Crash due to battery exhaustion

Flight Log:

I’m fairly new to Ardupilot so perhaps you can help me understand what went wrong in this flight.
What happened:

  • I was running a test flight to test some new parameter settings, and also to see how the battery failsafe and RTL works.
  • I let the battery voltage drop too low while the quad was still in the air and it tumbled out of the sky quite dramatically.
  • The BATT_LOW_VOLT is set to 13.5 and BATT_LOW_TIMER is 10. So by my understanding, it should have activated the battery failsafe when the voltage is below 13.5 for 10 seconds (at around 10:50 flight time)
  • I manually activated RTL around 11:00. I was expecting it to descend to the RTL_ALT of 15 m, but i guess it maintains it’s current altitude.
  • At 11:37, the battery failsafe finally activates and goes to SMART_RTL and causes the quad to climb. This is the most confusing part.
  • I manually took control into ALT_HOLD mode and commenced the descent, but it was ultimately too late.

Lessons Learned:

  • Set the battery failsafe voltage higher
  • Don’t fly so high with low battery

I guess my biggest question is why did SMART_RTL cause a climb? It was well above the RTL_ALT.

Hi @jackhenderson101,

SmartRTL retraces the path that the vehicle flew but shortens it slightly by cutting out loops. RTL_ALT is not used in SmartRTL. I have not looked closely at the logs but my guess is that it climbed because just before SmartRTL was engaged it was descending.

13.5 might be the lower limit for your battery, it really takes a nose-dive just after then.
12 volts is definitely too low for your critical voltage level - by the time the battery gets down to 12 volts it’s already too late.

Looks like Randy is correct, it was descending when it changed to SmartRTL, so it tried to ascend and go back the way it came.

What battery is it? Lipo, Liion? 4 cell?
Use the spreadsheet attached below to set the correct values, or press Alt A while connected to MissionPlanner and follow the prompts.
Also check and calibrate your voltage sensor if you havent already. Test and set it for the lowest voltage you expect to see, not the highest voltage.

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Thanks xfacta for the info. It was a 4S LiPo battery.
Will adjust the parameters as per the spreadsheet.
I will also switch to just RTL mode as that seems to be the safer option.
I’ll have a go at calibrating the voltage sensor to see if there was any major discrepancy