My P200 I ran for 10 years and had to replace one motor and get this, it still has the same batteries! MK are the best! Now I've heard a lot that I did not use it very much, or go very long distances, etc. but believe me I did. And the same thing with my TDX, MK batteries 10 years. However that one I did not ride real hard and and for the past 5 years it's been sitting behind my couch as my backup. And by roommate turned it on the other day and it was still fully charged.
Heres how it works.
MK are about the best for long term float service life. (most high quality batteries kept fully topped up will have a stand by service life or 10 to 12 years) MK are among the best here. But this means that they must be regularly topped up to 100% and if this is true, then they will have 80% of the original capacity remaining after 10 years. This is called stand by or float life. If they are NOT kept topped up, regularly, they will have much less than 80% of capacity remaining. Regardless they will still say "full" if charged on a powerchair, even if most of the capacity or range has vanished...
You get only a couple of hundred really deep discharge cycles. So best not to do that. Heavy users get less than 1 years use from a set of MK batteries.
MK gel offer 500 cycles if discharged by 80 percent, and correctly charged. So if you really cover long distances, then you will get 18 months before they go in the bin. And are good deep cycle batteries too. This is as good as lead batteries get.
You get several THOUSANDS of shallow discharge cycles of 30 to 40% used. So if you dont go too far, and regularly charge FULL, they can last a very long time.
However just because your chair that is stored says full, certainly does not mean it is. It may read full even if its basically a ruined battery with only half its original capacity remaining, and if its 85% charged. So dont rely on that chair gauge to tell you anything useful. Only measuring capacity can tell you if its any good. And only a full charge cycle, can tell you it is actually fully charged. And you should do that in storage approx once every 2 weeks or every month at bare minimum. Or the battery will sulfate internally and die faster.