Common issue with Li-ion batteries. They hold voltage output for a long time so the device has no idea they are getting low until they are nearly dead.
If the devices are not mission critical I would use this app: [RELEASE] Device Activity Check - Get notifications for "inactive" devices
Setup activity AND battery alerts for all the battery devices. That way in case the battery dies suddenly you will get a notice the next day the device has not been reporting in anymore.
