It's a 12V SYSTEM. If you have a shorted battery or a battery has a bad cell, when you try to charge it with the converter, the converter will try for awhile until the excessive draw causes either the converter to blow it's own fuses, the converter burns up, or it trips the circuit breaker which powers it.
Like mbopp stated, if you can't measure voltage with a meter, it's difficult or impossible to diagnose what is happening. With a $10 meter, it becomes easy. I would guess, that now, your converter is not functioning.