If a battery has dropped below a critical voltage threshold (usually under 10V) or has an internal short circuit, the charger will not recognize it or push current through it. Test the battery with a multimeter. The Charger Gets Very Hot
Clean the battery terminals to ensure a good connection. Bosch Kl 1206 Manual
The manual emphasizes a specific sequence for safe operation to prevent sparks or damage to vehicle electronics. Connection If a battery has dropped below a critical
Remove the from the grounding point or post. Remove the Red positive clamp from the battery terminal. Bosch Kl 1206 Manual