I am no battery expert either. But one thing I have learned is when a battery is rated at a certain voltage. Say 15.2 volts. That is when it is discharged. Empty.I'm no expert but reading these forums, especiall recently about battery issues/voltages etc - here is my 2cents
Take off battery shows 97% - after 31secs of flight the voltages read:
3.637v 3.66v 3.638v 3.561v
1 cell shows quite a bit of deviation after 31secs - I'm thinking it's a battery/ voltages issue (ie taking off on a battery that isn't fully charged)
The above voltages look way off after 31secs of flight from a '97%' charged battery - (edit to add) I'm usually nearly landed when those voltages show & the battery is showing anything between 20 - 35% depending if I've been hard on the sticks
Was this battery lying around for a few days/ week prior to use? When I charge a battery fully it only reads 98% in the app - unless I turn the battery on & then connect to the charger - then it goes to 100% (this is important imo)
Were you flying agressively at all? Also some strong gusts & you got some scary compass readings (last second of flight) which could indicate a bird strike - Just trying to hopefully find you some answers
Sorry for your loss
Same with car battery at 12 volts. That is empty. Batteries must have higher voltage than rated or they are discharged.
A 15.2v battery with 4 cells should have at fully discharge state. 15.2 / 4 = 3.8v per cell. This battery were below that.
These battery's should be more like 4.3 volts / cell at full charge.
The fpv may still work as it may require less voltage than the motors. I don't understand why you did not have a low battery warning.
Very sorry to hear of this happening to you. Hopefully we all learn something from your experience.