For those of you that are even remotely interested...
I got one of these. It allows around an extra 3 days standby obviously depending on what your hammering the phone with.
When charging it charges the phone first then the added battery. When the phone battery needs power a press of the button on the battery pack then transfers the charge to the phone. An added bonus is that it protects the phone too. I find it makes the phone more aesthetically pleasing.