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My knowledge applies to AGM batteries - I think Lithium are very similar.

There are two considerations with batteries. State of Discharge and Depth of Discharge.

State of Discharge - a battery would be 100% or close when you buy it - over time it loses it's ability to charge - ie say after 4 yrs it might only be 80%.

Depth of Discharge - AMG down to 50% and Lithium down to 20% (I have read). You can go lower on both but you stuff the battery a lot quicker. Not as many charges. Shortens the battery life.

So after 4 years - AGM can go from 80% to 50% and Lithium from 80% to 20%.

Late last year I took my motorhome AGM batteries (2x140Ah) to an auto electrician - he tested them and said they were down a bit but pretty ok. Then I had a full load test done on them - they were stuffed.

Just a question - I heard 6 months ago - lithium batteries only last 7 years in an EV and they cost $30,000 to replace them - (today's cost). Anybody have any real info on that. I know battery prices are and will come down over time.