Voltage drop triggering batt_fs

New to the scene. Built the quadcopter to fulfill school project requirements and possibly compete in a competition for autonomous UAVs between schools.

Cube Orange
4x 920kv motors
9450 dji props
30a ESC’s
CC 10a BEC (plugged into AUX1, intended to power rpi later)
3s 3200mah 30C lipo

Battery failsafe set to 10.5, full voltage is around 12.6. Battery monitor has been setup in different configurations, per the Ardupilot documentation, with no luck.

No issues arming the quad. Assuming I’m reading the logs correctly, as soon as I lift off, I get a little less than 1V drop in about 5 seconds, and works down from that new value, The drop seems like more than it should be from my research. So even if I have 11.5V, I can’t fly the quad which doesn’t seem right. Current at its highest, during multiple tests, is around 40amps.

Tried multiple batteries and issue persists.

Faulty power module? Does the power module need to be vibration resistant? As of now, its mounted to one of the arms and secured with zip ties. Been trying to diagnose this issue all day.

Prefer not to turn off the failsafe if possible.

Any help is appreciated.

It is normal for voltage drop as current consumption goes up. When your copter lift up the motors draws more power thus making the current rise.

You are discharging your batteries at up to 12.5C (40 A / 3200mAh). That is a lot.

Check this plot (source: lowest safe voltage per cell on lipo - Discussions - diydrones):

So, the big voltage drop is not unexpected at all. You say it drops to about 11.5 V and the failsafe is at 10.5 V. So why is the quad unflyable? It seems fine.

Do the manual calibration of the voltage sensor in MissionPlanner. Use a low reference voltage like 9.0 or 10.0 volts, and get the sensor reading accurate and reliable at that voltage. It’s not as critical to read the voltage accurately at high voltages.
You are correct in saying you shouldn’t change any of these:

If you have to lower those voltage params then you are not fixing the problem (battery voltage sagging) you are actually making it worse.

If you are confident of the battery capacity and quality, you can increase BATT_LOW_TIMER to 8 or 10

Alex, the highest draw I saw from the logs was at 40A, average is around 15-20A, assuming I’m reading them correctly. I was saying if I have 11.5V and I try to fly, the voltage will drop to 10.5 triggering the failsafe.

xfacta, I did the calibration and I believe my current readings are correct but will do another to make sure. Should I look into other power modules? From looking around, it seems the Mauch Electronic ones are the best.

Ended up killing a cell in the 3s trying to configure a RPi with the cube smh. Lesson learned. So picked up a couple 4s 4000mah.

Oh I see what you are saying, unloaded 11.5. But still, the voltage drop is to be expected and though you may be able to go a little lower on the voltage failsafe, based on the loaded discharge curve, I would not take my own word for that because I’m not an expert on battery safety/health.