Been using phantom vision 2 for 6 months with no problems. I took it out the other day and battery showing 90%. All of a sudden warning alarm came on and  battery life went really low. phantom started to land and i lost all control. Luckily i wasnt over water so recovered it. I tried draining the battery to about 8% and recharging but the same thing happened again when i tried to take off. Almost immediate loss of all power.
The assistant shows that i still have 89% life left in the battery.
Luckily i never lost my phantom. Another battery ordered so helpfully this will be ok. My problem is what if this happens next time and im over water. This must be a design fault. Anybody had the same problem ?   

Can you post a photo of the battery readings in the assistant program.
Sounds like its dead, but it would help others to see the readings.
Might be worth contacting your dealer although the battery warranty is only 3 months, some people have had replacements sent free of charge.

Rob,
It's not likely a design fault as much as it is a random failure (albeit potentially a permanent random failure).  Do you have only one battery?  And if multiples, did you try another battery?

DJI: What element of the system senses and controls the emergency sequencing?

Look like it is  not related to design.
It should be the battery defect.
Please try another brand new battery, check everything is fine.
If yes, please contact your dealer for a replace if it is still in warranty.