Dual Alternators

[AnotherMike]

I'm reading that the charge line from the truck battery and back to the 7 way trailer connector has a 15 amp fuse. What gauge wire is it? It probably has a measurable voltage drop, and the more current pulled the lower the charging voltage at the trailer battery...

What gauge wire from the trailer 7-way connector to the trailer battery?

Are there any additional unintentional voltage drops between the truck battery and the trailer battery?

In my opinion your goal should be to minimize the voltage drop between the truck battery and the the trailer battery... this will maximize the charging from the truck alternator.

Note that 3/4 of a volt is roughly the difference between fully charged and 50% charged on a lead-acid battery... so if you have 3/4 of a volt drop between the truck battery and the the trailer battery that means that the trailer battery will never get above 50%... severely limiting the run-time of the trailer loads (i.e. refrigerator, lighting, etc).

One way to resolve this voltage drop is to use a fatter wire (lower gauge number, i.e. #8 instead of the #10, #12 or #14 that is there now) as the charging line, another way is a device called a DC-to-DC charger that you could add inside the trailer battery compartment... where the trailer connecting cord meets the battery.

Here's a table I cribbed a long time ago from another posting...

Volts %Charge

12.7 to 12.8 100%
12.6 90%
12.5 80%
12.3 70%
12.2 60%
12.1 50%
11.9 40%
11.8 30%
11.6 20%
11.5 10%

Below is a JPG of another table cribbed from a posting... the numbers are somewhat different.

Mike

[shane wood]

Originally Posted by Calicajun

Mostly just wondering about traveling down the road for six to 8 hours a day. The truck we are looking at comes with the option of dual alternators for a price of course.
Don't plan on a lot of boondocking but will be some and sometimes maybe a stay in a parking lot over night.
Do have a 2000 watt generator now and plan on adding solar 300 or 600 watts. But solar is a question for another thread.

Go with the dual alternators
Truck manufactots came up with this option do to people adding all kinds of electronic goodies to their vehicles

[theoldwizard]

Originally Posted by twinboat

A single alternator will be fine.

Originally Posted by tuffr2

I had a truck with two alternators and two batteries. It did not seem to charge the trailer battery very fast. I tried it once and remember it took much longer than I thought it should for just a partial charge.

Save your money on the second alternator and buy a DC-DC battery charger for your house bank. Several different brands on the market. They actually BOOST the marginal voltage you get from your vehicle to the appropriate voltages to do a multi-step charge.

And you do NOT have to install any OVERSIZED charging cables !

[theoldwizard]

Originally Posted by AnotherMike

I'm reading that the charge line from the truck battery and back to the 7 way trailer connector has a 15 amp fuse. What gauge wire is it? It probably has a measurable voltage drop, and the more current pulled the lower the charging voltage at the trailer battery.

What gauge wire from the trailer 7-way connector to the trailer battery?

Are there any additional unintentional voltage drops between the truck battery and the trailer battery?

Whoa, whoa, WHOA ! You are going the wrong direction !

All car/truck manufacturers has been using a "smart charging system" on all vehicles since around 2000. Smart charging limits the voltage delivered to the starting battery to about 13.2V within a few minutes of the engine starting. This is inadequate to PROPERLY recharge a house bank.

HD alternators, or second alternators, are primarily for vehicles that have a large loads when the engine is running (multiple warning lights, electric pumps, etc) and no "house bank" that are low on charge.

What you need is a DC-DC battery charger like the Renogy DC to DC Battery Charger with Multi-stage Charging. That is the 20A model but they make ones go up to 60A. These do a MUCH BETTER JOB of recharging a house battery bank because they actually boost the votage that is being applied to the battery bank. Watch this video.
Renogy DC-DC Battery Charger Review (similar products are available from other manufacturers).

[CharlesinGA]

Contrary to what OldWizard is saying, a DC to DC charger will need a separate set of rather heavy cables to do its job properly. The smallest DC/DC chargers are 20 amp and the largest common ones are about 50 though I am sure there are larger. You would want to run a separate set of properly sized cables to the charger, expect them to draw about ten amps more than their rated output and then use an Anderson Connector at the bumper to make a connection to the trailer (mount the DC/DC charger on the trailer near the batteries). Good ones also have a connection for solar input. This setup is quite common in Australia and has come over here in the past few years.

The problem with expecting the vehicles alternator to directly charge the batteries is 1) the way undersize charge line to the 7 pin, really only capable of keeping charged a small SLA battery for break away purposes on a cargo or car hauler, etc. 2) and most importantly the fact that new computer controlled charging systems, which are devoid of traditional voltage regulators, do not run at the high 14.7 volts or so that traditional regulator systems of the past did. To conserve fuel and since the system only is designed to operate the VEHICLES accessories and devices, typically run at a much lower voltage closer to 13.5 or less, barely enough to keep the vehicle's battery charged and not enough to fully top RV batteries. The DC/DC charger overcomes this by bucking the voltage up to as much as 14.7 to bulk charge a deep cycle battery and then top it at 13.6 or so and even dropping to 13.2 or so if the batteries do become fully charged during the drive. It is truthfully not anything different than your built in converter, except that it is powered by a 12v DC system and has the capability to buck the voltage up beyond what it receives.

As someone noted the Anderson connector, at the rear bumper also makes a good place to connect a winch if needed on the rear hitch. The Blue Sea chart in this article (click to enlarge the chart) will show you that a #6 or possibly a #4 sized cable will be needed to provide power to the Anderson connector. For the short run on the trailer you could drop down a size and not compromise the systems operation.

https://www.bluesea.com/support/arti...r_a_DC_Circuit

Powerwerx is one good source of siamese zip cable and Anderson connectors, and all things DC electrical. https://powerwerx.com

Renogy is a good source of DC/DC chargers and Redarc is also. Theirs are weatherproof and can be mounted in the open on a trailer tongue if needed. https://redarcelectronics.com/

I eventually intend to install a Redarc 40 amp DC/DC unit on my Bigfoot when I upgrade the battery system. I'm not interested in Lithium and Bigfoot made space for large batteries but didn't give the height for a regular golf cart battery and I will end up using a shorter battery like the Full River Group 27 AGM deepcycle batteries.

Charles

[Calicajun]

More very informative answers, giving me lots more to think about and read up on.



Just is case any one was wondering, it is the wife that talked me into the Residential fridge. It if were me I still be in a 28' TT with no slides and a RV fridge. Yes, I'm a bit behind the times.


Again thanks for all the answers,
Craig

[theoldwizard]

Originally Posted by CharlesinGA

Contrary to what OldWizard is saying, a DC to DC charger will need a separate set of rather heavy cables to do its job properly. The smallest DC/DC chargers are 20 amp and the largest common ones are about 50 ...

I don't know about your vehicle, but the trailer battery charge wire on my vehicle has a 40A fuse ! So yes, I would stick with the 20A version.

Originally Posted by CharlesinGA

I eventually intend to install a Redarc 40 amp DC/DC unit on my Bigfoot when I upgrade the battery system.

Before spending a lot of money and time and effort on oversized wiring, check to see what your battery charge wire is fused at !

[theoldwizard]

Originally Posted by Calicajun

Just is case any one was wondering, it is the wife that talked me into the Residential fridge. It if were me I still be in a 28' TT with no slides and a RV fridge. Yes, I'm a bit behind the times.

First, the old saying, "If Mama ain't happy, nobody's happy !"

Second, when it is in the 90°s with matching humidity, there is nothing like a 40° beer or plenty of ice for your other adult beverages ! Also, non-mushy ice cream !

I personally have never seen an evaporator refrigerator to is capable of doing that when the ambient temperature is above 90°.

[tuffr2]

Just an idea, probably a good idea I think. But what if you sprung for a RV Litium Ion battery. The last I saw they were no longer $1,000 but more like $700 from Costco.

Not sure what may need to change with the converter.

[theoldwizard]

Originally Posted by tuffr2

Not sure what may need to change with the converter.

Most "stock" converters do not have the proper charging profile for LiFePO4 batteries. If you are going lithium, no would be a great time to remove the old convert, leaving the DC fuse panel, and install a combination inverter/charger/transfer switch.

[tuffr2]

I double checked the cost of an RV Litium battery. They are too expensive. You can buy a small one for $700 but the high capacity batteries are $2,000.

[Calicajun]

Good thing we have some time, about 8 months) before we buy both the RV and truck. Because you folks have given me lots to think about and investigate. We are pretty sure we want the residential fridge, as we always seem to end up in the hot part in the summer and cold area in the winter.

Thanks,
Craig

[CharlesinGA]

Originally Posted by shane wood

Go with the dual alternators
Truck manufactots came up with this option do to people adding all kinds of electronic goodies to their vehicles

The idea behind dual alternators is to provide more AMPERAGE to operate specialized equipment directly connected to the vehicles electrical system. Things like Ambulance boxes with all kinds of lights and medical equipment, electrical winches, hoists, electro hydraulic pumps, etc.

The full current of the second alternator will not be available to the trailer due to the very limited stock electrical connection, and due to the computer's charging profile, which maintains too low a voltage for proper charge and top off of the batteries on the trailer, makes the second alternator a poor choice for this type of rig.

The stock alternators on most modern trucks has more than enough capacity to charge the trailers batteries thru a DC/DC charger however.

Charles

[hamm2018]

Originally Posted by Calicajun

We are in the market, looking for a 5th wheel and new truck. Thinking about getting a 5'er with a residential fridge for the extra space. Wondering if we should get a truck with dual alternators in order to keep the batteries charged while driving down or up the road? Planning on (2) 6v batteries in series for the fridge and (2) house batteries. Or will one alternator be enough for this load?


Thanks,
Craig

We have a 2018 Road Warrior with a residential fridge and our 2017 Ram does have factory installed dual alternators. No issues so far with any charging or running while on the road. Road Warrior has 2 12 volt batteries and we generally drive about 300 to 325 miles a day.
All equipment in Road Warrior and Ram factory, have not added any extra batteries, chargers.