Mark,
That is reason I asked. I am almost certain a LiFe 1800 pack will outlast a 2500 NiMh pack by maybe even two times.
These packs are rated in mah but it is tested differently than NiMh. You would think it is a load times time it draws that load and mah ratings are the same. They aren't and you cannot compare mah of packs of different chemistries.
If a 2500 nimh ever had enough then a 1800 cube would surely. Not sure if that size fits for you.
I have 1300 packs you could parallel to get a pack less than 3" long and have 2600 mah of LiFe. If 1800 cube will fit I would give that a try first. You may be surprised.
Everyone that tries these first time can't believe the difference.