Thank for your help. I finally found the trouble. I checked the charger and understood it work that way: If no battery is connected, AC is completely disconnected by a master relay. Controller board is feeded by the 48v from the batteries when connecting the lift batteries. Then, the controller make an analysis of the battery state, end if they need charge, it turn on the master AC relay that feed the transformer. It is this master relay coil that was melted. It work fine now. Your suggestion guided me on the right path!