We have had success with using portable power battery packs designed for tablets with the AudioBox iOne and AudioBox iTwo and support iOS devices.
The specific requirements you need to look for in when selecting a battery pack is that it specifically states that it will work for an Apple iPad.
The minimum power requirement is 2600mA (also read as 2.6A, 2.6 Amps, 2600 milliamps, 2600 millamps per hour or the most common method of display is 2600 mAh)
If the mAh (milliamp per hour) is less than 2600, it will not work reliably. You can however find battery packs with ratings up to 4000 mAh and above.
Performance will vary based on battery selection and method of use. With phantom power enabled with an M7 running with an iTwo we were able to run for more than an hour on a full charge on the battery we tested with a 2600mAh.
We found our battery for an affordable price at a local electronics retail store. The 2600 mAh battery started to falter after that before requiring a recharge.
Note: Not all phantom powered microphones pull the full 48V DC power from the AudioBox iOne or AudioBox iTwo. Your battery performance will differ based on the microphone(s) you are using.