I think that's the part that bibilit had in mind. This is a very common problem with these (and other similar) chargers, apparently.
That's also common, especially if the battery is original to the computer (the newest of which would be at least 12 years old now).
The only way to fix your particular problem, I've found is to replace the cells in the battery, or replace the whole thing. I've had a similar problem, and I think it's due to individual cells in the battery going bad over time.
I rebuilt one of mine, and it worked well for awhile. Until I left it unused for a month or two. Back to square one...
There's also a possibility that the computer itself is the problem. For example, I couldn't get the aforementioned rebuilt battery to charge at all in a Tangerine iBook I had, but then I swapped out the logic board with another I had on hand, and it worked perfectly. A "beyond dead" battery is much more likely, though.
c