The measurement of the battery voltage is a BS implementation, see Popp Smoke Detector and Siren - #27 by Celaeno1.
Quote from the manual:
“Low Battery Alarm (when the battery of the wireless modules goes low), indicated by yellow LED”
It might be necessary to add your Z-Wave controller to association group 2 (shouldn’t be necessary though) and to add the “low battery alarm” to the Z-Wave database.
Problem:
How to find out the notification type/notification for “battery low alarm”? Just a wild guess: 0x01 (Smoke Alarm) / Replacement required (0x04) - you could use an almost empty battery (make sure the yellow LED in on) and watch the debug log to find the values for “battery low alarm”.