The process to re-calibrate the full battery gauge:

1) Charge the battery for 8 hrs in the charger.

2) With NO battery, lens or viewfinder installed on the camera, press the orange on/off button for a few seconds to make sure the camera is fully discharged.

3) Press the flash and menu buttons at the same time on the camera grip, and hold them as you install the freshly charged battery.

You should then here a rapid series of beeps(three short beeps within a second or so on H3DII/H4D) indicating the PMU on the battery has been reset. If not, repeat steps 2 and 3.

With the battery installed, press the button just above the on/off button and hold it to see the battery status. It should read 100%

http://www.luminous-landscape.com/forum/index.php?topic=38924

http://www.hasselbladdigitalforum.com/index.php?topic=101.0

2013/04/05 Update:
Here is an instruction from Hasselblad UK with some pictures: http://www.hasselbladbulletin.com/uk/sep-2011/tips-tricks/h-system-battery-handling.aspx