Every BL-5J has a chip inside which keeps track of the amount of charge a battery can hold. It seems like yours was off by a large amount or simply defect.

You can re-calibrate it yourself or purchase a new battery if the one you're currently using does turn out to be defect.
http://talk.maemo.org/showpost.php?p...&postcount=846
http://talk.maemo.org/showpost.php?p...postcount=1176