Nissan change out batteries that drop below (IIRC) 75% of capacity under warranty (96 months / 100,000 miles for the electric bits).
And batteries are not "built in" - they are replaceable, though you would probably want to be sat down when you get the quote. There are also companies now that will rebuild battery packs with new cells for a lot less than a new OEM battery. I still don't want one, though.
My brother has a Nissan Leaf - the battery gets a "health check" annually at the service. IIRC his was at 95% of new capacity at 4 years old.