I recently ignored my warning when my NiMH batterys’ charge drops below a threshold in HA, forgot about it, and sure enough it died before I got home. Yale does have 2 contacts that you put a 9V battery on and it wakes up and you can enter your code. I’ve tested this to make sure.
This time the 9V backup did not wake the lock. What seemed to happen this time is the NiMH cells got low enough, but perhaps not low enough for the lock to let me use the 9V backup? I had no way to enter the code (I do have a key box for another door luckily).
I took the batteries out and tried again, and sure enough the 9V then woke it up.
Has anyone else experienced this?
Yeah I know, I may be the only one dumb enough to let it happen 
Rechargeable NiMH cells work great otherwise, just so you realize the percentage charge shown in HA is going to be below Alkaline cells. I switched about a year ago and it’s been fine. Not that I go through a ton of disposable cells, but enough to annoy me.