I also picked up the AA battery pack so I can run it off AA batteries if the power is out. And although the manual doesn't say so, it does work with NIMH rechargeables too.
AAs are almost always available no matter what the disaster, so that's a good move. NiMHs are good for rechargeable if there's still power. The problem is that the radio won't transmit at 5W; it drops to 0.5W or less.
I bought some 7.2V 1.2Amp Hour batteries from a toy store that sells R/C model airplanes and made a connection with a Radio Shack plug that lets me plug the battery into my DC-in socket on the radio. The radio still works at 5W, and the external battery charges the FNB-80LI if you leave it in.