Fast charger for smart phones

This reminds me of the stand-alone charger that my wife had bought. She is using 2 batteries for her Samsung J7, that was before the emergence of the power bank or portable charger. That fast charger would take just 30 minutes for a full charge of the battery. Looks good. However, as days pass, my wife noticed that the charge is getting quicker to consume. The usual 1 day for a full charge now lasts for half a day. And after investigation, the fast charger has been discharging the battery.