Attopilot 180A wrong voltage via telemetry

This is a weird one.

I’m using a APM2.5 board with a 3S battery, 3DR 915Mhz telemetry radio and an Attopilot 180A to measure voltage and current.

When I am connected to the APM via USB, then Mission Planner reads about the correct voltage, about 12.4v for a fairly full battery charge, this checks out against a multimeter. But when I switch over to the 3DR radio, the voltage reading drops to about 8.5 - 8.6v, which is not what the multimeter reads.

Does anyone have any idea whats going on? Could it have anything to do with the fact that Attopilot output is scaled to 3.3v? The ratio between the voltage via USB & via 3DR radio is almost 3.3/5v, but not an exact fit.

Can you verify that your grounding is correct?

Specifically, that the ground of the APM is galvanically connected to the power module, in both cases.
Both ground and Vcc wires from the power module should be connected to the APM

I finally got the chance to test my wiring. The grounding checks out ok.

But I am getting some weird readings from the Attopilot. When the USB is connected, I read 5.53v between the Gnd & V sensor terminals of the Attopilot (MP reports 12.9v). When the USB is disconnected I read 5.24v between the same terminals (MP reports 8.3v).

I suspect this it might have something to do with my power setup. I have a tricopter with:

  1. 3x 20A Afro ESCs putting out a nominal 5v
  2. 1 of the ESCs is powering the APM 2.5 board
  3. JP1 is connected
  4. Another ESC powers the tail servo

When USB is disconnected (MP reporting 8.3v) the voltage between Gnd & +ve on:

  1. PWM output connectors is 4.94v
  2. PWM input connectors is 4.6v
  3. A1 pins is 4.6v

The PWM input connector voltage is out of spec. I think.

With the USB connected (MP reporting 12.9v) I read for the same terminals:

  1. 4.99v
  2. 4.88v
  3. 4.88v

Does anyone have any idea what is going on?
Does the A1 pin of the APM read the Attopilot sensor voltage relative to gnd or to the +ve rail?
Should I be running with JP1 connected or not?

Muchly confused.