Alternator to trailer wiring gauge - too small?

[michaelrider5]

I am about to install a Renogy Charge Controller in my trailer. It combines an MPPT solar controller with a function that maximizes the potential out put to the trailer batteries from the truck alternator. The 2008 Tundra 5.7 tow vehicle has a 150 Amp alternator, however the wiring to the 7 prong trailer plug all the way at the back bumper is 10 gauge at best, as is the wiring on the trailer side to the batteries. I have twin 100 watt solar panels, but on rainy or cloudy days or even weeks, the truck alternator barely keeps the house batteries charged when I'm on the move, and I end out having to drag out my generator. On a previous vehicle I ran a 8 g wire direct from the alternator to the 7 prong plug to resolve a similar issue. Any recommendations? I'm sure others have run across this issue. The ads and videos for the charge controller don't address voltage drop or wire gouge at all.
I'd be grateful for any advice!

 

[Utclmjmpr]

It sounds as if you need to determine you usage and shed some of it when not needed,(as in when your traveling ) >>>Dan

 

[Mark_K5LXP]

What controller is it? If it's the Renogy DC-DC charger there's a wire gauge and distance table in the unit manual. If not, it would be useful to know what kind of currents this charger/controller is capable of drawing from the vehicle.

Mark B.
Albuquerque, NM

 

[michaelrider5]

Thanks for your replies. The DC-DC Charge controller is a 30A unit. I did some more research. The trailer pig tail from a junction box on the trailer frame to the 7 pin connector has 10 gauge power and ground wires. The truck has a 40A dedicated fuse at the battery, followed by a relay that cuts power to the bumper mounted 7 pin connector when the key is off. It also has a 10gauge wire.

 

[michaelrider5]

Thanks for your replies. I did some more research. The DC-DC charge controller is a 30A unit. The trailer has a 6-7' pigtail running from a junction box to the truck bumper with 10g wires for the power and ground. It is about another 4 feet to the battery terminals, a total of 10-12' feet with a couple of junctions. The truck has a dedicated 40A fuse, a relay that cuts the power with the key off, and about 20' of 10G wire to the 7 pin connector at the bumper.
Blue Seas Systems, a supplier of marine electronics has a wiring gauge chart that indicates 10 gauge wire is good for 30 feet at 30 amps for "non-critical" systems. There is a 10% voltage drop at that gauge.
So the truck battery is about 30' total from the "house" battery, right at the maximum recommended distance. For "critical" systems, the same chart recommends 4 gauge wire, which gives '0' voltage drop over that distance at 30A. Based on those numbers, in an ideal world, if I wanted to maximize the benefit of the charge controller, I ought to run a 6 or 4 gauge wire from the truck battery to to the charge controller, with a dedicated connector at the bumper or install new 7 pin connectors that will accommodate the larger gauge. Better get out my wallet! It just happens that cable for arc welders is 4 gauge and is a third the price of "automotive" or "marine" rated wire.
Thanks again for reading my post!

 

[Mark_K5LXP]

The reason to use the DC-DC is it gives you the capability to maintain a given current irrespective of that wiring voltage drop. The alternator puts out a nominal 14.2V but with voltage drop the current delivery to batteries after 30' or whatever of cable will be fairly diminished. Unlike the house batteries that are very voltage senstive, the DC-DC converter can operate over a wide input voltage range, which translates to delivering it's full current even with a few volts of drop at it's input. So if you were to put the converter right at the batteries it would likely deliver the full 30A it's capable of, where without it you might get only half of that or less. So choosing wire gauge vs current the formula for acceptable voltage drop changes with the converter due to it's wide input voltage range.

Running some rough numbers based on full bulk current delivery for a 30A converter it needs at least 10.8V, so about 3.6V or less of drop between the alternator and converter would be within the current rating of the 7 pin connector. If your measured drop at the converter input is less than 3.6V with the cable and connectors you have, it will work OK.

Mark B.
Albuquerque, NM

 

[CharlesinGA]

Run new 4 gauge cables from the truck to the DC-DC charger using an Anderson connector at the bumper. This is quite common. If the DC-DC charger has a low voltage disconnect, it will disconnect shortly after the engine is shut off and won't drain the two vehicle battery. If you are uncertain, a relay with a voltage sensing board to switch the relay can be installed at the truck battery to shut off the power going down the 4 gauge as soon as the alternator quits putting out and the battery voltage drops, again disconnecting your battery from the trailer so it doesn't go dead while hooked up.

Don't come off the alternator, you tap off at the battery. Also don't be surprised if your Tundra is using half of the alternator output, or more on a regular basis, running computers, A/C clutche and blower, fuel pump, and a whole host of other small items.

Charles

 

[Lou Schneider]

You said you have not installed the Renogy yet? What is it's model number? If it has provision for alternator charging it may automatically boost the low voltage coming from your existing charging wire so you may not need to replace it.

 

[Alontheway]

I also used an Anderson plug. Forget the 7-pin for charging, though keep it wired into the system as normal because things change...
WIth the Anderson plug I used heavy gauge wire, like pinky finger size. I ran both positive and negative wires. The neg I just had attached to the truck frame right near the plug and to the camper frame near the plug as the frame is ground.
Off the truck I connected the pos wire to the starter terminal positive as this is closer than going further forward with expensive wire to get to the battery or alternator. The Starter wire is heavy gauge and already run.
I installed a solenoid close to the starter positive terminal and was able to turn it on or off so it could be off when not towing (no 12 volt live wire when not needed). I also turn it off when towing in the sun as I do not need alternator power with the solar working. It rarely was on in reality but was there when needed and provided better power than the little trailer plug wiring.

I have not tried DC-DC charging through the 7-pin plug so can't give feedback there. That, if it works as great as they say, would be easier.