Iphone6 battery draining
Recently my iphone6 (yes, I know, old but I can’t afford upgrade) battery has been draining erratically. Eg, Up at 90%, then down to 20 and then to 1. I’ve turned it off and on and percentage jumps up but then it starts all over again. Screen has been going black as well. How can this be fixed? I really can’t afford a new phone