Good idea...
... if the battery really does deliver 6hrs battery life, surely there won't be much of a reason to swap it out, short of there not being a Starbucks within 100 miles of your location?!
The point is here - and it's been made about price - is that there must be a sweet point when it comes to battery life.
I'll put a fiver on it being, er, 8hrs or above...