I got this idea from Zigbee2MQTT which sends the voltage, so you probably have it already.
I just got surprised by my third battery failure. I think because all three have been rechargeable NiMH, which has a different discharge curve:
Example curves I found quickly are on this page
https://michaelbluejay.com/batteries/rechargeable.html
My guess is that the batteries looked okay and then suddenly died. You could give us the option to select battery type, but I suspect easiest would be just to send voltage in the Node Red Battery status object so we could find a curve that fits our battery chemistry.
I already set up a warning in Node Red should a battery get low so I’m sure I didn’t miss a low voltage.
And a second TRV surprised me by running out of battery. I know just 3 days ago I looked at the battery icon and it was full. Didn’t look at the number:just picture.
Alternatively tell us a rechargeable battery chemistry that is monitored accurately for the TRV? I’m using NiMH.
Was thinking of using a “lithium Ion” or Lithium Polymer, but then those cells have a natural voltage that is higher so there must be a battery management PCB that probably tricks the Sonnoff TRV too?
I already discovered the Sonoff Zigbee motion sensors V1 dislike rechargeable coin cells. I’ve had a couple refuse to join the network that joined as soon as I put in a non-rechargeable battery. Door/window, TH and button all seem fine with rechargeable coin cells.