Review phones not charging

I bought a hi-sense phone at cash crusaders in Albertina Sisulu in Jhb cbd to my surprise the phone battery when charged stays at 100% for 2 minutes then it keeps on dropping without me touching it then at 44% it switches itself off I'm really disappointed I bought a phone because I needed it because mine was stolen but for them to be selling phones that are not working then when you complain u r told that they can't assist you that's not the good service