For a battery PSU fro something like this I would not recommend using the High-capacity NiMH batteries.
They have a notoriously short shelf life and die very quickly after a certain discharge threshold. It would mean your mixer would suddenly drop out in the middle of a recording.
A better option is to use an SLA battery (a slim 3AH 12V battery) which has a much more forgiving discharge curve and a much longer shelf life when charged. The reason NiMH are used in some equipment is that they can supply very high peak currents, which is often required for camera shutters, flashes or motors.
Another idea is to use Lithium batteries, such as those used for video cameras and mobile phones. These are normally in packs with a terminal voltage of 3.6 or 7.2V and have really good capacity to weight ratio. I have used these on portable mic preamps (used for field recording) with success.
A couple of 7.2V batteries are usually all that is required. If your signal chain is designed to work from 10V (or +/-5V) min then you get about +12dB output before clipping when the batteries are low (otherwise about +15dB with charged batteries). So if you are recording very dynamic material you will probably need a limiter or compressor, or reduce your average recording level to accommodate the peaks. VU meters are a waste of time on this kind of recording device, you should use a few LED's configured as a peak meter to show you how close you are to clipping the preamp. Usually don't need more than 4 LEDs to show signal and peaks, since they chew up battery power as well.
