An oldie, a divisive one at that, but here is my experience.
My first 2 cheap
at $US200 for 130AH each Chinese batteries
lasted 2 1/2 years. Stupid me then bought 2 more offered at a discount because the first two didn't go the distance. Now, after 4 years, they do died.
I tested them with an Ancel B101 analyser, and they showed about 30% capacity, with a "replace battery" readout. I went through several charge/discharge cycles with a 16 amp load, but always got a "replace battery".
With nothing to lose, I thought I'd add distilled water
The cells are covered with a thin plastic plate glued into the battery
. The plates stop the rubber fill caps directly under it from popping out during charging
I put in 20 millilitres (2/3 of an ounce) of distilled water
into each cell, and recharged the batteries
This time I got a "good battery" reading, with a consistent percentage good in the mid 60's %.
Even after a few more charge/discharge cycles.