Battery became low

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

Use Battery became 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.

For a visual overview of all battery statuses, open the Maintenance dashboard.

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 became 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 became 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. For how an automation is built in YAML, refer to Automations in YAML.

In YAML, put the settings that are specific to this trigger under options. Keep target, if the trigger uses one, and general settings such as id, alias, enabled, and variables at the top level, next to trigger.

When this trigger starts an automation, it passes data about what happened to the automation. You can use this trigger data in the templates of your conditions and actions. To see which data is available, refer to Trigger data in templates.

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

trigger: battery.became_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.became_low
target:
  area_id: living_room
options:
  behavior: all

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 low battery sensor behind that target.

• Entity: one specific low battery sensor, such as binary_sensor.phone_battery_low.
• Device: every low battery sensor that belongs to a device.
• Area: every low battery sensor in a room or area.
• Floor: every low battery sensor on a floor.
• Label: every low battery sensor 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.

To learn which entities Home Assistant includes for a device, area, floor, or label, and how it handles several entities, refer to Automation targets.

Trigger variables

When the trigger reacts, the trigger variable identifies the entity that made it react. This is useful when a target contains multiple entities, such as a device, area, floor, or label.

The following trigger-specific variables are available:

• trigger.entity_id: Entity ID of the entity that made the trigger react.
• trigger.from_state: Previous state object of that entity.
• trigger.to_state: New state object of that entity.
• trigger.for: Timedelta object of how long the trigger criteria were met, if applicable.

For example, use {{ trigger.to_state.name }} in an action to include the name of the entity that made the trigger react in a notification.

The trigger variable describes the entity that made the trigger react. It doesn’t contain all the entities of the target.

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 (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: the trigger fires only on the first transition in the targeted group, then waits until none of the targeted entities is in that state anymore 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: 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

• Use a binary sensor entity with the battery device class.
• Use a label to group battery-powered devices across different areas, and target that label to monitor them all from a single automation.
• For battery percentage sensors, use Battery level crossed threshold instead.
• 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, and then make the trigger react. For example, change the state of the targeted 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], turn on a light that the trigger monitors, or change its brightness. To check whether the automation ran, open its traces.

More examples

Real scenarios where this trigger fires in automations and scripts. Copy any example and adapt it to your setup. In the visual editor, select the UI options listed in the example when they are available, or paste the YAML code.

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 became 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.became_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.