That's a good question. You never know whether it is the charger, the us bcable, or the battery. Intuitively, the battery would seem to be the most likely to fail, but actually, the charger is more likely, with usb cable a close second. It is not a bad idea to have an extra usb cable anyway, so I would get another one if you do not already have one. However, the fact that it went from fully charged to 4% overnight would seem to indicate that the battery has failed. You can get a replacement battery on amazon or ebay at a very low cost. However, overnight shipping is expensive. Since chargers can be unreliable, I would not buy one online. If you go to a phone store, they will test your components for you, and you can get whatever you need right away. batteries can be expensive at the phone store.
I would turn off your phone and remove the back cover. Get the part number of the battery and price it online. Then go to a phone store and take your charger and usb cable with the phone, so that they can all be tested. Then, if you need a charger, I would get it from the phone store. If you need a cable or a battery, you can decide.
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