You mentioned that you had water damage a few years ago. My first thought would be that your previous damage is acting up again. If there was any corrosion around the ISL charging chip and it was not thoroughly cleaned after liquid damage, then now - years later + humidity + heat + any other variable could have cut the thread that was holding the system together.
If you are comfortable with it, I would recommend opening the system up and looking around the ISL chip and its resistors that is on the back side of your motherboard - with a magnifine glass or better yet, microscope.
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Damage can be easy or hard to spot.
It really sounds like a bad feedback resistor on the battery side of the power in circuit. If you do find damage here, you might want to consider sending your computer out for micro soldering repair, or if you are confident in your own skills and can get the right supplies, you can give it a go by your self first! There are some good videos on line showing what this chip does an how it can be replaced.
Good luck!