generic MeshCore telemetry path. See Which Temperature Is Which

Battery Level & SOC Workaround¶

MeshCore currently transmits only battery voltage on Channel 1 — there is no native SOC% field. The MeshCore app converts this voltage back to a percentage using a hardcoded Li-ion discharge curve. This works well for Li-ion cells but produces wrong readings for LiFePO4, LTO, or Na-ion chemistries (which have a much flatter voltage curve).

The MR2 works around this limitation:

SOC State	What `getBattMilliVolts()` returns	App displays
SOC not yet valid	Real battery voltage from INA228	Percentage based on Li-ion curve (may be inaccurate for non-Li-ion)
SOC valid (coulomb counter calibrated)	Fake Li-ion OCV reverse-mapped from true SOC% (`socToLiIonMilliVolts()`)	Correct percentage — the app's Li-ion curve decodes back to the original SOC%

OCV = Open Circuit Voltage — the battery's resting voltage without load. The OCV curve (voltage vs. SOC%) is characteristic for each battery chemistry and is used here as a lookup table to reverse-map SOC% back to a voltage the app can interpret.

The SOC becomes valid as soon as a reference point exists — either manually via set board.soc <percent> or automatically on a "Charging Done" event (which sets SOC to 100%).

Without set board.batcap <mAh> a chemistry-typical default capacity (1500–2000 mAh) is assumed. Setting the real capacity is what makes the displayed percentage and the Batt-TTL accurate.

The reverse mapping uses a piecewise-linear Li-ion OCV table (3000 mV at 0% → 4200 mV at 100%). This ensures the app displays the correct coulomb-counted SOC regardless of the actual battery chemistry.

Channel 2 — Environment (BME280)¶

Data from the BME280 environment sensor (always present on the MR2).

Field	Unit	Source	Description
Temperature	°C / °F	BME280	Ambient temperature
Relative Humidity	%	BME280	Relative humidity
Barometric Pressure	hPa	BME280	QNH (sea-level pressure) when `board.altitude` is configured; otherwise station pressure
Altitude	m / ft	BME280	Configured installation altitude; otherwise altitude derived from pressure

Set the installation altitude once with set board.altitude <metres> (valid range: −500 to 9000 m). The setting is persistent. The firmware then reduces the measured station pressure to QNH using the ICAO standard-atmosphere formula and transmits that directly comparable sea-level value. get board.altitude shows the active value.

Use set board.altitude clear to remove the stored altitude and return to uncorrected station pressure.

Compatibility: Until an altitude is configured, telemetry keeps the previous behaviour: uncorrected station pressure plus an altitude derived from the fixed standard sea-level pressure of 1013.25 hPa.

Channel 3 — Battery (INA228 / BQ25798)¶

High-precision battery data from the INA228 coulomb counter and BQ25798 charge controller.

Field	LPP Type	Unit	Source	Description
Voltage	Voltage	V	INA228	Battery voltage (20-bit ADC, ±0.1% accuracy)
SOC	Percentage	%	INA228	State of charge via coulomb counting — optional, only when calibrated
Current	Current	A	INA228	Battery current. Negative = discharging, positive = charging
Temperature	Temperature	°C / °F	NTC on the BQ25798 TS pin	Battery temperature — optional, omitted when unavailable
Batt-TTL	Distance	days	calculated	Estimated time-to-live (remaining runtime) — optional, only with valid SOC

SOC & Batt-TTL¶

SOC and Batt-TTL only appear when the coulomb counter has a valid reference point — either a manual SOC set (set board.soc) or a "Charging Done" event. The percentage is based on the configured battery capacity (set board.batcap; a chemistry-typical default of 1500–2000 mAh is assumed otherwise). Until the SOC is valid, these fields are omitted.

Batt-TTL Encoding¶

The Batt-TTL is transmitted as a CayenneLPP Distance value in days, since CayenneLPP has no native "duration" type. The MeshCore app displays it as a distance (e.g. "42 m"), but the value represents days of remaining runtime.

Condition	Transmitted Value	Meaning
Finite Batt-TTL	`ttlHours / 24.0`	Estimated remaining days on battery
Surplus (charging > consumption)	`990.0` (sentinel value)	Effectively infinite — device is gaining charge
Unknown (SOC not yet valid)	not sent	Batt-TTL cannot be calculated yet

Battery Temperature¶

The temperature on this channel is measured at the battery, by the NTC on the BQ25798 TS pin. The BQ25798 reports the TS pin voltage as a percentage of REGN; the firmware decodes it through the Inhero divider (RT1 = 5.6 kΩ, RT2 = 27 kΩ) with a Steinhart-Hart equation, adds the set board.tccal offset and runs a plausibility check before the value is transmitted.

Four conditions must hold for a battery temperature to appear:

The BQ25798 ADC one-shot completes.
The TS ADC channel is enabled. It is switched off while the INA228 reports a battery voltage above 0 and below 3200 mV and no input source is qualified — with TS enabled the BQ25798 ADC needs VBAT ≥ 3.2 V in battery-only operation, and switching TS off drops that threshold to 2.9 V so the solar readings keep working. With an input source qualified the channel stays on, so a cold and nearly empty cell reports its temperature while it is being charged.
The decoded value lies within −50 … +90 °C.
The value passes the BME280 plausibility check (below).

The battery chemistry plays no part in this decision. The TS channel runs for every chemistry, so LiFePO4, Li-ion, LTO and Na-ion all report a battery temperature when an NTC is fitted and VBAT allows the channel.

Condition 2 is the one that shows up in the field: at a nominal 3.2 V (LiFePO4) or 3.1 V (Na-ion) these chemistries spend much of their discharge curve below 3200 mV, and without solar input the battery temperature reads N/A there even with an NTC fitted — on a solar node, at night. LTO 2S (4.6–5.4 V) stays above the threshold throughout, and Li-ion 1S is above it over most of its curve.

BME280 Plausibility Check¶

A missing or open NTC can decode to a value the window check accepts. With RT1 = 5.6 kΩ and RT2 = 27 kΩ the divider sits on the RT2-only pole. Exactly on the pole the decode returns −99 °C, which the −50 … +90 °C window catches. One TS ADC step (0.09765625 % of REGN) off the pole the value lands around −46 °C, inside that window and indistinguishable from a real deep-cold reading. Each accepted reading is therefore compared against a fresh BME280 measurement:

BME280 reading	Difference (calibrated NTC value vs. BME280)	Result
Above −100 °C and below +100 °C	≤ 15.0 °C	Value is transmitted
Above −100 °C and below +100 °C	> 15.0 °C	Replaced by −999 → N/A
Unreadable (returns −999)	not evaluated	Check stands down, value is transmitted unchanged

A difference of exactly 15.0 °C passes. The comparison uses the calibrated value (raw reading + set board.tccal offset).

A rejected reading also does not refresh the cached temperature used for SOC derating; after 5 minutes without an accepted NTC reading, the derating falls back to the BME280. The set board.tccal auto-calibration is unaffected — it reads the BQ25798 driver directly with the offset zeroed and skips only the driver's own error codes.

Temperature Sentinel Values¶

The BQ25798 driver produces these values:

Value	Meaning
−999 °C	I²C communication error
−888 °C	ADC not ready: the one-shot did not complete, the TS register still read 0 or 0xFFFF after three retries, or the TS channel was switched off
−99 °C	NTC open (not connected), or the decode landed on the RT2-only pole
+99 °C	NTC shorted

Anything outside −50 … +90 °C is converted to −999 in the board layer, and a reading rejected by the BME280 check becomes −999 as well. −999 is therefore the only sentinel that leaves that layer, and it carries three causes at once: an I²C error, a driver error code, or an implausible reading. The three are not distinguishable from outside.

Values ≤ −100 °C are not transmitted: the CayenneLPP temperature field on the battery channel is omitted entirely, and get board.telem prints N/A in its place.

Channel 4 — Solar (BQ25798)¶

Solar input data from the BQ25798 charge controller.

Field	LPP Type	Unit	Source	Description
Voltage	Voltage	V	BQ25798	Solar input voltage (VBUS)
Current	Current	A	BQ25798	Solar input current (IBUS)
MPPT 7-Day	Percentage	%	Firmware	MPPT activation over the last 7 days. Shows what percentage of time the MPPT regulator was actively harvesting solar energy.

Note — Solar current accuracy: The BQ25798 IBUS ADC has a resolution of 1 mA (15-bit mode) but exhibits significant measurement error at low currents (~±30 mA). Values below approximately 150 mA should be treated as rough estimates. For precise current measurement, the battery side uses the INA228 instead.

Note: The MPPT percentage is a rolling 7-day average. A low value (e.g. 1%) means the panel rarely delivers enough power to activate the MPPT regulator — e.g. during overcast conditions or suboptimal panel angle.

Which Temperature Is Which¶

The MR2 reports four temperatures from four different sensors:

Where it appears	Value	Sensor	Notes
Channel 1	MCU die temperature	nRF52840 on-chip sensor	Added by the generic MeshCore telemetry path (`getMCUTemperature()`), alongside the base battery voltage. Runs warmer than ambient under load
Channel 2	Ambient / board temperature	BME280	Also the reference for `set board.tccal`, the plausibility check and the SOC-derating fallback
Channel 3	Battery temperature	NTC on the BQ25798 TS pin	Steinhart-Hart decode + `tccal` offset + plausibility check; omitted when unavailable
`get board.cinfo`, field `TDIE:`	Charger die temperature	BQ25798 on-chip sensor	Junction temperature of the charge controller, from the last completed ADC one-shot, so up to one telemetry period old. Runs warmer than ambient while charging. Not on any LPP channel

Only the Channel 3 value is measured at the battery.

Channel Assignment in Code¶

Channels are assigned dynamically:

Channel 1 (TELEM_CHANNEL_SELF) is statically defined and contains the MeshCore base data (battery voltage and MCU die temperature).
querySensors() assigns each active sensor its own channel starting right after Channel 1 — the BME280 therefore lands on Channel 2.
The battery channel is determined by queryBoardTelemetry() as the next free channel (findNextFreeLppChannel).
The solar channel = battery channel + 1.

querySensors() assigns the BME280 to Channel 2 before queryBoardTelemetry() runs, so battery data lands on Channel 3 and solar on Channel 4 in practice.

Order in CayenneLPP packet:
┌──────────────────────────────────────────────────┐
│ Channel 1: Voltage (INA228 / SOC fake)           │  ← MyMesh.cpp (getBattMilliVolts)
│ Channel 2: Temp, Humidity, Pressure, Alt.        │  ← BME280 (querySensors)
│ Channel 3: VBAT, [SOC], IBAT, [TBAT], [Batt-TTL] │  ← queryBoardTelemetry()
│ Channel 4: VSOL, ISOL, MPPT%                     │  ← queryBoardTelemetry()
│ Channel 1: MCU die temperature                   │  ← MyMesh.cpp (getMCUTemperature)
└──────────────────────────────────────────────────┘

Fields in square brackets are optional: [SOC] and [Batt-TTL] need a valid coulomb-counter reference point, [TBAT] needs an available battery temperature.

Permissions: Channels 2–4 are only sent if the requesting client has the TELEM_PERM_ENVIRONMENT permission. Guests (Guest role) receive only Channel 1 with base voltage and MCU temperature.