I plan to install a 18 amp DC DC isolated charger to charge LiFePO batteries in my trailer while driving my truck. Does the 7 pin ground wire get connected to the negative input? And the negative output get connected to the trailer's negative bus? If so will the trailer lights work correctly?
The normal wiring for 12 volts to the 7-pin connector may not be sized to allow your expected charging current. The Orion DC-DC chargers will reduce the charging current to keep the voltage at it's input terminals above a preset value. The positive lead in my tow vehicle was #12 and caused way too much voltage drop for a useful charging current. I had to run #6 wire from the battery to the 7-pin.
Also make sure the ground for the 7-pin is a low resistance also. I had to rewire that also but found a chassis connection that was low resistance back to the battery. Otherwise, you may need to run a separate ground wire from the battery to the 7-pin.
Your trailer will connect to chassis to the truck's chassis via the hitch (don't rely on this!) and the ground in the 7-pin. Connect the +12 and ground in the 7-pin to the input of the DC-DC charger. I was able to use the trailer chassis for the negative connection.
Locate the DC-DC charger as close to the battery as possible since any voltage drop in the output wiring will raise the charger's measured battery voltage and may drop out of bulk charge too soon.
If you have a battery monitor, the DC-DC charger negative goes to the system side of that, not to the battery negative terminal.
Don't forget to provide a low resistance connection between the 7-pin +12 volt and the input of the DC-DC charger. This connection adds to voltage drop at the charger's input so if you want 18 amps of charging this will need to be a fairly large wire also. I used #6 here also.
The DC-DC charger should not interfere with your trailer lights.
