Different model of Resound, but my experience with 4 iterations of Resound and size 13 batteries, it is the 3 beeps and 15 minutes to go. The zinc/air batteries tend to hold maximum voltage to near death. Makes it hard for them to predict.
If they’re your old aids then I guess you know around how long the batteries usually last. If so then change them the morning of the day you expect them to die, or the morning before that to be conservative.