I also took apart my old sealed concorde that was not working great anymore a couple months ago.
The water in the battery was low, it evaporated out.i ended up buying a new lead acid gill
because you can top them off as needed with distilled water.
Ive noticed that the key to keeping the battery lasting longer is to ALWAYS keep it on a trickle charger when not in use and keeping water topped off.
The plates inside tend to “rust” (sulfonate) when not always topped off.
Ive had good results with this product recovering batteries but it takes time.
https://www.recovermybatteries.com/ex01-12-24-200.html
On another note,This is an interesting video of
a guy rebuilding a lead acid battery.
It gives you a visual on how these things are built
https://youtu.be/kNGg0P7B5fI?si=CPfyqDVV8Vq5csY4