Just bought Surface Pro 2 a week ago, and so far I am very happy with it. But the battery never reaches the full designed capacity of 42,000mWh. It is always at about 40,800 mWh when fully charged, and I am getting about 7 to 8 hour usage. If a complete run-down of battery can help boost the capacity, how do I do it? The surface pro shuts itself off at 5%. Thank you.