For one application, I need to transmit data using 2 XBee modems (I am looking for XBee Pro XSC 900 modems since we need a long range). This is going to be outdoor at temperatures between -20C and +30C. I need an autonomy of about 1h30.
NiMH , NiCads and even lead-acid bateries don’t do well at cold temperatures. They act as if they have been discharged, voltage and capacity decreases and this is only at 0°C.
An app note I found shows that NiMH have only 20% capacity at -20°C so you would need 5 times the battery capacity.
Now if you only need 1.5 hours of operation time you could over size the voltage and capacity of the battery as you mention. Experimentation may be required to determine how much more is required.
You could put the batteries in an insulated container and place a chem-pack hand warmer in the container to keep the battery temperature higher. This should increase how long they last.