Why the charge time is so long?

Hello, i have a problem with my asus zenfon2. I bought it 1 year ago and from some months the charge time is became too long. To charge my device 20 h is requested and furthermore often during the charge the device stop to charge it look likes the charger wire disconnects from phone. I use original charger and i have tried with many charger and wires but the problme is always the same! Help me. The phone is never falls in the water or had some damages.