This:
and this:
Are not the same, or the first does not imply the second. 
Many sensors report battery levels. I can only think of a couple I use that don't. But the information tends to not be reliable. This is partly because most of these use lithium batteries with a non-linear discharge curve, with voltage remaining fairly steady until the very end, when there's a sharp drop.
IMHO, a better approach is just to see when the sensors stop reporting and then take a look to see if there's a problem. That's why I wrote Device Activity Check, though there are alternative approaches you can use.
There's no reason to pick one approach or the other. You can also use battery levels, either in this app, as new optional column in the "Devices" page on hub update 2.3.8, with a Rule or Notifier app or even HSM, or a number of other approaches. But, of course, an app can't know better than the device itself, so you're still stuck with the issue of how battery reporting works in the first place, and it tends to not be great (I haven't seen reports of anyone claiming a particular device reports notably well compared to others, for example).
