Q regarding PM and voltage displayed under load

I’ve been working with the Pixhawk now for about 6 months of off an on flying of a hex based on the Tarot FY680 Pro frame (extended to 815mm or so).

Last night I was calibrating the PM on mission planner for volts and current. At resting voltage I got a good reading on a 4S Zippy 8000 maH battery (about 16.7V) and calibrated using the mulitplier. The current draw calibration seemed pretty close, so I left it there.

While doing the current draw test to calibrate, I noticed that the voltage reported to MP on a load was not accurate based on the tester I used to check the load. The MP would report numbers in the 14.9V range on a full throttle load, however, the tester reported about 15.9V during this same period.

Initially, I had a failsafe for low battery for this very reason, but also for this same reason I turned that failsafe off and started using one of the little LiPo battery alarms as every time I would fly I’d receive a battery failsafe fairly quickly. Sometimes when the battery reports about 80 percent, a full throttle load reduces the reading to 14.0V when the meter reports closer to 15.0V.

Could this be a defective module? I’ve purchased a smart port voltage meter for my Taranis setup since it reads voltage from the balance ports (like the little LiPo alarms) and I can have the telemetry sent back to the radio to monitor voltage this way.

Thank you.

Did you calibrate the current and voltage measurements of your PM module by changing the offsets in mission planner ? (youtube.com/watch?v=tEA0Or-1n18)

Yes, I calibrated those. And the current measurements were almost spot-on as they were. But as I said, under load, the tester reported much higher than what Mission Planner was reporting. Baffling.

I have a similar problem: I calibrated the Voltage as indicated without load (it was off about 1V!), then I calibrated the current while powering my quadcopter around 5Amps (I used my bench PSU which puts out max 5A). However, when I pull the throttle down, the voltage readout is completely off, reading 13.7V while it’s 12.6V (I didn’t touch the voltage settings, just edited the read Amps). Now I may understand a faulty shunt so the multipliers are off on the analog Voltage x Amp conversion, but the normal output voltage should be precise down to the 100th of a volt and stay unaffeccted by the Amp calibration but here it’s useless. My PM is a 90A capable, 5.3V 3Amps with 6pin connector which plugs directly to the PM port on my APM 2.6. Yes it can be defective, but why would the voltage readout be so off? Looks more like a problem on the ADC port of the APM. Does anyone have any heads up?