I was using my Z9 this last week near the Seal River on Hudson Bay. Ambient air temps were from +5 to -5 Fahrenheit, dropping as low as -20 to -30 F with windchill.
The Z9 and my Z7II worked well -- used the Z 100-400 and 500 mm PF, in some cases with their respective Z or F mount 1.4 TCs, and the Z 20 mm f1.8 for northern lights.
I had 3 EN-EL 18D batteries for my Z9. All new this year. They seemed to hold up well in the cold. Using firmware 3.0.
On the second to the last day of my trip, the EN-EL18D that I had been using that morning triggered the calibrate light when I put it on the Nikon charger. I assume I should calibrate it. Does this indicate any damage or other concern with the battery, other than the time to recalibrate it? Any chance it was triggered by the cold (although it did not happen to the other two EN-EL18D batteries that were equally used in the cold). I tended to put the batteries on the chanrger while they were still pretty cold -- perhaps that was a bad idea?