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Please help debug drone failure logs

Dear Forum,

My drone suddenly goes down and I need your expert opinion to debug the reason using logs

I downloaded the logs and this lines exists in the log:

Test: Event/Failsafe = FAIL - ERR found: FS_BATT
Test: NaNs = FAIL - Found NaN in CTUN.DSAlt
Test: OpticalFlow = FAIL - FAIL: no optical flow data
Test: Parameters = FAIL - ‘THR_MIN’ not found

So most probably this is batter issue, as in other topic someone explained me that the reason is 2S battery. I should use 3s. Please see also my flight logs in that thread.

However I want to debug this using Mission Planner flight logs. And I am thinking to debug by this steps

1)Find the point when drone has crashed i.e. when the altitude dropped to 0.
2) Check the voltage of battery.

Can someone please teach me how to see drone altitude?
And how to see that batter voltage is not enough for drone fly.

Can someone please explain me this graph:

BAT Volt is Battery Voltage BAT VoltR is a calculation for “Resting voltage”. It attempts to caluclate what the battery would be at without sag from supplying current. You can ignore it. The fact is your battery was low.

thank you for the answer!

battery would be at without sag from supplying current
excuse me, what is sag here?

It shows FS-Batt: Detected error, do you know why is that error?
As i am understanding when battery voltage is low the fail-safe should be activate and drone must return to launching point.

And last question~
How I can see drone altitude values?

Batteries drop in voltage when they are supplying current. When they stop supplying current they recover. As I said VoltR is a calculation of this recovery voltage.

It depends on how you have the battery failsafe set. The voltage and the action taken. In the log Baro will give you altitude.

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