Reply
 
Thread Tools Search this Thread Display Modes
 
Old 01-09-2010, 01:16 PM   #1
Wise Elder
 
Jammer's Avatar
 
2010 30' Classic
Vintage Kin Owner
South of the river , Minnesota
Join Date: Dec 2009
Posts: 4,119
Charging Airstream batteries from tow vehicle

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.
__________________

__________________
Jammer is offline   Reply With Quote
Old 01-09-2010, 01:38 PM   #2
Site Team
 
azflycaster's Avatar
 
1975 25' Tradewind
Dewey , Arizona
Join Date: Mar 2005
Posts: 12,125
Images: 62
Blog Entries: 1
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?
__________________

__________________

Richard

Wally Byam Airstream Club 7513
azflycaster is offline   Reply With Quote
Old 01-09-2010, 02:07 PM   #3
2 Rivet Member
 
1972 29' Ambassador
Fort McCoy , Florida
Join Date: Aug 2009
Posts: 33
Images: 16
Blog Entries: 1
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.
__________________
Mrwaterway is offline   Reply With Quote
Old 01-09-2010, 02:43 PM   #4
CLOUDSPLITTER "Tahawas"
 
ROBERT CROSS's Avatar

 
2003 25' Classic
Zanadude Nebula , WNY
Join Date: Jun 2007
Posts: 11,153
Images: 1
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.
__________________
PFC.....

ďAfter all these years the reason I continue to love Thanksgiving.....I still sit at the kids table.Ē
RLC

Sandra wanted to go to Cleveland on vacation,
but Iím the Husband, so we went to Cleveland.
RLC
ROBERT CROSS is offline   Reply With Quote
Old 01-09-2010, 04:03 PM   #5
Wise Elder
 
Jammer's Avatar
 
2010 30' Classic
Vintage Kin Owner
South of the river , Minnesota
Join Date: Dec 2009
Posts: 4,119
Quote:
Originally Posted by azflycaster View Post
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.
__________________
Jammer is offline   Reply With Quote
Old 01-09-2010, 04:07 PM   #6
Maniacal Engineer
 
barts's Avatar
 
1971 25' Tradewind
Menlo Park , California
Join Date: Jan 2010
Posts: 1,223
Images: 1
Blog Entries: 4
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
__________________
Bart Smaalders
Menlo Park, CA
http://tinpickle.blogspot.com
http://smaalders.net/barts
barts is offline   Reply With Quote
Old 01-09-2010, 04:18 PM   #7
Wise Elder
 
Jammer's Avatar
 
2010 30' Classic
Vintage Kin Owner
South of the river , Minnesota
Join Date: Dec 2009
Posts: 4,119
Quote:
Originally Posted by ROBERT CROSS View Post
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.

Quote:
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 is offline   Reply With Quote
Old 01-09-2010, 04:21 PM   #8
Wise Elder
 
Jammer's Avatar
 
2010 30' Classic
Vintage Kin Owner
South of the river , Minnesota
Join Date: Dec 2009
Posts: 4,119
Quote:
Originally Posted by barts View Post
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.
__________________
Jammer is offline   Reply With Quote
Old 01-09-2010, 05:02 PM   #9
Rivet Master
Airstream Dealer
 
Inland RV Center, In's Avatar
 
Corona , California
Join Date: Feb 2002
Posts: 16,499
Images: 1
12 volt DC charging

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
__________________
Inland RV Center, In is offline   Reply With Quote
Old 01-09-2010, 06:59 PM   #10
2 Rivet Member
 
1989 32' Excella
Limestone , Alabama
Join Date: Sep 2009
Posts: 38
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
__________________
war eagle is offline   Reply With Quote
Old 01-09-2010, 09:03 PM   #11
New Member
 
1976 31' Sovereign
Cumberland , Rhode Island
Join Date: Apr 2007
Posts: 4
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.
__________________
The only person who does not make a mistake, is the one who isn't doing anything.
Mr Stuff is offline   Reply With Quote
Old 01-10-2010, 07:27 AM   #12
CLOUDSPLITTER "Tahawas"
 
ROBERT CROSS's Avatar

 
2003 25' Classic
Zanadude Nebula , WNY
Join Date: Jun 2007
Posts: 11,153
Images: 1
Quote:
Originally Posted by Jammer View Post
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..
__________________
PFC.....

ďAfter all these years the reason I continue to love Thanksgiving.....I still sit at the kids table.Ē
RLC

Sandra wanted to go to Cleveland on vacation,
but Iím the Husband, so we went to Cleveland.
RLC
ROBERT CROSS is offline   Reply With Quote
Old 01-10-2010, 07:50 AM   #13
Rivet Master
 
65CV's Avatar
 
1959 26' Overlander
Western , Massachusetts
Join Date: Sep 2005
Posts: 1,449
Images: 6
Good thought

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 is offline   Reply With Quote
Old 01-10-2010, 06:32 PM   #14
Rivet Master
 
65CV's Avatar
 
1959 26' Overlander
Western , Massachusetts
Join Date: Sep 2005
Posts: 1,449
Images: 6
My last post

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.
__________________

__________________
65CV is offline   Reply With Quote
Reply


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Tow vehicle charging batteries safari 28 Batteries, Univolts, Converters & Inverters 25 11-15-2009 11:28 PM
Battery charging from tow vehicle ? rwnash Batteries, Univolts, Converters & Inverters 3 07-29-2008 09:17 AM
Charging the batteries using tow vehicle bci 2006 - 2010 Classic (all lengths) 22 06-09-2008 10:57 PM
Charging AGM trailer batteries from vehicle mswartz Batteries, Univolts, Converters & Inverters 5 05-15-2004 03:34 PM
tow vehicle isolated from A/S batteries?? BOBHASTINGS Electrical - Systems, Generators, Batteries & Solar 13 02-05-2004 01:50 PM


Virginia Campgrounds

Reviews provided by




Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -6. The time now is 12:02 PM.


Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, Jelsoft Enterprises Ltd.

Airstream is a registered trademark of Airstream Inc. All rights reserved. Airstream trademark used under license to Social Knowledge LLC.