Crash apparently due to Battery failsafe not working

On my Pixracer using latest firware. I did 3 tests for RTL, 2 first where ok and the third wanted to check if failsafe RTL low battery was ok. Before I increased the default failsafe battery to 10% and warn to 20%. But the xcopter try to come back and crashed due to low power. The batttery 3S was only at 9.3V when I measured it. In QGC Power I set it to Batt min 10.2V. I assume it’s my fault but still do not understand why no RTL was activated. I’ve the log, but when I try to upload, not authorized format…

HOW TO upload my logs?

Upload the .ulog files from your autopilot SDcard here:

Can you provide a little more information about your set-up? Are you using some type of battery power module to provide voltage and current measurement to your Pixracer? I think if you upload your log it will show what parameters you used for the battery model.

LiPo batteries can sometimes drop off very fast depending on the temperature or condition of the battery.

@chris66 do you mind posting the link to the log here in the discussion thread so I (or others that might know more than I know) can take a look?

@Antiheavy, did you had a look at the log?

Hi @chris66

I have a few Points…

  • Apparently your Craft is rather heavy or the motors/props to weak as your Motors need 80% throttle to keep the altitude.
  • Pixracer ADC is 3.3V Based
  • The Voltage seems to have dropped rather low, as your Flightcontroller Power Supply struggled to keep the 5V Supply.
  • The Voltage Divider seems rather odd… did you calibrate by using a Voltmeter and QGC? Currently I is set for a PDB that measures a max of 33.4V while the Amperage Divider is set for a Probe that goes up to 120A.
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The system thinks your starting voltage is ~10.8 volts. So either you started with a nearly empty battery (unlikely?) or your voltage divider calibration is wrong. Based on @Andreas_Hoffmann comment it sounds more likely your voltage calibration is wrong. I also plotted current for reference (scaled down 10x, so peak current of 5 = 50 amps).

If I scale your voltage up by 1.15 then pack voltage shows closer to 12.4 volts before takeoff. You can see the battery voltage die off sharply below 10.5 volts which is typical for some LiPos, especially under heavy load. I’m not sure how much flight time you were expecting based on your battery pack size. It is possible your battery is undersized or maybe you just have a bad battery.

@Antiheavy, you’re right, voltage is low but right, it was the 4th small flight, I just wanted to test failsafe battery. I think this was not the right way. I was sure the FC couldn’t arm with low battery, but I was wrong… Thanks.