Re-check your connections. Ensure red goes to (+) and black goes to (-). If connected correctly, the battery may be deeply discharged or damaged.
Dictates that the connected battery is deeply discharged and drawing maximum current. bosch kl 1208 battery charger manual exclusive
If it hums but the ammeter doesn't move, check for broken internal wiring or fractured copper wire inside the insulated jumper clamps. Re-check your connections