Phone appearing to take a charge, but the percentage is decreasing?

The percentage charge on the battery of my Samsung Galaxy A3 (2016) is decreasing, rather than increasing whilst charging (it does so worryingly rapidly, even when on battery saving mode)- the only time it doesn't decrease, and the number remains completely stationary, is when the phone is shut off.

I've tried three different USB cords and plugs (two chargers were fast-charging and by Samsung, the other was not) and multiple different wall sockets, so I know that those options have been eliminated. I've checked for visible swelling from the battery (there is none) and tried cleaning the USB port.

I'd had the same issue with the A3 about two weeks after buying it (this was early January), and sent it off for repairs on warranty by my carrier. I also had a friend who had the same issue, but with an A5, and his phone was immediately replaced (he's with a different provider, so things all came along fairly quickly).

My carrier is Vodafone, so the warranty lasts 12 months (or an extended length, if the maker's warranty is longer, so mine would last 24 months, as it is a Samsung phone), so I'm worried that I won't get it repaired on warranty, as they may think it could have been to do mistreatment, and there are no Samsung repair shops near to me (they're all down the other end of the country), or very many phone repair shops in my area, really.

Do you have any idea what the problem could be, and, if so, the solution to this problem? I know it's not from water damage, or from dropping it, as I treat all my devices fairly carefully and delicately, so what do you think it could be? The charging port possibly?