There is no requirement for a battery in modern converters except of course 12 volt
power when away from shore power.. That is not the reason for your trouble, guaranteed.
The first thing to do is place a simple volt meter on the output lugs of the converter and see if you have 13.6 volts. If not, check the circuit breakers on the end of the unit. If they are good too, it's dead and you can stop chasing fuses and breakers. If there is 13.6 volts coming out out of the converter, your problem is down range from the converter.
When you said "mains" I automatically knew to look at your location. How are you transforming 220 "mains" to 120 for the converter? Just curious.