The idea is a good one but the problem is that the 17 battery is a totally different shape and size and won't fit in the 15. So, it is physically impossible. The way it is now with the 15 and 17 is that you really need a second battery. I have 3 for my 15 and 2 for 17! Until battery tech improves and or energy efficiency goes up, this is the only way to achieve mobility.
Good luck and welcome to the boards!