2200-2500 mAh NiMH AA cells especially those that are rated or recommended for digital camera use are very good general purpose power source for arduino projects. Just series them up for desired output voltage and parallel them up for more duration if desired. They can provide very good peak current loads.
I will confess that I also still use some non-rechargeable AA alkaline batteries but do buy them in bulk quantities at my local Costco warehouse store, seems to work out to be about 24 cents per cell. Note that your average drugstore wants around $2.50 each for those small 9 volt batteries that 1.50 worth of AA cells would last probably 50 100 times longer, or whatever the math is and would have much larger normal and peak current capacity.
Once educated about it, one should laugh at anyone still using a 9vdc battery in anything other then a smoke alarm or garage door remote or other device that uses just microamps of current.
Lefty