# 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`) — WSDCGQ12LM
- **Temp Office** (`0x54ef441000a547bd`) — WSDCGQ12LM
- **temp_server** (`0x54ef441000a564b6`) — WSDCGQ12LM

The **Temp Bedroom** sensor (`0x00124b0025495463`) is a SONOFF SNZB-02 and reports battery correctly.

## 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 WSDCGQ12LM sensors that always report 0% battery.

## Implementation

### Solution 1: Calculate battery from voltage in Zigbee2MQTT (Implemented)

Override the Home Assistant battery entity's `value_template` in Zigbee2MQTT device configuration to calculate battery percentage from voltage.

**Formula:** `(voltage - 2100) / 9` (maps 2100-3000mV to 0-100%)

**Changes in `services/home-assistant/default.nix`:**
- Device configuration moved from external `devices.yaml` to inline NixOS config
- Three affected sensors have `homeassistant.sensor_battery.value_template` override

**Expected battery values based on current voltages:**
| Sensor | Voltage | Expected Battery |
|--------|---------|------------------|
| Temp Living Room | 2710 mV | ~68% |
| Temp Office | 2658 mV | ~62% |
| temp_server | 2765 mV | ~74% |

### Solution 2: Alert on sensor staleness (Implemented)

Added Prometheus alert `zigbee_sensor_stale` in `services/monitoring/rules.yml` that fires when a Zigbee temperature sensor hasn't updated in over 1 hour. This provides defense-in-depth for detecting dead sensors regardless of battery reporting accuracy.

**Alert details:**
- Expression: `(time() - hass_last_updated_time_seconds{entity=~"sensor\\.(0x[0-9a-f]+|temp_server)_temperature"}) > 3600`
- Severity: warning
- For: 5m

## Post-Deployment Steps

After deploying to ha1:

1. Restart zigbee2mqtt service (automatic on NixOS rebuild)
2. In Home Assistant, the battery entities may need to be re-discovered:
   - Go to Settings → Devices & Services → MQTT
   - The new `value_template` should take effect after entity re-discovery
   - If not, try disabling and re-enabling the battery entities

## Notes

- Device configuration is now declarative in NixOS. Future device additions via Zigbee2MQTT frontend will need to be added to the NixOS config to persist.
- The `devices.yaml` file on ha1 will be overwritten on service start but can be removed after confirming the new config works.