Since most users have lost the manual, here is the standard operating procedure for this class of Bosch charger:
Check the Bosch KL 1208 power cord, alligator clamps, and chassis for fraying, cracks, or damage. Do not use a damaged charger. bosch kl 1208 battery charger manual
Check and replace the fuse. If the fuse is intact, the battery may be sulfated or dead beyond repair, requiring replacement. Since most users have lost the manual, here
Look for a broken filament inside the glass or plastic housing. and chassis for fraying