I was out testing some new Lithium Ion batteries today in my quadplane, it is running an original pixhawk, arduplane 3.8.0 with emlid reach RTK GPS (which to note I did not have the Base station injecting GPS to it). I was hovering around fine, QLOITER was a bit squirrely I’d attribute that to me forgetting the base station and range pole. Main reason was to test and see how long the Lithium Ion batteries would hold a hover as they drop voltage quite quickly when in Quad mode, but recover once in pusher mode very well. My Voltage was hovering around 19.5 volts still well within the range of these Ion bats had no issues still plenty of power then all of a sudden it takes a loop over its self and lands on its top. I have looked over the log but will admit that I am not the greatest log analyzer! Here is a link to the DF file.
DF File
Here is a short video of what happened, sorry for the poor quality had to have it sent over cellphone.
Flip Video
Here also is a parameter file from the flight.2017-10-03 18-48-53.log.param (16.2 KB)
Looks like your front left ESC might of hit the low voltage cutout. Otherwise the ESC might of overheated and thermaled out. Note Lithium Ion have a lower voltage than LiPo, and most ESC’s are setup for LiPo. Disable the ESC low voltage cutout (if it has one) and try some hover tests in ground proximity.
When doing propulsion performance tests its always a good idea to monitor thermal activity of various components, like motor, ESC and Batteries in intervals before committing to a full flight. That s one of the most common indicators for something not working as it should. There’s also an issue with Atmel based ESC’s that drop PWM frames when used with the Pixhawk, that results in the exact same behaviour. What ESC’s, motor’s and batteries are you using?
I think it’s something more than just a low voltage cutout. The BAT.Volt logged voltage suddenly drops from 18.8 volts to 10 volts then recovers to 20 volts. That looks like a bad solder joint or bad connector, or possibly a short (current went up suddenly, which could mean a short circuit)
Could well be. The current mostly coincides with the RC5out and voltage dip however. Lithium ion are not good at all for peak current and sag dramatically. Poor wiring doesn’t help. I’m not sure if it was the cause or the consequence though.
dropping to 1.8V/cell is more than just sag I think. That voltage level should come with links to websites to buy a new aircraft 
Lol I think they do. The link only displays on the battery after impact… 
After digging deeper I did see that very big voltage drop (so many parameters to choose from when looking at logs where does one start?), I have flown this many times in auto and using RC with 6s multistar 5200mah lipos x2 without an issue, I will look over all my wiring tomorrow but my suspicion is that it did hit its low voltage failsafe on the one esc. I am running Tiger motor 4010-9 580kv with 15" tiger motor CF props, using a tiger motor 40amp air esc, the battery I was trying out was one of those titan 6S 10500mah batteries. The Tiger Air 40amp esc does not have the ability to program anything unfortunately (can someone recommend a good 40 amp esc that has the ability to do so), or are these ion bats all hype unless using them for fixed wing, which is the reason why I have them, just had light bulbs go off when I looked at mah and weight compared to lipo, although the very low C rating scared me but the quad motors are used so sparingly. The nice thing was that nothing broke in this little test other than a motor mount getting twisted from where it impacted. My overall weight on this aircraft with 2 6s multistar 5200mah lipo, random sundries and sony a6000 is about 4800kg. I believe I have read somewhere about using two different batteries one for the quad and one for the fixed wing mainly a lipo for quad and a ion for pusher but slim down the quad battery because it is only used for a short amount of time (except when things go wrong hahaha) but that sounded like a not good idea, I may be wrong to many hours pouring over this forum!! I seem to not be breaking the 40 min of flight time with my quadplane, although I have zero background in this except I bought a fpv racer with a seriously pro f3 and made that fly have built a firefly (ava was a horrible software), and have no programming experience although i am taking a programming class at the college here hopefully that helps. Appreciate you two taking a look at that for me!! Will be putting in the Lipo tomorrow for a similar test just to verify it was the Ion batteries!
normally when that happens the initial voltage change is a rise in voltage, as one motor cuts.
The sudden drop by such a large amount looks to me more like a hw failure of some sort, not just a low volt cutoff.
According to the manufacture spec of those batteries I wouldn’t recommend them for hover use on your quadplane either way:
Energy
- Voltage Range: 15.0v - 25.2v
- Capacity: 10.5Ah / 233.1Wh
Discharge
- Burst: 105A / 10C / 1800W (lasts a few seconds)
- Full Throttle: 60A / 6C / 1170W (60 seconds full throttle, will get hot unless cooled)
- Constant: 22A / 2C / 360W (32 minutes, maximum constant discharge rate)
According to the log it’s using over 25A for hover with peaks to 35A. (excluding the failure where it’s 45A)
You can use them for a fixed wing, or add a extra battery for the quad motors on the quadplane.
I’d recommend a extended hover test using the Multistar batteries which are a bit better, and measure the temperature of your motors, ESC’s and connections . Typically I do a 10, 30, 60 and 120 second hover tests with temperature and physical tests in between. Ideally using a IR temperature sensor of the motor core works best.
Regarding where to start with looking at logs: This depends on the failure experienced. There’s a bunch of presets you can use in the pulldown menu on MP. In this case it looks like a power related issue so in MP under BAT you can select Volt and current, to see what the PXH told the ESC’s to do select the appropriate channels from 5-8 in RCOU.