I’ve been testing with Mayfly 1.1 and LiSOCl2 D batteries. Our package is using a Mayfly 1.1, and SD card, and a sensor on the switch power that pulls ~350 mA in pulses when its on.
I initially tried using 1S4P configuration with 95sq015 protection diodes with the intent of preventing heating from cell imbalances. That puts the Vin at about 3.4 V. At temperatures down to about -10C it works fine, but below that the voltage would drop to the point that the 1284p will brownout. The internal battery measurement shows less than 3.1 V, but as the powersupply is not a buck/boost, I doubt this is accurate and I think the voltage is sagging to 2.7V and triggering the BOD (herm, note to self: try measuring battery voltage using internal 2.7 V VREF). After reading some papers and datasheets about LiSOCL2, I wrote the company that makes the LiSOCL2 (Xeno) I’m using, they said for the use case where the cells are the primary power source and replaced as a group, the diodes aren’t needed. I removed them, and, at least in the short term these are able to operate at -20C.
Something else I noticed was that the LiSOCL2 cells have an activation period: they couldn’t power the Mayfly for a few moments after I first connected them (reboot loop) after a few minutes it started working. Several datasheets mention that the lithium electrode gets deactivated when the cells are not used and that some current draw is required to reactivate it.