I agree it would be nice to have this built in, so would support this as a feature request.
While we wait... Aside from @jabecker 's suggestion, if you haven't found it already, you may be interested in the Community-based Device Watchdog app by Bryan.
Alternatively, you could include battery-based devices in a dashboard to monitor them manually / visually.
A third option (I am assuming) is to setup rules to warn you when battery levels drop below a threshold.
