how can I get "alternator only power" from truck?

[ducks13]

I'd add a relay same as the AC relay if you have a space in the relay block. Heck, there may already be a wired relay socket for some accessory (heated seats or something like that) that you don't have in the relay panel, just add a relay and hijack its output to the trailer.

 

[Dr Dave]

My ford is switched with a relay and protected by a 30 amp maxi fuse. These came in a packet in glove box, with instalation directions, also had a pigtail for a brake controler that plugs in under center of dash.:thumbsup: This circuit is only hot with key in run position. Charges battery on camper, and runs winch on boat trailer.

 

[buickanddeere]

Add a mechanical oil pressure switch that energizes a HD relay only when the engine is running. Fail safe and avoids the 0.7 voltage drop that diodes cause.
On these low voltage/high current applications, the wiring can't ever be too large of an AWG. The (-) circuit has to be low resistance too.

 

[Travelover]

What you want to do is very common for travel trailers. The best way is to use a relay wired so that it turns on with ignition. If you use a solid state isolator, you lose voltage across it automatically, which is a problem because getting as close to 14.5 volts to the second battery ensures that it charges.

As has been mentioned, if second battery is a long way from the alternator, you need to use heavy gauge (4, 6 or 8) wire or the voltage to the second battery will be too low to charge properly.

I buy heavy gauge wire at the recyclers, usually new off the ends of very long rolls. Be sure to put a 30 or 40 amp fuse or circuit breaker on BOTH batteries.

 

[RobertN]

On my old Ford F250, I put a relay in. It was same relay as used for the starter on Ford's for decades. It was pretty simple. The second battery was in the cab-over camper. It only charged the camper batteries when the truck was running

When we got the new truck(gosh, it is 11 years old now...) I got an electronic isolator.

Charging that second set of batteries is a pretty common setup for campers, travel trailers, RV's.

RV Battery
DC battery isolator and split charge relay for RV, car, and truck applications
RV Battery Isolators, Seperators and Accessories

 

[jwstewar]

I wouldn't want to wire a plug up that didn't fit "standards." That will then keep you from pulling other trailers that are wired to standard as well as keep others from pulling your trailer (that part may not be a bad thing). If anything, add a second connection in to do what you want to do so that you still have a standard connection as the center pin is usually for back up lights. Don't see them too often any more, but we used to have a camper that had them. They actually helped a bit when back when dark, but were a lot more useful in letting someone know you were backing up.

 

[rsewill]

There is one other important issue in a circuit like this, and that is if you run the battery down in the trailer, and then reconnect to the truck for charging, it will draw a potentially very large current until the battery voltages level out. So your hot wire for this circuit should be at least a #8, and an equal sized ground wire from the back of the plug to the truck chassis.

This is the reason they put a small dc-dc chargers in battery breakaway circuits.

This would be a fire issue on the truck, so make sure you think it through completely.

This is your biggest problem. Your wiring won't handle the possible current.

I had a generator go out on a truck. To get where I wanted to go I ran a #12 wire from the reefer unit on the trailer which worked fine until it got dark and I turned on the lights. Melted the insulation and the wire.

 

[sldva00]

As mentioned by RSEWILL, wire guage is your problem. I would use the Power Interrupt kit similar to Warn Industries - Electrical Accessories for Jeep, Truck & SUV Winches.

You can use a relay with 12 gauge wire to switch the solenoid on/off. To power the relay, tap into the "Run" position of your ignition key switch with a wire that has an inline fuse. Just open up your steering column and see which wire is hot only when the truck is on.

I would not use the trailer light plugs for a connection. You can't connect to the terminals using a proper gauged wire, they are just too small. You can use the Warn Quick Connect to supply power to the trailer's battery. I have a plug for my winch in front and back of my pickup using this setup. It works great.

 

[jeffsw6]

Your information is incorrect you don't need a second battery in the engine compartment. The battery isolator "isolates" each battery from the other.

I didn't know there were different kinds of these kits. I see what you mean now. I think I can buy this device RobertN suggested and put it on the trailer:

DC battery isolator and split charge relay for RV, car, and truck applications

It seems like the wire size could be a real problem if the trailer battery is low and draws a large current to charge itself. I could put larger wire on the truck and trailer easily EXCEPT the 7-wire trailer plug, since the wire opening of them is typically not huge.

Would the easiest way to solve this problem be to use the Battery Isolator above to supply charging current to another device, a 12VDC to 12VDC charger? The same site sells this DC Input Battery Chargers for Lead Acid and SLA Batteries, for charging batteries from vehicles and other low voltage sources I know I don't really need the voltage regulation / step-up function of this thing, but it does limit the current draw so wire size would not be a problem.

The relay device is $74 and the other one is $154 but if I used those things, I could simply use the black wire (12V battery) power from the truck, not worry about depleting it, and not have to customize my trucks' wiring.

I could mount both devices to the outside of the under-floor battery box on the rear side and maybe put a wire screen around them to protect from road debris strikes.

 

[Travelover]

...............It seems like the wire size could be a real problem if the trailer battery is low and draws a large current to charge itself. I could put larger wire on the truck and trailer easily EXCEPT the 7-wire trailer plug, since the wire opening of them is typically not huge ...............

This is what I did when I wired my truck to pull a travel trailer. At the plugs, I down sized to 10 gauge wire. You can still pull 30+ amps through the thinner wire without overheating it and it is doubtful you are going to hit the trailer battery with that much current for any length of time.

It is a little tough to wrap your head around this, but the reason for the large gauge wire in a two battery system is to prevent voltage drop at the far battery, not to prevent the wires from overheating. Look at the gauge of wire running from the alternator to the tow vehicle battery.