Selling second hand/ bad bad customer service

I brought back my laptop last month as the battery was not working well, I charge it and it get full very quick and when I use it within 30 mins again it off, they said it was fixed and I took it this week been trying to use it and it still does the same thing I don't even feel it was fixed. Hifi is just a **** and selling bad things that don't last I am very very frustrated because I need to use the laptop for my school work