my f150 charges my trailer house battery fine.
i tried to charge a battery for my trolling motor from the aux terminal and blew 20 and 30 amp fuses.
is there a current limiting device to limit current to 20 amps or less?
sp

 

You probably have everything connected ok except the battery you are trying to charge is taking more current than the circuit protector is allowing to pass. Some things to keep in mind, whatever set up you have to charge the house battery most likely has heavier gauge wire that can handle the hi current going to the battery. That aux wire is probably only 12 gauge and it's current carrying capacity is limited to around 20 -30 amps. That battery, if low, or run down will accept a lot more than what the wire should carry and the fuse will handle. You might install a current limiting resistor in the line to keep the current limited to 20 amps. A wirewound or ceramic resistor will generate a lot of heat , but will do the job. Using ohms law I came up with a value of approx .6 ohms.
12 volts divided by the 20 amps did give me .6 ohms.(point 6).