I know a lot of people here run field radios using battery packs, so I thought the group might find this helpful, especially if running lipo batteries. If you need a low voltage shutoff for your radio, or your pi for that matter,? I just received a couple of these low voltage disconnect switches that I found on Amazon and they do the trick. Shutoff can be set anywhere from 7.0V-80V in tenths of a volt. For those that use batteries that can be charged while operating the radio, or a solar bank that might also charge back up while you are away, you can also set a diff voltage, so it has to charge back up to a certain level before switching back on, and won't just cycle on and off as it barely goes back above the shutoff level. It is about the size of half a pack of cigarettes:
edit: Actually, the shutoff level is too high to be useful for the pi, but great for most radio battery packs.
Bryan Waller