Battery low

The Battery low trigger fires when a battery sensor reports that the battery is low.

Use Battery low to send an alert before a device goes offline, pause automations that rely on battery-powered sensors, or keep a log of which devices need attention. Combine it with an area or label target to monitor all battery-powered devices in a room or across your whole home at once.

Using this trigger from the user interface

If you prefer building automations visually, Home Assistant walks you through this trigger step by step. You pick what to watch, tweak a few options, and save. No YAML knowledge required.

To use Battery low in an automation:

1. Go to Settings > Automations & scenes.
2. Open an existing automation, or select Create automation > Create new automation.
3. In the When section, select Add trigger.
4. Select what you want to monitor. Under By target (see Targets), pick the area your battery-powered device is in (like your hallway or garden). You can also select a device, a specific entity, or a label.
5. From the triggers shown for that target, select Battery low.
6. Under Trigger when (see Behavior), pick Each, First, or All to control how the trigger behaves when multiple sensors are targeted.
7. Under For at least, enter how long the sensor must remain reporting low battery before the trigger fires. Leave it at zero to fire immediately.
8. Select Save.

Options in the UI

Using this trigger in YAML

If you work directly in YAML, or you want to know exactly what Home Assistant does under the hood, this section has the technical reference. It lists the field names you use in YAML, their types, and which ones are required.

In YAML, Battery low is referred to as battery.low. A basic example looks like this:

trigger: battery.low
target:
  entity_id: binary_sensor.front_door_lock_battery

This fires the moment binary_sensor.front_door_lock_battery reports a low battery.

To watch all battery sensors in a room and fire only after every sensor in the area has reported low battery:

trigger: battery.low
target:
  area_id: living_room
options:
  behavior: last

Options in YAML

Targets of the trigger

This trigger requires a target. The target is the object that Home Assistant will watch. You can select a single entityAn entity represents a sensor, actor, or function in Home Assistant. Entities are used to monitor physical properties or to control other entities. An entity is usually part of a device or a service. [Learn more], a device, an area, a floor, or a label as a target, and Home Assistant will watch every matching battery entity behind that target.

• Entity: one specific battery entity, such as battery.living_room.
• Device: every battery entity that belongs to a device.
• Area: every battery entity in a room or area.
• Floor: every battery entity on a floor.
• Label: every battery entity that shares a label.

You can also select different target types in one trigger. For example, you can add a specific entity and an area as targets in the same trigger to monitor both of them at once.

Behavior with multiple targets

When you target more than one entity (or select an area, floor, or label that contains several), the Trigger when option controls how the trigger responds:

• Each (any in YAML, default): the trigger fires every time any one of the targeted entities transitions. For example, if you monitor three motion sensors in the living room and someone walks past sensor 1, the automation fires. When they walk past sensor 2 a moment later, it fires again. Every individual event counts.
• First (first in YAML): the trigger fires only on the first transition in the targeted group, then waits until all targeted entities have reset before it fires again. For example, if you monitor the same three motion sensors, the automation fires when the first one picks up movement (someone entered the room). The other two firing afterward are ignored, so you get one notification per “someone walked in” event instead of three.
• All (last in YAML): the trigger fires only after the last targeted entity in the group has fired, meaning all of them are now in the expected state. For example, if you monitor the lights in the living room, bedroom, and hallway, the automation fires only once all three have turned off. This is useful for scenarios like “start the robot vacuum only after every light on the floor is off,” so you know the room is truly empty.

Good to know

• This trigger works with binary_sensor entities that have the battery device class. These are separate from battery percentage sensors (sensor entities with the battery device class). If your device only exposes a percentage sensor, use Battery level crossed threshold instead.
• Use a label to group battery-powered devices across different areas, and target that label to monitor them all from a single automation.
• Combine this trigger with a notification action to get a push notification on your phone the moment any sensor runs low.

Try it yourself

Ready to test this? Go to Settings > Automations & scenes, create a new automation, and add this trigger. Save the automation, then change the state of the targeted entity to watch the trigger fire on your actual entitiesAn entity represents a sensor, actor, or function in Home Assistant. Entities are used to monitor physical properties or to control other entities. An entity is usually part of a device or a service. [Learn more].

More examples

Real scenarios where this trigger fires in automations and scripts. Copy any example and adapt it to your setup.

Automation: get a notification when any sensor in an area runs low

Sensors in out-of-the-way spots like a mailbox or garden shed can run out of battery without you noticing. This automation sends you a notification the moment any battery sensor in your garden reports low battery, so you can replace it before it stops responding.

• Trigger: Battery low
  • Target: Garden area
• Action: Send a notification message
  • Target: My Device (notify.my_device)

alias: "Notify when a garden sensor battery is low"
triggers:
  - trigger: battery.low
    target:
      area_id: garden
actions:
  - action: notify.send_message
    target:
      entity_id: notify.my_device
    data:
      message: >-
        {{ trigger.entity_id }} is reporting a low battery.
        Time to replace it.