MIDAS CENTURION BAD SERVICE UNDER BATTERY WARRANTY

I bought another battery while my battery was under warranty now they are saying my car its killing the batteries it has a faulty ulternater,There is no good customer service there , i took the battery there and they charged it for two hours now its over charged and they are taking the blame that my vehicle its faulty while the car its moving very well with the new battery .my query is why do they said the battery its under warranty if they cant give me a new one without paying another 1800 ,This is very bad because now i need to wait for another 6 weeks after they decided to take the battery to the LAB they say .