What amp fuse for charging circuit from truck Alt?

[Arktikos]

I have a new (to me) 2006 Chevy 2500 and a camper to go with it. This truck has a factory tow package but the red wire "battery feed" on the factory plug at the bumper is dead, even with the truck running. Instead of trying to track down where that red wire goes under the hood I want to run another wire from the truck battery back to the camper and was wondering what amp fuse I should use to protect this circuit?

I am ordering this isolator switch Smart Dual Battery isolator which is rated for 140 Amp. Was thinking about a 200 amp fuse so that it wont blow when there is a low battery condition in the camper, coupled with say, lights and the DC fridge running starts drawing a lot of amps from the alternator.
Thanks for any advice!

[time2roll]

I would sooner pull wire down from solar panels before I rewired my truck.

[pianotuna]

Hi Arktikos,

Thanks.

Do consider #8 or even #6 wire for the charging path. 20 amps won't do much in the way of recharging.

[Arktikos]

pianotuna wrote:
Hi Arktikos,

Maximum measure load on the DC side for my inverter was 276 amps. That happened by accident.

That sounds like an impressive system!

[pianotuna]

Hi Arktikos,

Maximum measure load on the DC side for my inverter was 276 amps. That happened by accident.

[Arktikos]

pianotuna wrote:
Hi smk,

No, I don't mind automatic in some cases. My charging solenoids are automatic, but I do have manual control over them. What I don't like is the 12.9 volt cut out and the 13.4 volt turn on.

I don't think I'd install wire that was only capable of 20 amps. I changed over to 50 amp automatic circuit breakers after blowing 2 60 amp fuses on the OEM charging path. (I admit I'm a power hog and I push my system HARD).

I have seen 12.7 volts on my chassis system, for example when running the water heater on the electric setting. I admit that most folks would not do that.

Are you talking about running an electric water heater through an inverter off the 12V battery? If so that is a lot of amps! If it is a 1500W element, that is 12.5 amps@120v. And at 12v is 120 amps plus whatever the inverter uses. My little 8 foot cabover TC doesn't even have a water heater so I expect very little load other than the furnace.
Anyway, thanks for your perspective.

[pianotuna]

Hi smk,

No, I don't mind automatic in some cases. My charging solenoids are automatic, but I do have manual control over them. What I don't like is the 12.9 volt cut out and the 13.4 volt turn on.

I don't think I'd install wire that was only capable of 20 amps. I changed over to 50 amp automatic circuit breakers after blowing 2 60 amp fuses on the OEM charging path. (I admit I'm a power hog and I push my system HARD).

I have seen 12.7 volts on my chassis system, for example when running the water heater on the electric setting. I admit that most folks would not do that.

[time2roll]

The relay will work fine. For his own reasons pianotuna tries to stay away from automatic things. I like the relay and I prefer things automatic. It would have to be very odd circumstances where the relay remained open when you expected it to be closed.

[Arktikos]

pianotuna wrote:
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.

I don't quite understand. How this isolator works is it makes a connection between the aux and main battery if either battery reaches 13.4v. If I am driving somewhere the main battery will be over 13.4 (more like 14.4V) and there will be a connection, charging the rv battery. If there is something in the coach drawing so much power that the truck charging system wont handle it, dragging down the alternator to under 12.9v then the connection will be cut, but the 20 amp fuse on the aux circuit to the camper will have blown anyway so that will never happen.

[kaydeejay]

wa8yxm wrote:

wolfe10 wrote:
Fuses are sized to protect the WIRE, so the wire gauge will determine the correct fuse.

THIS is the correct and proper answer... LISTEN to this man

Putting a 40 amp fuse on a 12 ga wire is a good way to show of the last RV fire I saw.. This was a TT.. The TT had smoke damage to the outside, A good bath (perhaps with a bit of Clorox) is all it needed.

The Truck Pulling it was TOAST (Do not know why, Did not stop since Fire and Friend were both on scene already so no need for me to stop)

But putting a big fuse on a small wire is one way to do it.

The correct fuse for the trailer feed on GM trucks IS 40A. The OEM wiring is sized correctly.

[wa8yxm]

wolfe10 wrote:
Fuses are sized to protect the WIRE, so the wire gauge will determine the correct fuse.

THIS is the correct and proper answer... LISTEN to this man

Putting a 40 amp fuse on a 12 ga wire is a good way to show of the last RV fire I saw.. This was a TT.. The TT had smoke damage to the outside, A good bath (perhaps with a bit of Clorox) is all it needed.

The Truck Pulling it was TOAST (Do not know why, Did not stop since Fire and Friend were both on scene already so no need for me to stop)

But putting a big fuse on a small wire is one way to do it.

[pianotuna]

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..

[Arktikos]

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.

[pianotuna]

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.

[Arktikos]

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.