What we have done in my store is:
The myDevice designated for RFID is permanently stored in the STLs office.
We requisitioned an item called 'Outdoor Tech Kodiak Power Bank.' What this item does is, when connected to a myDevice, it extends the life of the myDevice so that it will maintain a full charge throughout the shift. We found that a fully charged myDevice registers scans more quickly (whether or not this is beneficial is TBD). Both the power bank and myDevice fit comfortably in a myDevice holster.
The catch?
You must remove the actual ipod from the red scanning cartridge so that is can be connected to the power bank. None of the bits our PMT were small enough, so, we visited the local hardware store to find the correct size. Apparently removing the myDevice is against some sort of Target law, but, WHATEVER!?!!?... We are getting our scans in unimpeded by the errors associated with a myDevice running on a low battery.
What I don't understand is why our STL thinks he has quarentine that one myDevice in his office for the whole week. We can plug any myDevice into the power bank and run on full charge!