Sri lekshmi tata motors - Vehicle not start

Tuesday my vehicle Tata Tigor is not started and start vehicle by jump start and brought the vehicle to showroom ( Sri Lekshmi Auto enterprise -Ambattur chennai ) . They checked and found its a battery issue . They repalced the battery with new one and found ok .when i see the battery physically it was old one .Even i took the vehicle to my location ,on the way in road ,i stopped vehicle for break and again i tried to start ,vehicle was not started ,by using help of people by pushing the vehicle and started to reach my location ... They are not diagnosis the real problem. In my point of view Alternator have issue . Need to take action