BC-900 will not charge a completely drained cell (display says 'null' after you insert the cell). Follow the instructions here to 'jump-start' the dead cell, so that its terminal voltage is restored to about 0.5-1 volt. Then the BC-900 will charge it:

UNPLUG the BC-900!
Place a good cell and a 'dead' cell next to each other
Short the (+) terminals of the two cells together with a paper clip for a few seconds
Remove short, plug in the BC-900