Assumption based on experience from laptops, phones and other devices with LiPo/LiOn/Lixxx. Maybe teslas batteries are better? In 2 years we will find out...
Why would you base your assumption on experience with laptops when you have access (through the magic of the Internet) to perfectly good data about the Tesla Roadster? They've been on the road for over four years now.
It says that the batteries have, on average, over 80 percent capacity remaining after 100,000 miles. 80% of the original 265 mile EPA range is still well over 210 miles of range.
Cars generally cycle the batteries far more gently, as their weight and size constraints are far less tight. 10-year-old Priuses have NiMH batteries chugging along just fine (http://en.wikipedia.org/wiki/Toyota_Prius#Battery_life_cycle), but most 10-year-old gadgets are in pretty bad shape battery-wise.
Hybrid vehicles have a significant advantage over PEVs when it comes to battery life, which is that they don't need to utilize the full capacity of their batteries. The control system on a Prius tries to keep its main battery at an average 60% charge, which significantly lengthens its life. That's not such a big deal when you can recharge the battery at any time off of the gas engine.
Depending on heat, a fully charged Li+ battery will lose between 15-30% of its capacity per year. If you keep its charge between 40-60%, it will lose only around 4-10% per year.
Tesla estimates that after 5 years, their battery packs will still have 70% of their original capacity, which would seem to indicate that this is indeed what they're doing (70%^(1/5) = 93%, or 7% loss per year). That sucks for a car which is entirely powered by its battery, since it means that a Tesla could, at a long-term cost to total capacity, potentially have at least twice the range its control system will allow. It seems like that would be a decent feature to give for people making occasional long trips.
