The minimum size ANL fuse we have is 35 amps. It would be proper to use a 35 or even a 40 Amp fuse with a 30 Amp charger. The wire must be capable of carrying 35 or 40 Amps. The fuse is a safety mechanism in the event of a short circuit, we don't want to make its trip so close to the running amps that it gets hot or nuisance trips.