My New Battery Is Not Working Properly

[web_designer 7]

Hi everyone, I have this problem with my new battery for Dell Inspiron 1520, I just bought it two weeks ago and it should work better than that. The whole problem is, every time it reaches 66%, the whole computer sleeps even if it plugs to power, I mean it is charging, and I even couldn't turn it on until few seconds more. I know it is not that big thing, but it is really annoying, sometimes it sleeps when I was working on something really important, so it bothers me a lot. I checked the settings but couldn't find something weird, so any suggestion on what to do?Thanks in advance.