What (AGM, Gel, Standard Lead-Acid) are you trying to charge?
Corrosion or rust on battery terminals blocks electrical flow. Clean the terminals with a wire brush before attaching clamps. Bosch Kl 1206 Manual
However, if it smells burnt or becomes too hot to safely hold, unplug it immediately. Ensure the ventilation slots on the casing are not covered or blocked by dust. Frequently Asked Questions Does the Bosch KL 1206 have automatic shut-off? What (AGM, Gel, Standard Lead-Acid) are you trying to charge
You have connected the Red clamp to Negative and the Black clamp to Positive. Unplug the unit immediately, replace the blown fuse, and correct the connections. replace the blown fuse
Never connect or disconnect the alligator clamps while the charger is plugged into the wall. Doing so creates a spark that could ignite hydrogen gas.