Charging issues with my Samsung smart phone

Hello there,
I have recently bought Samsung Galaxy S5 smart phone and it is amazing. But I am facing charging issues with it. It is taking very long time to charge. It is taking 5-6 hours to charge up to 80%. After unplugging it from AC power, for 1-2 hours it will be fine. In the middle suddenly the charging is going down like from 70% to 30% like that even though there are no applications running on the backend. I have tried replacing the battery, but no much use. I am not sure how can I troubleshoot this issue.
Could this be an issue with the phone itself? Did anybody experience the same issue? Please advise if you have any Suggestions. Thanks in advance.
