This problem has of occurred with my phone to. This is a simple fix 80 percent of the time it's just the cable of the charger has a fault or is dirty in the phone charge socket or wall socket. Which can be cleaned with an ear bud or tooth pick. If the wire of the charger has a fault just have it replaced with a brand new one or a spare (if you have one). The other 20 percent of the time it's the storage on your phone which can drain battery power. The best way to see what's draining it is to get this recommended app called cleaner. This will show the drainage on your battery power. Finnaly the last thing to do with this 20 percent of the time is in the settings go on to developer settings and put change the SUB to charge only at socket. Simple right.