Monitoring a second battery with APM 2.5

ArduPlane Plane 3.4
1

Hi,

Could someone help to clarify how to monitor a second battery with APM 2.5 and 3.4.0 firmware.
I have tried to read the voltage with Frsky and Attopilot sensors but can’t get any reasonable readings to Mission Planner.

2

I’m going to put some information here.
viewtopic.php?f=113&t=14382
Thanks, Grant.