Some early models of the Sound Devices MP2 and possibly MixPre (and the Shure FP-24 re-branded Mixpre) don't work with rechargeable AA batteries without a slight mod. Pop in a couple rechargeable AAs - if they work, you're all set. If they don't, you'll need to tweak your preamp. Sound Devices has historically fixed this problem swiftly and cheaply, but if you're inclined to do it yourself, thanks to Marc (leegeddy) for this one:
you could do this... take an exacto and cut away the plastic cover to expose more of the metal on the (+) nub on the AA. that's all it is.