As a test I recently bought a pair of Li-ion AA batteries, that are recharged through a micro USB connector in the battery. They are rated 2880 mWh and are advertised to provide a flat 1.5v during the whole discharge cycle (they have internal circuitry to convert from the native 3.7v).

I wonder how those compare to my current rotating stock of Ni-MH ones that regularly stop working. Jury is still out, they were considerably more expensive though (something like €4.50 a piece).