Bosch Kl 1208 User 39-s Manual |link| Info

: For standard (non-maintenance-free) batteries, unscrew all screw plugs to allow gases to escape.

Charging creates flammable gas; always ensure adequate ventilation and keep away from sparks or flames. Bosch Kl 1208 User 39-s Manual

In newer "Smart" variants, this may indicate the battery is too hot to charge. A wiring diagram or specific part numbers : For standard (non-maintenance-free) batteries

This usually indicates a load is still connected (like a car clock or alarm). Disconnect the battery from the vehicle for the most accurate charging. Bosch Kl 1208 User 39-s Manual