I solved this by creating a “Helper” sensor which “Combine the state of several sensors”. Set “Statistic characteristic” to Minimum and Precision of 2.
Then I added an automation to send an alert if any battery in all of my battery operated devices gets below 25%.
alias: Alert order batteries for hydrometers
description: ""
triggers:
- entity_id:
- sensor.minimum_battery_level
below: 25
trigger: numeric_state
conditions: []
actions:
- action: notify.notify
data:
title: Battery Low
message: Order batteries
mode: single
I get a single alert. If you wanted a daily alert, then you can also add a condition paired with a daily timer to get both instant alert and a daily reminder.
You can also create a “Group” of batteries based on their updated state. If any one in the group becomes “Unknown,” then the group becomes unknown. It means the device disconnected or died. You can create another alert based on this.
So you have the flexibility of two types of alerts. When any device becomes unknown you get an alert to repair it. However, you don’t lose the ability to track the remaining device battery levels.