Another note is that internal battery mods/AIO have a finite shelf life. Think of them as being good for no more than a handful of years,with some loss of capacity after only a year (whether you use them or not). This is a fundamental chemistry thing - storing with a partial charge, and relatively cool, helps, but they do degenerate.
If you plan on vaping longer than a handful of years, consider getting mods with replaceable batteries. The odds are pretty good that 18650 batteries will be available for a fairly long time, and mods that can take a 21700 battery will have even longer battery availability (since they can use any of 21700/20700/18650, the latter with a small adapter).
I agree that lithium, cells internal and external, lose some capacity over time even if they aren't used. I do have one istick TC 40W that is probably 6 years old and has remarkable capacity for its age. It's not a daily use mod, but I grab it sometimes when I want a small, inconspicuous mod for out and about. It can go for well over half a day and still be at about half charge.
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