I'm selling the batteries from my 2008 wrecked Prius. They only have 5k miles on them and should last you quite a long time with this low mileage. Here is the link to my auction on eBay... eBay Motors: Toyota Prius Hybrid Batteries (only 5000 miles!) (item 180270896544 end time Jul-31-08 15:53:10 PDT)
Off the Wall Questions from one of the "electrically clueless:" If you put it in the ubiquitous, "cool, dry place," what would the "shelf life" on one of these be? What if I hooked it to a trickle charger?
If you're getting at you're thinking of buying it as a spare... I'd say although I don't have an educated answer for you, I will say this period of time is more than likely measured in months, rather than years. Since your '08 is warrantied for 8 years/100k miles I'd say it isn't worth it. Now that's assuming that was what you were thinking. If not, please disregard.