People have missed a key point.
The back powers up using any of the three batteries. All of the menu functions work. However, the battery icon in the lower center of the screen on the starting menu always blinks and contains no bars. Whenever the back is fired for a capture, the back displays battery level low: Capture cancelled" and no capture is saved.
If the back is turned on when the back is not mounted on the camera, the battery icon still is empty and flashes.
The camera is a Hasselblad 503. The only electrical contact between the back and camera is via the sync cord. Three different sync cords have been tried. Two of the sync cords are known to work with a Phase One H20.
The back functions correctly when used in tethered mode.
If the backlight is set to always on, and the camera is left on for an hour, doing nothing, the battery will show drainage. I drained one of the batteries from 100% to 30% by doing this. I then recharged the battery to 100% and put it into the back. There was no change in behavior.