There are lots of Garmin battery apps on the CIQ store. They do lots of different tasks but the new one that looks interesting to me is the Kurev CIQ app which, unusually, is also certified and in the thisisant directory.
Note there is both an app and a widget but not a data field so, for example, it is visible in the widget glances. Only these types of sensor are supported Speed, Cadence, Power, Radar, Shifting and Running Dynamics, and LightNetwork (partially) ie not TEMPE and HRM.
Thisisant says it’s certified for speed, power and cadence devices but when I have my Di2 or Varia lights enabled it can cause problems. Currently, BLE sensors seem to crash it too but they have been disabled for now (you can re-enable them in the CIQ store or Garmin Express)
Nevertheless, this is a potentially great way to keep on track of the battery status of what seems to be an ever-increasing number of battery-reliant sensors.
Source: CIQ Store