FYI my battery died on me as well just after having bought the car (997 carrera - second hand) and was luckily replaced under warranty. 

I asked my dealership to fit a stronger battery than the one I had (80 or 85 I think) and they told me it was not possible because the car's computer is only programmed to re-charge that particular type of battery. Putting something else in would not work...

I offered to pay the difference between the standard battery and the stronger one (suspecting that it might have something to do with the warranty only paying for a particular type), but this apparently did not change a thing.

I am not an electrician (and the above sounds a bit as a "dodgy explanation" from the dealership), but I wanted to share my experience in this respect