Battery life

bought a used iphone 5s on contract with cell c. noticed after 3 days that the battery life of a phone is not normal. as it last for 4 hours when fully charged . took the phone back to the store on the 15th October and they booked it in for repair, got the phone back on the 25th October at 14:00pm it was 100% charged, 16:00 the battery is on 45%. took the phone back again and the man told me that maybe the technician did not use the right charger to charge the phone after repairs but he advised that i must wait for the phone to be completely out so i can charge it with the original iPhone charger, \was shocked"followed the instruction and took the screen shot after every hour. and i took the phone back again on the 27th October for the same problem and i made it very clear that \""I do not want that phone again\"" Now the store people are telling me to speak to repairs and repairs is telling me to speak to the store people. Now i need to know who to speak to as i do not want the phone at all. If the technician could not fix the phone for the second time"