Pixhawk Power Module Reads Incorrect Voltage

Hardware Pixhawk 2
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1

Hi,

I purchased PIxhawk 2.1 and it came with its own Power Module (power brick). I assembled and configured a quadcopter. When I connected the battery that is fully charged everything works fine. However, if the battery is not fully charged (voltage slightly off) I get the Low Battery beeping.

It narrowed down the problem to the Power Module. Apparently, it reads voltage about 2 V below of the actual battery voltage. I even disconnected all components from Pixhawk and just left connected a buzzer, a safety switch and the battery through PM, but the problem with the low battery is not going away.

I’m using QGroundControl (on Mac). When I connect a USB cable and the battery, the PM reads 10.55V in QGC, for the fully charged 3S battery, which actually has 12.53V as tested with the voltmeter. At this voltage, I get a Low Battery warning from QGC speaking, but no buzzer.

When the actual battery voltage drops a bit to 12.45V, I get a buzzer beeping and QGC shows only 13% of the battery at 10.46V.

So, Pixhawk’s Power Module seems to lose about 2 V from the actual battery’s voltage. Keep in mind that there is really no load on the system and ESCs are not even connected - everything is at rest. Did I get a faulty PM with my purchase or did I do something wrong with PM’s calibration?

I searched google for straight 2 days but can’t find the answer. I would understand if PM loses some 0.2/0.3 V, but not the full 2V. Something seems oddly wrong here.

I’m using 3S 5000mAh 52C battery.
Pixhawk is fully configured with QGC and has the following parameters:
Number of cells: 3S
Full Voltage: 4.05V
Empty Voltage: 3.40V
Voltage divider: 10.17793941
Amps per volt: 15.39103031

Here are snapshots from QGC:

Battery Configuration.png580Ă—553 74.6 KB
Almost Full Battery.png1440Ă—630 152 KB
Full Battery.png1441Ă—654 153 KB

Please help!

2

The “Voltage Divider” parameter (shown in your screenshot) may allow you to calibrate your Power Module’s voltage measurement. Can you try adjusting this until the onscreen voltage reads correctly, then see if it tracks well from there for other levels of charge?

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3

Hunt0r,

Thanks for pointing me in the right direction to look into Voltage Divider value. After I carefully read the instructions I realized that in order to perform the battery calibration the actual battery must be plugged in. My rookie mistake! Once I performed the calibration with plugged in battery the number jumped to 12.23234653, which made QGC to correctly displayed battery at 100%.

Thanks again for your time.

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