What charger is it?
The original flyback charger is a 20 amp 2 bank charger. It's not a smart charger; it doesn't have a float mode or sensors to differentiate which bank to charge first. It should have a 20 amp fuse in line between the charger and the battery.
It also has an internal fuse that you have to take apart the case to change. From what inexperienced and read this charger is not very good. Once it starts blowing fuses its time for a new one. I tried to keep mine going and ended up ruining my 4D battery. Dumb move.
It might be time for a new charger but we need more info to really help.
My idea would be...... disconnect the wire at both ends of that you are talking about and do a continuity test on that wire.The fuse in the charger line to battery #1 keeps blowing.I don't see any shorts frayed wires or loose connections. Any Ideas?