What amp fuse for charging circuit from truck Alt?

Hi,

The 12.9 cut out and the 13.4 connect are why I don't think the "smart" relay is a good idea. Particularly if an inverter is in use the battery voltage may drop. At least one converter goes to 13.2 when it is in "float" mode. That means the "smart" relay will not maintain the chassis battery.

Those are some of the reasons I recommended a simple inexpensive continuous duty solenoid relay.

Arktikos wrote:
It connects batteries when it senses one of the two batteries has 13.4V available and disconnects at 12.9V. When it is connected, current can flow either direction depending on which battery has lower volts with no loss so you will charge the truck battery if the camper is plugged into shore power, on generator, or solar power..

pianotuna wrote:
Hi,

I would encourage you to use a continuous duty relay rather than the (expensive) "smart" isolator.

I replaced the 60 amp fuse with a 50 amp automatic circuit breaker.

Thanks, Yes I want all the amps I can get so that rules out diodes.. This isolator is the one I have ordered: Smart Isolator It connects batteries when it senses one of the two batteries has 13.4V available and disconnects at 12.9V. When it is connected, current can flow either direction depending on which battery has lower volts with no loss so you will charge the truck battery if the camper is plugged into shore power, on generator, or solar power.. I have never tried one of these, the type I have had before were relays that would engage when power was supplied to the relay. This type did have the advantage of putting in a bypass button to momentarily help get more power to the engine starter should the main batt become discharged. That doesn't happen that often tho, especially with a gas engine.

Hi,

I would encourage you to use a continuous duty relay rather than the (expensive) "smart" isolator.

I replaced the 60 amp fuse with a 50 amp automatic circuit breaker.

Yes it was just the absent fuse, now with a 40A fuse in the correct spot the camper batt is seeing 13.8V with the truck running. I'm still going to hook up an isolator but will connect it using the stock wire that goes through the plug at the bumper. Maybe a solar pnl someday as well, but the sun is pretty fickle in these parts and I do have a 1000w Honda Gen but it is nice to start out the evening with a full charge after a days drive.

If you must pull wire I would just use simple #6 wire and 60 amp fuse holder.

http://www.genuinedealz.com/maxi-fuse-inline-fuse-holder-w-6-awg-leads-up-to-60a

Honestly I would have the oem circuit fixed. Might be just a new fuse.

Either way consider 100 to 200 watts of solar on the TC to charge the battery. To rely mostly on the alternator is probably setting you up for disappointment.

Thanks so much! this info you have given has been so valuable to me. I opened up the fuse box under the hood (Nice I didn't have to stand on my head to look under the dash!) and found where Stud#1 is. Appears to be just a red piece of plastic plugged into it. Am heading to NAPA now to get a 40A.. Maybe we'll have power to the camper for the trip we got planned for Monday! If so I wont bother running another wire back to the camper as I just want the batt back there to charge up after a day's driving. Not really needing a lot of power for this little 8' cabover..
Thanks again.

Hi,

If you have #4 wire going all the way to the trailer, then a 100 amp fuse is the max size that should be used. You will not get more than 50 amps to travel between the two 12 volt batteries. You will need a very oversized connector, like a forklift battery connector to get more than a few amps from the truck to the trailer. Look for a 50 amp rated 'anderson connector'. It will handle 75 amps without getting warm, and is rated at 50 amps when plugging in and out all the time without a problem.

The wires going through the factory wiring harness are probably #12 wire - rated around 20 amps, and with that distance, probably will never see more than about 10 amps going into the trailer battery.

By connecting a wire directly to the alternator, you will get a full 14.2 volts, while the battery probably only has about 13.5 volts.

Good luck,

Fred.

Fuses are sized to protect the WIRE, so the wire gauge will determine the correct fuse.

It will be in the range of 10-30 amp depending on wire gauge It will be in the line from isolator (right at the isolator)to the camper.

Be sure to run a comparable gauge ground/negative wire from camper to truck frame.

The under-hood fusebox has a fuse called stud#1. It's 40A and feeds the trailer plug. There's no isolator other than pulling that fuse.

If you run your own wire, the fuse should be sized to prevent overloading that wire.

If you want to add an isolator to the existing wire, it's connected to a post (stud) just outside the fusebox and you can put the isolator in series there.

Take a look at the owners manual if it is still with the truck. Should be a place in the large under hood fuse box for a fuse to protect that line. Maybe the old owner never plugged one into it.