12-Volt Accessory Power Feed on 2008 Ford F250 is Not Charging Travel Trailer Battery

Updated 05/23/2024 | Published 08/09/2012

Question:

with truck running and trailer pluged in there is no 12volt charge going to the trailer battery.took cables off of battery in trailer ,used volt meter to check this .when the trailer battery goes dead and you start the truck nothing works ,you have to change to a charged battery and then you can use the slide ,jacks etc. 110v powers every thing and charges the battery back up. has a new paralax converter .

asked by: Tom H

Expert Reply:

In order to activate the 12-volt accessory power feed to the 7-way, there is a fuse and a relay that need to be installed in the fuse box under the hood.

Your owners manual or the legend on the fuse box lid will show you the correct location. If the accessory circuit once worked in the past, check for a blown fuse or a bad relay.

If the fuse and relay are already in place and are not faulty, please get back with me and will will look into some other less likely possibilities.

Keep in mind that the 12 volt feed coming from the 7-way won't have sufficient power to charge a dead or depleted battery. It can keep a charged battery maintained, but the vehicle alternator won't be able to push enough power through to charge the battery.

expert reply by: Mike L

Thomas

5/22/2024

I was watching a video on the 12-volt feed to keep the 12V camper battery charged. That video indicated that 12 V to 12 V charger such as the Renology one is needed for Lithium batteries. Their argument was that a relay does not protect the alternator. Wondering if you concur. Thanks

Mike L.

5/23/2024

@Thomas A DC to DC 12V charger is a great idea, because all the 12V circuit from the 7-way trailer connector will do is keep your trailer battery topped off. It won't charge it once it's dead. We don't carry Renology chargers, so unfortunately I'm not very familiar with them, so I couldn't say.