How heavy is your F550? Mine is at 2,380gr and I'm using a 3S 6400 mAh 35C lipo. This gives me 8:30 minutes flying time with 29% battery remaining. Here is what i use:
Voltage
No Load: 11.30v.
Loss: .50
Loaded: 10.80v
No Load: 11.00v
Loss: .50
Loaded: 10.50v
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