You could do this with Rule Machine, but there are lots of easier options. Hubitat Safety Monitor can monitor battery levels, but if you prefer to keep this separate from that, the Notifications app would be an easy alternative: just select that you want to get notified about battery, choose your devices, type in the level and other options, and choose your notification device.
That being said, an app I frequently recommend instead is Device Watchdog. It can monitor battery levels, but I don't use it for that; I just use it to monitor device activity--something it can do that no built-in option I know of can do. I know battery-level monitoring sounds nice and proactive, but the reality is that it's hard to make work well for most devices. First, Zigbee devices (and probably others; I just don't know as much about them) report battery level as belonging to a specific tier/range, which you can read about in this helpful post on the ST forum (different platform, but same here because it's the device that does this). Second, the lithium batteries that many current sensors take do not have a linear discharge curve (if graphing voltage); they generally plateau then quickly fall as they reach end of life, so estimating level based on voltage is tricky in the first place. If you haven't looked at your battery levels and the recent history when they are dead, you might be surprised by how tricky it would be to come up with a rule or battery level that works well for even one device every time, much less all your devices. Monitoring activity also requires some knowledge--how long you're willing to wait before you think a device must have died or fallen off (most of my devices check in at least once every few hours, and I have a few instances of DW installed to accommodate differences)--but I find it to be more reliable. It is, of course, not as proactive, so you certainly could combine this with battery monitoring if you want to, as well--just thought I'd mention that battery alone isn't as good as it sounds in many people's experience. 
