If it won't work when it isn't plugged in, then it sounds like the battery is done for and will need replacing. Nothing to do about that other than purchase a new one.
I would, however, recommend you avoid aftermarket ones. While they may seem like a good deal (price wise) they are also very much a hit or miss option. It is very easy to get one that claims it will work, doesn't or will not last as long as it should, or have other issues. So keep that in mind when purchasing a new one.