Lithium Ion batteries degrade over time. Even if they are brand new and never used, they still lose their capacity to hold a charge. I bought my laptop in 04 and it had 2.5 hours capacity then and barely has an hour now. This is why LiIon will never be viable in electric cars. The cost of replacing the battery pack every 4-5 years will cost you more than anything you save on fuel and maintenance costs.