MPG will drop and SOC fluctuate more. That aging process can take a long time though. The point of "failure" is not easy to predict. Basically, it isn't until the system detects something is outside of normal tolerances and throws a code.
The only real signs I saw was when I came back from driving the battery would be full. A few days of the car not being driven Id get in and start the car and the battery would be in the red like 2 bars left. It would sometimes show full then jump back lower but my mileage was always pretty much the same 42mpg. It eventually threw a code so I just swapped it. I called around and got a dealer price of $1800. They let me pick up the battery and drop the old one when I was done with the swap( perks of a small town dealer).
Lower mpg, battery draining faster than normal, engine running more often than usual to keep the battery near 6 bars. This is typical of the NiMH battery. No idea how the Li-ion ones will perform since they aren’t old enough (in the US).
