My 5c worth on this ....
I have been testing rechargeable batteries in Spirit TRVs and Horstmann/Secure Thermostats.
I have found the Eneloop white and black batteries seems to do well - I have a long term test running - but of rthe last 6 months they appear to have done well.
Recently started testing Ikea high end rechargeable batteries - specifically the Ladda 2450 and 900 - which appear to be doing as well as Envelops but at less than 33% of the cost.
Lessons learnt:-
I have found that when the Spirits and particularly the Hortsmann / Secure Thermostats fall off a click when they report about 35% battery and that is when you need to replace them, they will hover at this reported charge for a fews days then collapse to 5% in one big drop.
I have set triggers to watch every monitorable battery which sends me a text once a day telling me that one of the batteries is low accompanied by Matplotlib Battery graph and I can see instantly the device that needs replacing.