Problem with battery

Good afternoon,

I have problem with my battery: although I charge my computer and it signs that the battery is charged, when I use just the battery without power supply after five minutes the computer turns off. It is possible to use just with the power supply - I am using a not original power supply, maybe this the problem. 

I did the diagnostic to evaluate if there are some errors but the results demonstrated that the battery and the other parts are ok, without errors or problems. 

What do you think that it is the problem?

Thank you very much for attention!