While at it, and looking at that -00.7, I jacked converter voltage back to 15.4 and battery voltage instantly went to 15.4 too, and amps stayed at -00.7 like nothing happened. Too weird. Back to 14.8 no amps change. I think that might be the "wall"??
Yes and you are right to be suspicious. comparative inspection (using something valid as a reference) is one of the best ways to pass/fail a battery. Mr Pianotuna is similarly right on the point. I refuse to present more than 13.0 volts to a battery at a temp of 40c or higher. If amperage starts creeping up it is bad juju.
I have a Uni-T inductive ammeter with 40 amp scale that is remarkably accurate at <2.0 amperes. Your panel meter may use a 50mv shunt at 500 amperes scale?