I recently upgraded to 3.1rc4 of the firmware and noticed that the battery voltage and current was always zero in the MAVLink status messages. I went back to 9/18/2013 and confirmed that the batter voltage and current was correct. I checked the last rc2 revision on 10/10/2013 and the battery voltage and current were zero. So somewhere between 9/18 and 10/10, it appears that a bug was introduced regarding the battery voltage and current readings.
I grabbed the latest 3.1rc4 firmware source and uploaded the newly compiled firmware. It appears that my battery monitor configuration was not setup and the voltage was not showing up. Once I configured the battery monitor correctly, the correct voltage showed up. I still only get 0 amps of current until I turn on the motors, but maybe that is because the APM draws so little current. I wonder why the rc2 version worked?
There was a change to the battery monitoring - we moved it into a library so that the code could be shared between copter, rover and plane. That change meant you’ll need to upgrade to the latest mission planner.