I've built a temperature sensor module that runs off of 4xAA batteries and sends data back to a computer via XBee. Works great, until the weather gets cold. At that point, the rechargeable AA batteries (I've tried duracell and eneloop) die very quickly. At 40 or so it seems like they can hardly muster the 3.3V needed to power the wireless.
This needs to be battery powered - running a wire to it isn't an option.
I'm trying plain old alkalines right now, but I was wondering if anyone had any tips on getting enough power out of batteries during the winter. Do I just need to double up and use 8 batteries? Do NiCad or LiPo cells work better than NiMH? Would switching to a different wireless module that can handle lower voltage solve the problem?