I bought a bunch of eneloop pro, but using them in connected thermostats is always displaying “low battery” even after just fully charged. This is when I discovered that they are actually 1.2V
It really came as a surprise, is there a catch? Are they only good for low power stuff like remote controls?
Edit: it seems they do exist in lithium. Question remains why are the NiMH only 1.2v and why are they the most widespread?
Different chemistries produce different voltages per cell. Alkaline produces 1.5v, NiMH produces 1.2v, Li-Ion produces 3.6v. These are averages, the actual voltage varies over the current charge level of the cell. This variation in voltage is how the low battery alarm actually works, although Alkaline cells produce 1.5v initially, once they are nearly empty they are producing 1.1-1.2v. Your thermostats will likely work fine on NiMH batteries, if you can live with them continually complaining about the batteries being low.
There are, or at least were, rechargeable alkaline batteries, but they don’t last many cycles.
The 1.5v Li-Ions have a tiny circuit board on them that regulates the voltage down to 1.5v, which takes up space so the capacity is reduced. You could do that with NiMH, but it would have less capacity than the Li-Ion version, so there’s little point.