I've said this before :
1) You have no need to worry about the battery for 8 years (warranty),
2) after that IF a module fails it can be replaced individually
3) There will be LOADS of spare batteries available out of crashed PRII within 8 years, and i would be perfectly happy with a good wokring second hand module
4) In 8 years the volume of batteries produced will be so enormous that it will be almost a commoditty by then, and al ot cheaper than now.

Bottom line : don not worry about the battery.