i read:

"In fact alkaline batteries only supply 1.5V during the early stages of
their discharge and this drops to below 1.2V for most of their life.
NiMH batteries actually sustain a higher voltage than alkalines during
the bulk of their discharge."


So im surprised if its voltage. Maybe something to do with different
capacitnce / impedence of the batteries. My electronics is a bit rusty.

David.