Lost Mavic 2 Pro due to Critical Battery / Forced Landing

[garypcook]

Hi,

I've sadly lost my Mavic 2 Pro to the deep waters of a Scottish loch (lake) as it unexpectedly went into a Critical Battery Warning and forced landing, see log files attached. It's obviously irretrievable and well out of warrantee but can anyone advise what caused this, and what if anything I can do to avoid it in the future once I purchase a replacement?

Background :-

I haven't flown for a while, but fully charged all the batteries about 3 days earlier. I had a flight the day before without any problems for about 5 minutes or so and the battery still had plenty of charge (around 65%) and as I was only planning another brief flight I didn't replace this with a fresh battery. The drone took off ok and flew across the loch for about 35 seconds when suddenly it appears to has lost battery power (dropping from 61% to 0% in the logs at 35.5 seconds) and I got a warning "Critical Battery - Landing". I couldn't see how to override this and so immediately turned the drone around and started to fly back to home and also tried to increase altitude rather than drop. No mater what I did the drone continued to reduce altitude and try to land. After a few seconds I realised that I wasn't going to make it back and turned around again to head to a forested headland but didn't make it and watched hopelessly as it dropped into the water and sank. It still had power as the props were still turning.

In hindsight if I hadn't panicked I could have taken my chances earlier and tried to land amongst the trees on the headland but by the time I realised that this was the best option it was too late.

I intend to purchase a replacement (used) mavic 2 pro (can't afford to upgrade to the mavic 3) as I still have the smart controller and 2 other batteries etc. But (other than not panic and look for the best landing spot) is there anything I could / should have done to avoid this? What caused the critical battery power loss? Is there anyway to test the batteries and determine whether this is like to happen again? Is there anyway to override the forced landing if it does?

Many thanks,
Gary

 

[Kilrah]

One of the cells reached critically low voltage during the climb, the others not being that far either.

I had a flight the day before without any problems for about 5 minutes or so and the battery still had plenty of charge (around 65%)

5 minutes shouldn't use more than about 20-25% so that was probably a first sign something was wrong, but this battery simply seems to be on its way out and not able to sustain full load anymore. Having a look at the voltages in the app could have helped notice an issue on that last flight.

 

[Meta4]

I haven't flown for a while, but fully charged all the batteries about 3 days earlier. I had a flight the day before without any problems for about 5 minutes or so and the battery still had plenty of charge (around 65%) and as I was only planning another brief flight I didn't replace this with a fresh battery.

When you launched your battery was showing 64%.
It had been discharging for some time to have reached that level.
The % indication is only accurate when you start with a full battery.

At 8 seconds under the load of launching, the individual cell voltage levels were already down to 3.2 volts (Critical LOw Voltage level).
As you flew off at full left and right stick, the cell voltages dropped to 2.7 volts.

At 26 seconds you were getting Not Enough Force/ESC Error warnings.
AT 35 seconds the warnings changed to: Critically Low Voltage Warning__Aircraft will be forced to land. Not Enough Force/ESC Error
and
Propulsion output has been limited to ensure battery health.

Launching with a battery that's been sitting around for a while discharging is never a good idea.

 

[Kilrah]

It had been discharging for some time to have reached that level.

Launching with a battery that's been sitting around for a while discharging is never a good idea.

He said the 64% was a short flight the day before, not self-discharge.

 

[slup]

...anything I can do to avoid it in the future once I purchase a replacement?
...Is there anyway to test the batteries and determine whether this is like to happen again?

You will never be able to avoid that a battery fails ... all batteries will do that eventually, they don't last forever. The tricky part is to know when it's time to take a battery out of flight duties. The age of the battery (your battery was manufactured in March 2019) , the amount of charge cycles, how they have been treated & stored in between use... all plays in.

If you use your batteries between approx 15-100% on a regular basis, store them in room temp between 40-60%, don't have them laying around longer than like 2 days full or empty, never let them overheat ... laying around in the sun or in a hot car, don't fly them in extreme cold with a lot of amp draw ... you've set a good foundation for a long battery service life.

If you also add in a regular monitoring of the total capacity, how cell deviations behave & how the flight times holds up ... all over time through out all flights with the battery you will be able to see when the battery will start to misbehave & can take it of flight duties before it fails.

...suddenly it appears to has lost battery power (dropping from 61% to 0%
...What caused the critical battery power loss?

That the percentage suddenly drops to 0% means that the BMS chip doesn't have a clue about how much is in the "tank" anymore ... this as the battery doesn't behave as expected ... all this due to that one cell in the battery have failed.

...Is there anyway to override the forced landing if it does?

No ... if it's nothing left in the battery it will soon start to free fall, DJI prevent this by commanding a forced auto landing.

 

[Meta4]

He said the 64% was a short flight the day before, not self-discharge.

The voltages show that the 64% was a false indication.
It was really <10%

 

[Kilrah]

The voltages show that the 64% was a false indication.
It was really <10%

Voltage wouldn't have gone back up that much once the load lightened if it was, the 64% estimate was likely correct but the battery has degraded to a point where internal resistance is way too high.

 

[garypcook]

Looking back at the previous flight that was actually 7min 15secs rather than my guess of 5mins, the battery level started at 95% and gradually dropped to 66% at the end of that flight, can upload the logs from that flight as well if it helps, but doubt it will.

 

[garypcook]

Is there anything I can do to test batteries before a flight?

 

[slup]

Is there anything I can do to test batteries before a flight?

In general ... no.

Even though you start up your flight with a freshly charged battery ... you never know for sure that the cell voltages will keep up when you start to draw high current from the battery. With battery age, a lot of charge cycles & battery abuse in general the risk for cell failures increases.

Re-read post #5 ... you there have the procedure regarding use & storage that will put less strain on the battery increasing the possibility for a long service life. Additional to that is the performance trends from the battery ... this will indicate the battery degradation making it possible for you to take it off flight duty before the battery fails mid air.

 

[garypcook]

In general ... no.

Even though you start up your flight with a freshly charged battery ... you never know for sure that the cell voltages will keep up when you start to draw high current from the battery. With battery age, a lot of charge cycles & battery abuse in general the risk for cell failures increases.

Re-read post #5 ... you there have the procedure regarding use & storage that will put less strain on the battery increasing the possibility for a long service life. Additional to that is the performance trends from the battery ... this will indicate the battery degradation making it possible for you to take it off flight duty before the battery fails mid air.

Okay thanks, when you say "If you also add in a regular monitoring of the total capacity, how cell deviations behave & how the flight times holds up ... all over time through out all flights with the battery you will be able to see when the battery will start to misbehave & can take it of flight duties before it fails." do you do that through downloading the flight logs or is there anything on the smart controller that can help with this monitoring?

What battery / cell voltages & deviations should I be looking for as a warning level?

 

[slup]

...when you say "If you also add in a regular monitoring of the total capacity, how cell deviations behave & how the flight times holds up ... all over time through out all flights with the battery you will be able to see when the battery will start to misbehave & can take it of flight duties before it fails." do you do that through downloading the flight logs or is there anything on the smart controller that can help with this monitoring?

The parameters that's easily available through the flight logs is:

-Remaining battery capacity which you compare with the stated capacity of the battery
-Cell voltage deviations
-Flight times (theoretical ... like if you should have used every single percent of the battery)

It's only a matter of compiling these KPI's through out all flights you have with that battery ... so you can see the degradation trend & by that draw conclusions about the status of the battery.

Just looking at one single flight doesn't say much ... perhaps it was extra cold & you used Sport mode a lot & by this the cell deviations was a bit larger than usual & the flight time shorter. You need to widen the picture over several flights to notice any degradation.

You can compile these trends either by looking into every log manually & get used to how it should behave & take notice if it were any special normal circumstances that made any of the KPI's be bad just this flight... or compile everything manually in for instance Excel. Or you can spend a bit of $ & get a paid Airdata subscription (as an insurance) & let them compile all battery data for you & show it in charts with defined critical levels for the different data.

If you don't monitor this data ... & just fly, it's just a matter of time until you get caught airborne with another failing battery ... & this even though you care for the battery by the book, as said earlier, batteries doesn't last forever & the only visible sign of a sick battery is swelling & that doesn't occur for all batteries.

 

[Kbviper]

Unfortunately this has happened a number of times. Mavic 2 models and associated models (Mavic Enterprise), have a history of this type of battery failure and forced landing. I had it happen to me twice and it happened to a work colleague once. In all three cases, the weather was cold (below 32 degrees F) , the batteries were warm, and near fully charged, and the drone was flying into a strong wind while going forward and up into the wind. I watch my batteries condition in Airdata and there were no previous issues with the ones that had failed, including any swelling.

Both times I was over water - in the first case I managed to get into a sandy beach and augered in, in the second, the water was iced over, and I hit the ice hard not far from shore and damaged the two front legs. When it happened to the colleague, he was over land and not far away, and did a straight down landing.

See this thread of messages and my post on page 2 for further details of the beach landing incident: Has anybody ever had this happen to them?

Based on my experience, I am guessing that there are some M2 batteries that go into the "fail mode" when certain conditions are met including cold weather and a high load on them.

 

[eEridani]

AirData monitors the batteries in an easy to use format. Once you upload a log, you can explore the battery state, power flow during flight, voltage trends over time, etc. Well worth the subscription.

 

[garypcook]

I've subscribed to Airdata and checked my logs but struggling to see any trend from the previous flights which would have warned me of the impending doom for that battery. Battery life is showing as 98%, Health looks okay, Minutes per battery is declining slowly with one flight that looks abnormally low (warnings of high wind velocity on this flight), a fair few Minor Deviations but very few Major Deviations until the penultimate flight the day before - see charts below. Is there anything in this that I've missed?

I see a basic service is recommended every 20 charges or flights which seems to be just a controlled deep cycle discharge - which I will now do on my other batteries.

 

[garypcook]

Checking the logs on my other two batteries, these (unlike the failed one) show an increasing number of Minor Deviations and some Major Deviations, but both still show 97% - 98% Battery life, good health etc. Should I be worried about these?

 

[Faster]

This is what Air Data recommends. If you follow what they say I would stop flying with the last one and would be checking closely the first one.