Help Samsung tab 3 10.1 wont work unless plugged in Please read! topic

OK so i bought this tab 3 10.1 inch because they guy said it would not turn on i got it to turn on but only if it is plugged into the wall or my computer i have replaced the usb/charging port and then the battery and it still does the same thing the battery says its 100% and everything works fine but as soon as i unplugg the usb cable the tablet goes dead i have tried all the little button combos of holding this and that for so many seconds and nothing works if anyone has any idea of what could be the problem please shoot me some ideas because as of right now im thinking it could be something where the battey is not connecting to the tab but on the other hand it shows it charging and it at 100% so it must be working so could this be some sort of software issue or something where it has to be tethered to work or is the tablet messed up for good any help is appreciated i looked all over the internet and didnt find anything like this