Consumer grade AA have a poor output, though they may have plenty of capacity. Think, large bucket, small hole. You need a larger 'hole' (output).
FWIW the higher the capacity in NiMh (comparing similar size) the WORSE the output. 2500mA AA are mostly lousy batteries for about anything. You want good AA get 2000mA Eneloops (still not good for power applications though).
You are better off with something down around 1000-1500 sub-C or there about by guesstimate.
You can get suitable NiMh batteries for about any power application here:
Mostly though I agree with the suggestion to go lipo. Get a decent balancing charger and a large clay pot + some kind of lid, or dead pressure cooker from a thrift store to charge in-something like that.