Faulty car battery

My son bought a car battery on 3 September 2022.Nowadays whenever he wants to drive the battery is flat then he must recharge it at home because they don't want to exchange it although it's still under warranty.He tuc it back 2times already then the guy said he must put a small by the side of the battery..what kind of advise is tha***** unacceptable coz they can exchange it .