Car battery charger voltage

i have a car battery(74ah) 750a that i hooked up to a cheap 12v 4a car charger without any smart functions. At what voltage should i disconnect the charger when measured with a multimeter? Now it jumps around from 14.8 v to 15.0v. It’s probably too much and im already overcharging it, but maybe its normal? The battery is a few years old and not in the best shape and i will replace it soon so i dont really care if i overcharge it (without explosion i hope), but for the sake of knowing next time, at what voltage should i stop charging?