J/30 Class Association Forums J/30 Forum The Boat, Sails and Equipment Battery selector switch

I am fairly sure that their is a mains charger as one of the components in the battery locker has a sticker saying disconnect ac input before making dc connections. from the photos it just looks like a black box with no switches or lights, might be a screw in fuse on the top. It is ribbed presumably to dissipate heat.
Rich