snapster:
I'm not following...what would the advantage of using nimh be over using a lipo? I certainly will be putting the arduino to sleep when not in use though.
The 'project' as you have described it so far seems ideal for supply from AA Alkalines or indeed NiMh. They could be made to last years.
But since you have provided no information on what this project actually does, everyone is just spending time guessing.