This for sensors that use them, such as the exterior PM2.5 sensor WH41.
Here in the US we can get Duracell NiMH batteries pretty much anywhere, local stores, etc. However, one needs to closely read the package as some will have a capacity of 1250 mAhr , and some will be labelled 2500 mAhr. There is virtually no difference in appearance between the two except the fine print on the batteries.
In short, you are way better off with the higher mAhr version as it will sustain longer during cloudy events. And it is quite noticeable. I have the misfortune of buying the lower capacity ones and couldn't figure out why I needed more frequent external charging than previously. then i read the small print on the batteries.
These 1250 mAhr batteries will now serve duty in some remote controls instead, and I have replace with 2500 mAhr for those sensors. I guess it is a case of caveat emptor .
This more than likely also applies to other manufacturers, and ones that tell the truth on capacity (a whole different issue)