My most recent experience was that one minute the battery indicator on the ‘settings’ page for each individual camera showed “OK” and then a little bit later I see this.
Didn’t receive a push notification advising that the battery level was low, even though it was turned on, but did receive a notification later on that the camera was “offline”.
From my experience and what I’ve read from others, I wouldn’t expect much warning that battery replacement is imminent. If you’re leaving to go out of town for a period of time and have batteries that have been in a camera(s) for a year to year & half, or a camera(s) that regularly generate a lot of video clips, you may be well served to do a bit of preventive maintenance and put in fresh batteries prior to departing. If your luck is like mine, that would be when the batteries would fail, rendering the cameras(s) dead in the water.