My samsung grand duos gt 19082 is not turning on at all

my samsung grand duos gt 19082 is not turning on .

since 6 months it was like switching off suddenly and when it had turn on it was 100 % battery to 5%.

few days ago i had visited a marriage and my phone had switch off and left with 5% battery and after 2 days without battery the phone was like dead. when i had reach my home i charged it ,after 2 days of struggle i was finally able turn it on.

it was fine for 2 days with continuous switch offs. and the next day it was dead again . this time it had no sign of charging, i checked the battery in another phone and it was around 80% .today it turned on again but just for 20 minutes.please help me with my problems.