The Pro has a 5500mAh/7,4V battery, which equals 11000mAh of the Anker 3,7V battery. You charged 3 times 75% of the Surface with 50% of the Anker power bank? 3x8000 mAh out of 10000mAh of the Anker? (Assuming 100% efficiency, maybe 70% are reachable). Either is your Surface battery faulty or you...