Look inside the charger. Sometimes the wires come un-soldered from the diode plate. Plus there is usually a thermal protector, and they can go bad, cutting off the juice.
My son took cover off and we hooked to battery and plugged it in, it started smoking out of the metal plates the enclosed the wire coils, I guess that produces all the juice. It was literally black and burnt at the corner seems, not sure what happened, all wires were in tact. I can only refer to it like a large breaker contacts getting fuse together and not kicking out.