Is that the same when you charge your Mega through USB cable from PC? If yes, then it is phone's hardware problem, but if NOT, I would suggest to change charger.
Charging from PC is also possible, but the charging current is about 1/4 th of current from dedicated wall socket charger - the phone is charged although much less intensively.
Nominal charging current from wall socket charger is approx. 1.9 amp so if you need to find new one, choose wisely and accept only charger with 5V 2A specs.
Samsung Galaxy Mega GT-I9205 with MegaFire MJ2 V14X
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