# Zigbee Sensor Battery Monitoring

## Problem

Three Aqara Zigbee temperature sensors report `battery: 0` in their MQTT payload, making the `hass_sensor_battery_percent` Prometheus metric useless for battery monitoring on these devices.

Affected sensors:
- **Temp Living Room** (`0x54ef441000a54d3c`) — area: living_room
- **Temp Office** (`0x54ef441000a547bd`) — area: office
- **temp_server** — area: server_room

The **Temp Bedroom** sensor (`0x00124b0025495463`) is a different model and reports battery correctly (69% at time of investigation).

## Findings

- All three sensors are actively reporting temperature, humidity, and pressure data — they are not dead.
- The Zigbee2MQTT payload includes a `voltage` field (e.g., `2707` = 2.707V), which indicates healthy battery levels (~40-60% for a CR2032 coin cell).
- CR2032 voltage reference: ~3.0V fresh, ~2.7V mid-life, ~2.1V dead.
- The `voltage` field is not exposed as a Prometheus metric — it exists only in the MQTT payload.
- This is a known firmware quirk with some Aqara sensors that always report 0% battery.

## Possible Solutions

### 1. Expose voltage as a Prometheus metric
Enable the voltage sensor entities in Home Assistant (they may exist but be disabled by default). The HA Prometheus integration would then export them automatically.

### 2. Calculate battery from voltage in Zigbee2MQTT
Override the battery calculation using the voltage field. Approximate formula: `(voltage - 2100) / (3000 - 2100) * 100`.

### 3. Alert on sensor staleness instead
Create a Prometheus alert based on `hass_last_updated_time_seconds` going stale (e.g., no temperature update in 1 hour). This detects dead sensors regardless of battery reporting accuracy.