Changing from 4 pin to 7 pin

MikeRog

Hello All
Need some advice please
My truck is 7 pin, my trailer is a 4 pin. Want to change trailer to 7 pin
Presently there is no set up to charge the trailer battery from the truck while driving
Trailer has a WFCO Inverter with a 12v battery
The hot wire (+) from the truck 7 pin connects where ???
To the battery or to the WFCO Inverter

Thanks
mike

ras

To the battery with 30A fuse on one end or the other..