I've heard that the Prius needs battery replacement at 100,000 miles at a cost of $10,000.
You figure it out.
Toyota claims that not one has required a battery replacement due to malfunction or “wearing out.” The only replacement batteries sold—at the retail price of $3000—have been for cars that were involved in accidents. Toyota further claims that the nickel-metal hydride (NiMH) battery packs used in all Prius models are expected to last the life of the car with very little to no degradation in power capability. http://consumerguideauto.howstuffworks.com/hybrid-batteries-none-the-worse-for-wear-cga.htm
The Prius’ traction battery is warranted for 150k miles in California as part of the emissions-related equipment. I’d imagine the same would be true of the Volt.
The same “$10,000” canard floated around about the Prius, and might even have been true back when it was brand new (and covered by warranty). In taxi service the batteries have routinely gone past 350k miles, and replacements nowadays run about $1500, or less than the cost of a transmission in a similarly well-used car. The batteries require no maintenance.
This type of battery, properly manufactured and used, is very reliable. There are some tricks to achieving close to unlimited life; for example the Prius’ computerized controls maintain charge between 40% and 80% of full. When run in that range, they go practically forever.