Ahh, the wall charger. There might be your problem... If you forget to remove the battery from the charger just once, then that's it. If your wall charger is 50mA output then your charge time (from dead .8v/cell) is 20 hours... so 15 hours sounds about right, if the battery was partially charged to start with.

That Superbrain will allow you to correctly charge your batteries every time. You may even be able to revive your old battery. Enjoy.

-Mark