My thought was that the CB may be bad and the CB is cycling. Could also be a bad or reversed battery. When the CB is open the full converter voltage is fed to the lights. When closed the battery is pulling the voltage down.
I was referring to the auto resetting CB. Be careful with your multimeter when measuring the current. Many meters only read 10A max and more could damage the meter. The converter could be trying to supply more than 10A. You will have to open the circuit to measure the current unless you have a clamp-on DC ammeter.