Mainesail has forgotten more than I know.
After a LA battery only has 80% of its capacity remaining, the potential for a cell shorting out increases greatly.
Not that it is easy to figure out just when a battery crosses that threshold.
An Acurate test of a 100Ah battery is if it can privide exactly 5 amps for exactly 20 hours before voltage hits 10.5v. As the amps usually change when voltage drops, getting a constant 5 amps for that whole 20 hours is quite difficult without special equipment. Also, the battery is to be held at exactly 77f the entire test.
I am not sure if the 10.5v is the rebound voltage, after the 5 amp load is removed and X amount of time allowed to pass, or while the 5 amp load is still being applied.
In general the true 20 hour capacity test, is the only way to accurately know the remaining capacity and health, but it is also very abusive to the battery being tested.
With all the unknown and the difficulty of quantifying battery health, I recommend people just get them to as high a state of charge as possible whenever possible, and then there should be no surprises when they are worn out several years later, unless achieving full charge is a total pipe dream with their charging sources. "But But I drove for 15 minutes..........."
Only the cluelessly ignorant should be surprised by premature failure. Problem is they blame all the components other than the selfie taking overdepleting undercharger in the mirror.