There are a lot of teslas in the 10+ age bracket, mine included, which is outside of the warranty. And even more Nissan Leafs are up there. Typically what happens is one block of the series connected cells becomes weaker, and the BMS then limits the range of the car to prevent a safety issue.
You just notice the range reduce. At some point that gets to where you want to do something about it, because the cars seem to age very well otherwise - how many other modern cars really make it past 15 years before they meet the crusher because there are just too many expensive problems to fix?
Anyway the most common solution is to buy a pack from a wrecker. That is, a low mileage car which got totaled. Just slam it in and you’ve got original range back or better, if you upgraded. For example the original Leaf only djd 80 miles, but if you choose a battery from the late model you can get 220!
A salvage pack will cost you more in the 5-10k range. And even the dead pack has residual value. It’s a lot, but you appreciate never having paid for an oil change, filters, accessory belt, cam belt, plugs, clutch or dual mass flywheel, starter, alternator, exhaust, catalyser, DPF, EGR, turbo… etc. Even brakes don’t really wear on an EV.
There are shops that will repair the battery too, by swapping just the weak block. That’s a lot cheaper but I feel like you’re on borrowed time until the next one goes.
More unstable with age? Not unless the car is just parked in a barn for 10 years, I’d be a bit worried about it then, like your old iPhone in a drawer that puffs up. That’s due to the cells going “below empty” which is a known problem.
