Battery life for decades, 60's into 90's, was considered three years. That is how the life time warranty was started by Sears with the Die Hard - Average owner keeps a car for three years, person buys a three year old used car, they drive it for six months and replace the battery, two and a half years later they trade it off, a year later the batter goes bad, and the life time warranty is not transferable. Sears replaced very few batteries for warranty, and others learned how to market the idea.
Current technology is considered five years, and I don't know what gell cells are
I don't know, and the dealer does not know what the life of the Volt's batteries are. I know that with the Prius most owners expect to trade between three and five years. I also know some that have had the typical early failure at a year or so. I have not heard any firm costs, because everyone I heard of that had them replaced had it done under warranty.
When we figured cost of owning and using the Volt we figured owning for five years at 20,000 miles a year. The miles is based on max range between recharge without relying on gasoline engine. This would be your main local vehicle for commutes, errands, and close to home recreation. You would only use your other vehicle for long trips and heavy hauling. He figured a very low trade in value, and it wasn't too bad. The catch is that there is any market for a used electric vehicle with 100k, and that you don't need to pay for batteries. I don't recall what the warranty on the batteries is.