The starting voltage of a fully charged NiMH cell can be as high as 1.4v so you may briefly be running the processor at 5.6v.
The batteries ive got ive had for a while and tested them alot, after a full charge they sit at 1.2V. so i dont think ill be running the risk of running at over 5v. What is the likelihood of causing damage by doing this out of interest?
Because i already have loads (like 30) NiMH hanging around, ild be inclined to use them. If i can get a couple of months out of them a few times in a row, then ild probably go get some of them lithums. that datasheet was a good one, i like the look of the graph for Lithium vs Alkaline for low current continuous usage. very steady.
the last bit of info ill need before i go and make all these changes is to know for sure that the "void system_sleep()" function is from the nightingale is right for me and how to wake it up from the "ISR(ANALOG_COMP_vect )"