Sorry, Your Brother has quite the imagination or cant judge speed so well.
The max for a Spark in windless conditions is 50 KPH.
My DJI FPV with Gemfan props will do over 100 kph.
If you had Airdata you could see how each cell is working, your problem may be a bad cell, I had a M3E battery go bad with no swelling and few cycles. I checked the info in Airdata which also sent me an email about the battery issue. I could see...