Charging Airstream batteries from tow vehicle

[Jammer]

The conventional wisdom is that the batteries in the TT are charged little, if at all, by the TV alternator.

The main reason for this is that neither TV nor TT makers try very hard. It's a solvable problem. There are three parts to the problem:

1) The electrical path from the alternator to the trailer is indirect and, generally, high resistance. Most charging lines use 10 gauge wire, which has a resistance of about .001 ohms per foot. With a 60 foot round trip, that's .06 ohms, which provides a 1.8 volt drop at the 30 amp charging rate we'd (minimally) like to achieve. That's too much, because it drops the roughly 14.5 volts at the alternator output terminals down to to 12.7 volts at the battery, which won't result in any charging.

2) The alternator's voltage sense line is connected to the TV battery, or somewhere close to it electrically, so the voltage regulator in the TV won't adjust to compensate for the losses.

3) The 7-way RV connector most of use to connect the TV to the TT is rated for 30 amps, which would be underrated in some cases if we actually solved the problems above. The 10-gauge wire would be marginal.

Items 1 and 3 are easy to solve. Just run bigger wire and a better connector.

For wire, welding depot has welding cable (more flexible than battery cable and easier to install) by the foot in sizes from #4 to 4/0. As an example, the 1/0 cable would provide 1/10th the resistance of a typical stock 10 gauge wire, increasing the charging rate at a 0.2 volt drop from 3.3 amps to 33 amps even with a stock alternator and no change to the voltage sense circuit (with proper attention to connectors). You could buy enough to wire the TV and TT for less than $200.

For connectors, the choices are to use a heavier connector instead of the 7-way, or a heavier connector in addition to it. In the "instead of" department, the floating-pin 7 way connectors used mainly by truckers are rated for 40 amps. They are readily available and inexpensive. There is also a very similar 9-pin connector, which while uncommon, provides extra pins for a voltage sense function (more on that below). Photos and info in the 7-way and 9-way connectors here: Trailer Connectors In the "in addition to" department, there's the Anderson Power Pole connectors (Anderson Powerpoles & Accessories:), which aren't pretty and don't mount cleanly to a vehicle, but are inexpensive and are available in various sizes up to 350 amps, and also a two-pin connector the shape and size of the 7-way, rated for 100 amps and used in electric lift gate applications (Trailer Parts Superstore - Zinc Die-Cast Dual Pole Socket #15-326)

A remote ammeter connected through a shunt would complete the installation by showing that charging is taking place while the rig is being driven.

Another alternative would be to install a second alternator in the TV and wire it to (only) charge the TT batteries. Then, the voltage sense line could be connected to the TT batteries through a separate circuit (by using one of the extra pins in a 9-pin connector, for example, or by repurposing the battery line in the 7-way if adding a separate connector for charging). The voltage regulator would then adjust for the voltage drop in the wiring to the TT battery, within reason. Such a setup could easily charge even a large TT battery string at the C/4 bulk charge rate (around 60 amps for two group 31 batteries or two golf cart batteries).

I'm surprised more people don't do this, given that it could serve as an alternative to the expense and hassle of a generator in many cases.

[azflycaster]

If you are sensing the condition of the trailer battery, wouldn't you cook the trucks battery since it needs very little charge in most cases?

[Mrwaterway]

In my Chevy's main fuse box in the engine compartment there is a heavy 30AMP fuse dedicated to charging the AS battery when connected. Does not seem to affect amperage performance of the Alternator and charges a almost dead RV Battery in less than one hour. You have to add the fuse, it does not come installed.

[ROBERT CROSS]

Tell me again why I would want to charge two AGM's with one alternator.

BTW...you will have a very difficult time charging any dead deep cycle with the TV, certainly not... in an hour. The alternator would not charge fully at idle anyway & why hold 1500rpm for an hour.

[Jammer]

Originally Posted by azflycaster

If you are sensing the condition of the trailer battery, wouldn't you cook the trucks battery since it needs very little charge in most cases?

The TV battery would charge at slightly higher than desired voltage only during the time that a substantial amount of current is flowing to the batteries in the Airstream. There would be some gassing, so it would be important to check the water levels frequently.

[barts]

The charging rate at idle depends on the TV's alternator design, and how discharged the trailer's batteries have gotten. My diesel pickup has a 160A alternator; it charges the vehicle batteries very rapidly even at idle.

As the OP said, though, the voltage drop will cause slow charging of the trailer batteries in any case.

You can take a small inverter, drive it via the tow vehicle's charging lead and connect that to a battery charger in the trailer. This will correct the voltage issue. Make sure the inverter & charger won't draw more than the TV can supply; there may be issues w/ the charger not liking the inverter harmonics.

A Newmar 12-12-351 (factory modified as a charger) is a pretty fancy way of doing this; the car audio folks have designed some lower cost (and prob. lower reliability) units such as are seen here:

12 volt DC/DC converters regulator from 100 to 700 Watts.

- Bart

[Jammer]

Originally Posted by ROBERT CROSS

Tell me again why I would want to charge two AGM's with one alternator.

Because it saves you the weight and hassle of carrying a generator.

BTW...you will have a very difficult time charging any dead deep cycle with the TV, certainly not... in an hour. The alternator would not charge fully at idle anyway & why hold 1500rpm for an hour.

There are larger-frame alternators available for all full-size trucks that will produce around 75 amps at idle. They are a factory option in most cases and can be retrofitted if necessary, and are not particularly expensive; stock ones typically are rated at around 150 amps max and overwound ones with external diodes are available with ratings twice that or more. (See for example the CS-144 for GM products)

A deep-cycle battery should, ideally, not be discharged until dead nor be charged in a hour once this occurs (though AGMs will tolerate it). A more realistic use case would be to discharge to 50% and recharge to 90% over the course of two hours while moving to another campsite. Alternatively, the vehicle could be idled, as you suggest, which while something of a nuisance wouldn't be louder etc than running a small generator for that period of time.

[Jammer]

Originally Posted by barts

You can take a small inverter, drive it via the tow vehicle's charging lead and connect that to a battery charger in the trailer. This will correct the voltage issue. Make sure the inverter & charger won't draw more than the TV can supply; there may be issues w/ the charger not liking the inverter harmonics.

There are two difficulties in this case. One is that considerable care would be required in installation if such a setup were to be used while towing, especially in the rain, due to the 120v run from the TV to the Airstream.

The other is that, as you note, the system has to be sized so that everything matches. The charger in the trailer will set the charging rate, and if the inverter can't supply it the breaker will trip. If the alternator can't keep up with the inverter, the TV starting battery would be drained.

[Inland RV Center, In]

All modern tow vehicles, can easily charge the trailer batteries. Thousands of owners, do it everyday, without any problems.

However, there are some "must haves" in order to do so.

The charge line wiring within the TV, must be 10 gauge or larger.

The trailer/TV connectors must be in excellent condition.

If charging from 12 volts is inadequate, wow, we then have hundreds of thousand of RV owners that have dead batteries. NOT.

To go through the trouble of supplying 120 AC to the converter that's in the trailer, is to me, a waste of time trying, and a waste of money.

First off all, the TV alternator would "NOT" run at constant speed, therefore it cannot continuously provide 120 volts AC, at 60 cycles, which is required. Any frequency for the 120 volts AC much different than that, will more than likely, burn up the 12 volt DC converter.

Or, did I miss something in the original post?

Andy

[war eagle]

What I saw in the original post was discussion about potential use of larger than stock gauge wiring (with an upgrade at the connector interface between the TV and TT) on the charging lead from the TV to the TT to reduce the voltage drop caused by current flow - interesting. This voltage drop does in effect function as a limit to the current the TV can supply to the TT. His second discussion was the use of a second engine driven alternator, already optional on some (such as Ford F250) trucks, and a dedicated voltage sense line going back to the TT batteries. So the second alternator would be dedicated to charging the TT batteries (no connection to the +12 of the TV batteries) and could supply considerable current and charge the TT batteries quickly, if the wire gauge were adequate. The OP suggests these approaches as potential alternatives to purchasing and carrying a generator.

Steve

[Mr Stuff]

We seem to own Land Yachts. Changing 2 battery banks is done all the time on Sea Yachts. Engine battery House battery. They must be isolated, so that the engine will always start when you leave the light on all night and kill the house bank. Look at some of the marine electrical suppliers under voltage regulation. It can be done but costly.

[ROBERT CROSS]

Originally Posted by Jammer

Because it saves you the weight and hassle of carrying a generator.

Hassle to you....convenience to me.

Granted after a three hour trip to our favorite campsite we all charged up.

But two dead deep cycles fully charged in an hour?... idling at the campsite, waste of time & gas. A modern genset is much quicker and more efficient.

Rave-on..

Stream Safe..

[65CV]

I wish I had seen Jammer's post before closing off the belly pan. It makes perfect sense to deliver the proper charging voltage to the trailer batteries using beefed up cable and connectors -- not only for the initial charge, but especially in "topping off" the battery.

If you look at the charge curve for a battery, it gets tougher to top off as the battery approaches 90% or higher charge. Essentially, you're running the generator to power a trickle charger, often wasting power and making mostly noise. The Tow Vehicle could do this, essentially for free. (A 3-stage converter does improve the situation, if you have one.)

Mr. Stuff reminded me of the battery isolator we had on the big boat. We sold it years ago, and I can't remember the part used to protect one battery from another. It was something like this: ME-SBC Smart Battery Combiner

A combo of the two ideas might work really well. These forums work once in a while, don't they?

John

[65CV]

I made the comment in my last post that "These forums work once in a while, don't they?"

I was lucky to be right this time.

I received a PM from a long-term forum member who I really respect. He is a professional and happens to supply the part that I mentioned. He said that it would be perfect for the application. Jammer and MR. Stuff are right on track. PM me if you think it would work for you and I'll be more than happy to connect you.

I am concerned that sometimes, we weekend warriors pontificate on what we think we know.

I defer to people like Jammer. He seems to really know his stuff.

If you read anything from me, it is made in good faith, is based on a Masters in Engineering, NOT Electrical Engineering, and is based on discussions with my son, an apprentice electrician. I am NOT qualified to give professional advice, but can refer you to someone who can.

Good stuff, Jammer. You rock.

[Lumatic]

Perhaps my ignorance is my bliss, but I have never felt charging the tt battery while towing was a problem.

[Jammer]

Originally Posted by Inland RV Center, In

All modern tow vehicles, can easily charge the trailer batteries. Thousands of owners, do it everyday, without any problems.

However, there are some "must haves" in order to do so.

The charge line wiring within the TV, must be 10 gauge or larger.

The trailer/TV connectors must be in excellent condition.

If charging from 12 volts is inadequate, wow, we then have hundreds of thousand of RV owners that have dead batteries. NOT.

There is a general sentiment in the broader RVing community that the power from the TV cannot be relied upon to provide any meaningful amount of charging. Let's take a closer look. As you note, a typical tow vehicle has 10 gauge wire to the trailer, typically protected by a 30 to 40 amp fuse or circuit breaker, depending on the TV. Often this circuit also feeds the clearance lights on the trailer, through a relay.

Under optimal conditions -- clean, tight connections and relatively new batteries in the Airstream -- the math in my initial post tells us that we're going to max out somewhere below 30 amps, because at 30 amps the voltage at the batteries in the Airstream is going to be 12.7, and at 12.7 volts the batteries won't take 30 amps. What will happen, more typically, is that the batteries will charge at around 15 amps, and if there's a 12 volt fridge or another electrical load in the trailer, the charging rate will be reduced by something like half the amount of the load. As the batteries in the Airstream get older and their internal resistance increases, this rate will drop. It will also drop as the batteries approach full charge.

At this rate, with two group 31 deep cycle batteries, it will take around 8 hours of towing to get from 40% to 90% charge. So yes, the batteries will charge, after a fashion. But they will not charge enough that in a typical move from one boondocking site to another any substantial charging will take place, and they will not charge enough that it makes sense to idle the tow vehicle engine for a period of time just to charge the batteries. Hence, there are people who carry a generator, for no other reason than to charge the batteries.

Originally Posted by ROBERT CROSS

Hassle to you....convenience to me. [...] idling at the campsite, waste of time & gas. A modern genset is much quicker and more efficient.

Gensets have their advantages, among them the possibility of running air conditioning. They also have cost and weight associated with them and in many setups require their own fuel source and require that the generator be physically moved from a storage to operating location. While there exist well-integrated generator installations in Airstreams, that draw fuel from the propane tanks on the tongue and deliver power through a transfer switch, they are rare due to the various difficulties they pose. While it is true that there are efficiency tradeoffs, they are not as great as one might think, because modern pickup truck engines are more efficient at idle than the small gasoline engines in gensets. If we were willing to pay for a water-cooled genset engine with sequential multipoint fuel injection, oxygen sensor, computer, etc., the tradeoffs would be more considerable.

A well designed dual alternator setup on a tow vehicle could easily deliver 75 A to the batteries, at idle, more than any but the more sophisticated generator/converter combinations.

To go through the trouble of supplying 120 AC to the converter that's in the trailer, is to me, a waste of time trying, and a waste of money.

First off all, the TV alternator would "NOT" run at constant speed, therefore it cannot continuously provide 120 volts AC, at 60 cycles, which is required. Any frequency for the 120 volts AC much different than that, will more than likely, burn up the 12 volt DC converter.

Or, did I miss something in the original post?

Andy

Andy, I think we're in vehement agreement on that. ;-) That wasn't the suggestion in my initial post. I believe you're replying to a comment by barts.

[ROBERT CROSS]

Jammer,

So....... when your finished looking, do it your way.

I'll do it mine, and we'll both be happy.


Stream Safe...

[barts]

What I was trying to say was that the problem of reduced charging rate due to voltage drop has several solutions, some of them commercially available:

Newmar DC-DC Converters

These units will allow your towing vehicle to charge the batteries correctly if configured as a charger. The isolated units take 10-16V in and deliver 13.6V out.


- Bart

[Dwight]

Good info...