Battery voltage drop analysis

Hello Guys,

I am after a little help figuring out what the problem is with the battery voltage drop during this flight. I wasn’t expecting this and thought I would get a longer flight time. I received low voltage alarms almost straight away.

Link below, Thank you!

AUW - 3.6kg
Trust - 7.5kg
TWR - 2.08 (2:1)
Battery - 6s 12500mAh