I and a few others have noticed that the battery voltage displayed in Mission Planner is incorrect.
I am seeing about 0.4 - 0.6 volt difference from that measured at the battery input on the 3DR Power module.
The ability to calibrate this has been removed from the battery monitor setup page. The calibration used to consist of measuring the VCC on the board (at the fuse) and plugging that value into the first field on the battery monitor setup page. Refer to this wiki entry: http://copter.ardupilot.com/wiki/common-measuring-battery-voltage-and-current-consumption-with-apm/
I have found that I can roughly calibrate this by changing the VOLT_DIVIDER parameter in my case changing it from 10 to 10.3 brings it close. The value displayed in the HUD is very noisy and could do with a bit of filtering.
I did log a case in the Mission Planner Github. But this case was closed and I was told that the 3DR module was automatically calibrated.
Also as side issue, how do you read the VCC current value in MP when using the 3DR PM. hwvoltage used to show VCC but this is now mostly 0 with intermittent excursions to 30 - 40???.