Fun observation.
It seems like my Eneloops doesn't like -20 Celcius
I have some Temp sensors in the summerhouse, that are fed by 2 x Pana Eneloop (AA)
https://www.amazon.de/dp/B00392XX5U/I built some Arduino code, that combined with a Hope 868MHz module, keeps track of the temperatures reported from the sensors.
And poll the Arduino (every 5 sec) from a Linux box. I log to CSV every minute - Both Arduino + Linux are inside the summerhouse, kept at 10C.
If the Arduino don't detect a xmission from the temp sensor for 5 minutes , it reports the temperature as 0 degrees, for that sensor. In order to see a "malfunction".
It's quite cold in the summerhouse, and yesterday i noticed the sensor not reporting at several periods.
I actually think the Eneloops stop delivering power, when the temp goes below -19C.
The purple graph is the sensor "outside" by the Grill , the red is inside the (unheated garage).
I looked at the Eneloop DS , and it states they should work down to -20C , and i think i have hit the limit.
Seems like they wake again, when temp reaches around -14C.

I also looked at Alkaline, limit seems to be the same -20C.
Is there any "payable" batteries that works down to -30C ?
/Bingo