Forgive my ignorance on this.
Our portable mixer/recorder takes a floating 5-18v from an external power pack. I am looking at using NiMh RC packs, but here's the bit I don't quite get. The shop sells various capacities and various voltages, but the best fit in size and weight is a 9.6v @ 2500mAh (that's 8 x 2500mAh AA's)
The mixer takes 4 x 2700mAh AA NiMh internally and I get 4 hours of up time. So that's 4 hours from a 4.8v @ 2700mAh source.
The way I see it is that at 9.6v the mixer will demand less current than at 4.8v, so the 9.6 pack should last longer.....