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

handirifle · Jun 10, 2012

This is interesting to me. Why is one of the wires in the 7 pin plug designated the 12v battery charging wire, if it will not handle the load of charging a battery?

All of my RV 7 pin plugs have been wired this way and I have never had an issue. What makes his setup different? I am missing something I guess. It seems to me running the battery of an RV completely down is a very real possibility. Is there something in the RV limiting how fast it charges or how many amps it draws from the tow vehicle?

rasimmo · Jun 10, 2012

handirifle said:
This is interesting to me. Why is one of the wires in the 7 pin plug designated the 12v battery charging wire, if it will not handle the load of charging a battery?

All of my RV 7 pin plugs have been wired this way and I have never had an issue. What makes his setup different? I am missing something I guess. It seems to me running the battery of an RV completely down is a very real possibility. Is there something in the RV limiting how fast it charges or how many amps it draws from the tow vehicle?

The limit is the fuse on the truck protecting the circuit.

tcartwri · Jun 10, 2012

Travelover said:
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.

I agree, but the wire to the alternator is short and in open air. It's also usually 10awg or better.

Ampacity tables will give the rated current for chassis wiring. #8 is the minimum you want for this run.

Two batteries on the same circuit is always a compromise, and usually one battery will fail prematurely from over charging.

Travelover · Jun 10, 2012

tcartwri said:
.............Ampacity tables will give the rated current for chassis wiring. #8 is the minimum you want for this run. ..............

I agree with the wire gauge, but not for the why. The large gauge is to minimize voltage drop at the trailer battery, not to keep the wire from melting. Lots of people wire their trailer charging circuits with light gauge wire. The result is the trailer battery doesn't charge properly, especially if they try to run the refrigerator on 12 volts at the same time. The wire doesn't melt, as it is only creating a couple volt drop at most.

handirifle · Jun 10, 2012

OK, but the fuse is limited by the AWG, not the load at the end. I haven't ever looked to see what gauge wire goes to the batt terminal of my 7 pin, but I will. How many amps will the batt connector of the 7 pin handle? Will it handle the load of a #8, should the RV battery go onto full charge mode?

Travelovr
Given what you say, it seems reasonable to me the RV battery would likely last longer due to under charging, since I doubt #8 wire or similar is used. Am I correct in this assumption?

Diamondpilot · Jun 10, 2012

jeffsw6 said:
I would like to change my 7-wire center pin to provide charging power for an accessory battery. I only want it to be powered when the engine of the truck is running, that way the truck battery won't run down if things are left on while the rig is parked.

How do I get power from the truck only when the engine is running? I would rather not have to tear into the dashboards and spend all day poking at things with a VOM if I can just tap into the alternator cabling inside the engine compartment or something.

If this is on your Ford everyone I have dealt with are only powered when the key is on via a relay that must be installed in the fuse panel. I know on some other brands they are hot all the time so maybe some checking of when its powered first is in order.

Chris

tcartwri · Jun 10, 2012

Travelover said:
I agree with the wire gauge, but not for the why. The large gauge is to minimize voltage drop at the trailer battery, not to keep the wire from melting. Lots of people wire their trailer charging circuits with light gauge wire. The result is the trailer battery doesn't charge properly, especially if they try to run the refrigerator on 12 volts at the same time. The wire doesn't melt, as it is only creating a couple volt drop at most.

The problem will only occur when there is a big differential in the truck battery and the trailer battery. The truck battery will push as much current to the trailer battery as it can, until they equalize. The battery is not current limited like the alternator is.

RobertN · Jun 10, 2012

I have hooked up my 5th wheel when the two coach batteries were pretty low. I have never had a problem. I have the stock wire harness on my '01 Dodge. Looking at the book for my 5th wheel, it does not have anything to limit charging from the 7-wire connector.

handirifle said:
This is interesting to me. Why is one of the wires in the 7 pin plug designated the 12v battery charging wire, if it will not handle the load of charging a battery?

All of my RV 7 pin plugs have been wired this way and I have never had an issue. What makes his setup different? I am missing something I guess. It seems to me running the battery of an RV completely down is a very real possibility. Is there something in the RV limiting how fast it charges or how many amps it draws from the tow vehicle?

jeffsw6 · Jun 11, 2012

Diamondpilot said:
If this is on your Ford everyone I have dealt with are only powered when the key is on via a relay

I think I am going to go with that smart voltage-sensing relay plus the fancy charging device. Not cheap, but it seems like the most fool-proof way of setting things up.

Travelover · Jun 11, 2012

tcartwri said:
The problem will only occur when there is a big differential in the truck battery and the trailer battery. The truck battery will push as much current to the trailer battery as it can, until they equalize. The battery is not current limited like the alternator is.

The current flowing from one battery to another is determined by the voltage pushing it. For instance, if you take a fully charged battery and hook it to a dead battery, they will slowly equalize due to the small voltage difference between them and the internal resistance of the dead battery. No big inrush.

A fully charged battery is about 12.7 volts. A dead battery is around 11 volts. This is why an alternator has to push with 14.5 volts to charge a battery fully.