Well, the GTX 470 is relatively power hungry, and the total system power consumption of a Core i7 965 3.2GHz system with a GTX 470 would probably be around 350W to 450W while running Left 4 Dead, taken from Techreport.
At Idle your CPU should only be using around 30 or 40 watts so at the time you ran that test you must have been at Idle I guarantee you put some load on that and you will only get a few minutes of battery time.
Well, the GTX 470 is relatively power hungry, and the total system power consumption of a Core i7 965 3.2GHz system with a GTX 470 would probably be around 350W to 450W while running Left 4 Dead, taken from Techreport.