TLDR: Rapid loss of power due to battery failure (?) in a Mavic Air, auto-landed in tree; got lucky, easy recovery, minimal damage, but still wondering what-the-devil really happened, and implications regarding same-age batteries. Airdata link to the ill-fated flight: https://app.airdata.com/share/orjCoJ.
While camping in September, I took a short morning flight, to get an aerial view of our camp site in the mountains of Colorado. I took off with a freshly charged battery. It was one of my first flights taken with the FT Aviator v1 software/hardware as the controller, with an iPad Mini 4 for display. (Thus I'm unsure of the value of the FTAviator flight TXT file; it looks encrypted, but then so do the TXT files from DJI Go4, which do get used for analysis, so ...)
It took a while to get a good GPS fix, so I took off with 96% battery. Seconds into the flight I began getting a "Critical power level! Landing now!" message (I forget the exact wording, but that was the message, in red.), though I'd inadvertently muted the iPad so I think I failed to notice at the first instant. When it began autolanding, I still had minimal control, and cleverly managed to get the bird centered over one of the nearby scattered trees before it settled down for good. Fortunately, it was a young fir with soft branches & needles, and the drone gently tumbled down through it to settle on a branch within easy reach. One prop was nicked--on the rear edge (?)--but otherwise there was no hint of damage. Subsequent flights with a replacement prop have all been normal.
I'd recently read here about how the collapse of a battery cell could result in this sort of behavior. So I put the battery from this flight away for the rest of the trip, and am just now getting around to asking if an expert can offer any insight. I uploaded the flight to Airdata; there's a link to that above. I've also attached the FT Aviator TXT file, in case that's of use to anyone.
But I don't know what to make of the Airdata info. The "Cell Voltage" graph in the Power section (shown) is alarming, but how/why does the battery "recover" near the end of the flight? Furthermore, the "Efficiency" map in the Power section shows initial problems but "full power" at the end, when the MA was most definitely autolanding on account of power issues. Airdata tells me I had 94% power at the end, but that drone was not going to fly a minute more.
Finally ... this is one of three batteries from the FlyMore package I bought in Feb. 2018. It has 73 flights on it, and 79 charge cycles (static use would be downloading data and similar). I don't see any indication of impending doom in its prior history (via Airdata), certainly not in the prior half-dozen flights I closely examined. But I wonder now if this sort of failure is on the horizon for the other 2 "sister" batteries from that purchase.
Oh, and FWIW, I'd recently run a "battery maintenance" procedure against this battery (and the other five in my kit): see PPS below for details. This was the first flight subsequent to that.
==> Any thoughts will be of interest, particularly regarding the apparent inconsistencies (?) in what I see on Airdata and/or the prognosis for my other batteries.
PS: I flew this battery once since the incident, just around the back yard. The behavior was "normal", as was the "Cell Voltage" graph and other parameters seen in Airdata. But I daresay I won't be flying this battery again for anything other than test purposes! (And I definitely got lucky with the 'bad' flight; my next destination after the campsite selfie would have put me over the wetlands at the head end of a lake!)
PPS: This is the battery maintenance procedure I ran just before the mishap, taken--for better or worse--from Airdata recommendations:
- Fly until battery level reaches 25%-30%
- Allow battery to cool completely to room temperature
- Put battery back in and turn on aircraft (optionally starting motors with no propellers) and allow battery to discharge until it gets down to 8%, or until the battery can no longer be turned on. Launch the DJI GO app to check battery levels.
- Allow the battery to cool completely again to room temperature
- Recharge battery normally