Phone wont turn on except when connected to charger

Ok so first time i had problems like this was when my battery was like <30% and i take a photo with flash it turns off with no warning instant shutdown , then i had battery fluctuation if battery was like 8% when i offline charge like for 5minutes when i turned it on was 34 - 50 % . Now my phone wont go pass lg boot logo or it passes and get to homescreen it will insta shutdown , this happens even with 100% battery .Im on stock 4.4.2 v20a eu . Can someone help please ? i suspect the battery but i wan to know if someone else had problems like this before