Personally, I use Device Watchdog to let me know when devices have stopped responding. You do have to do a little guesswork and estimate how long you're willing to let devices not check in before you get concerned about them, so it's not as proactive as battery monitoring (which this app could also do but I don't use) might be, but I find it much more of a reliable method. I have several child apps--one that only lets devices not check in for a few hours, one for something like 24 hours, and one for nearly a week. I could probably break this down a bit more, but it just depends on your devices: some of mine check in nearly every hour (Xiaomi) or are at least likely to send some event in a few-hour timeframe (most temperature-sensing devices), whereas I have some like my Kevo Convert lock and smoke detectors that will go days without reporting anything at all, even battery levels.
I'll probably add battery-based reporting to this at some point (maybe for locks or things I might really care about), but for the rest, it works well for me. For example, if a motion sensor battery goes dead during the day, it's of little consequence for me, and if it goes dead at night, I'll notice because my automations aren't working. This helps me catch some of them before the latter happens.
