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Old 03-08-2016, 12:31 PM   #1
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Charging AS batteries from tow vehicle connection?

Should this be done or should I have the connection to the tow vehicle bypass everything except brakes and towing lights? I'll probably have solar on the roof too.
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Old 03-08-2016, 12:51 PM   #2
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Should this be done or should I have the connection to the tow vehicle bypass everything except brakes and towing lights? I'll probably have solar on the roof too.
There are a lot of dependencies in this scenario, but generally speaking you shouldn't expect to get much charge from your Tow Vehicle umbilical connection to your AS. The quick and dirty explanation is that voltage from your vehicle's alternator drops over distance and as the thickness of wiring carrying it decreases. By the time the juice gets to your AS batteries the voltage probably won't be high enough to charge the batteries.

There are ways around this typical scenario, it just depends on how deep down the rabbit hole you want to go. For example, you could run the proper gauge wire from your vehicle's 12v system back to you AS batteries, or you could do something more inventive like installing an inline voltage booster (check this thread: http://www.airforums.com/forums/f37/...op-130091.html).
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Old 03-08-2016, 01:22 PM   #3
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You can expect a charge current of 4 to 11 amps max, most vehicles I have had and measured ran about 6 amps. My current grand cherokee and front battery AS can run as high as 11 amps.

So, there is something, but not a lot available in most setups. As explained in post #2 above, the charge wire size and length make it difficult to change the situation.
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Old 03-08-2016, 02:02 PM   #4
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What would be the purpose of avoiding the small "free" charge from the tow vehicle, in order to keep the trailer batteries topped up?

If it ain't broken don't fix it . .
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Old 03-08-2016, 03:10 PM   #5
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A charge of 5-11 amps for 8 hours travel is 40-88 amp hours. Can't believe that is not a good idea. But we do not carry a generator and sometimes stop for the night with no hookups. So we like the TV charge. You can build a case that the alternator has a heavier load and therefore may have a shorter life. A replacement alternator can be a bit expensive and that might change your approach. Guess the logic would also shift if solar was installed to keep the bank charged. All a compromise, so build a case, decide what is best for your situation, and implement that plan. And if things change, you can adjust. Pat
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Old 03-09-2016, 01:33 PM   #6
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Go with the biggest wiring you can install from the TV battery back to the 7-way on the TV. I ran 10 gauge but others have used 8 and even 6 gauge. From the 7 way plug to the AS everything else is pretty much cast in stone since the umbilical is a standard size. Short runs of under size wiring don't hurt too much. The voltage loss is a combination of total resistances and the oversize wiring in the long run through the TV is important to keeping losses low. My Truck battery easily runs the tongue jack if the umbilical is hooked up with no house battery on board.
If you are using the TV as a charger, it might make sense to upgrade your alternator to deliver more amps. That way you will recover quicker when the house batteries are low.
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Old 03-09-2016, 06:27 PM   #7
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Perhaps faulty, but my recollection is that amperage is limited by the most resistive part of its run. Sending lots of amps through heavy gauge only to block them transitioning into light gauge leaves no appreciable increase in amps delivered to the intended destination. Electricians and electrical engineers, beam me some know how on this
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Old 03-09-2016, 10:36 PM   #8
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My concern would be for the health of the airstream battery bank. I wouldn't want the charging current from the TV to boil off the battery acid, assuming that could even happen.
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Old 03-10-2016, 04:34 AM   #9
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Tow vehicle (TV) alternators have built-in regulators, so there is no need to worry about a TV boiling an Airstream's batteries dry. Otherwise, TVs would be ruining their starter batteries, too.
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Old 03-10-2016, 07:04 AM   #10
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Tow vehicle (TV) alternators have built-in regulators, so there is no need to worry about a TV boiling an Airstream's batteries dry. Otherwise, TVs would be ruining their starter batteries, too.
Lifeline and other battery manufacturers prefer to have the charge line going to a trailer's battery bank further regulated or even disconnected. The voltage from the alternator, which is generally regulated at the alternator is still a single stage continuous voltage.

In the 2011 Sprinter that I use daily, my dash mounted Scan Gauge is set to read system voltage, along with a few other parameters. The voltage never varies from 14.1-14.2VDC.

Over a long duration drive with a trailer in tow, this would also be seen at the trailer's batteries. If you have a higher amperage output to the trailer after increasing the wire gauge to that trailer and you increase the amperage to say 15 amps, (even 11 saw Idroba has stated), you will probably overcharge your batteries at some point.

Example: you have the standard 160 amp/hour Interstate battery bank (80 usable amp/hours at 50% depth of discharge), which is depleted by 50 amp/hours after a night of camping with furnace use. Your TV alternator places 10 amps into your trailer while under way, and you drive for 8 hours that day.

After 5 hours (5 hrs X 10 amps = 50 amp/hours) your batteries are full and should be in float charge of 13.2VDC (approx.). Your alternator will still be placing that 10 amp charge into the batteries for another 3 hours, which will bring them into an overcharge state. The alternator will not recognize this and will still be outputting 14.2VDC @ 10 amps into the batteries for as long as you continue to have the engine running.

The simple answer is to have a battery monitor in the trailer so you know how much charge your batteries require at the start of your day and also know what the charge capacity of your TV is, and you do the math. A switch in the trailer on the charge line (simple to install) will give you the means to shut the charge line when you have filled the batteries.

More complex but better solutions include an automatic charge relay that will sense the battery voltage at the trailer and automatically disconnect when they are full (similar to the BIM that Airstream uses in the Interstate vans, or a unit from Blue Sea Systems) or have solar with a quality 3-stage charge controller. Many boats use systems like these to regulate the engine alternator output to the on-board battery banks while under way.

Ya pays yer money and ya makes yer choice.
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Old 03-10-2016, 07:45 AM   #11
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Tow vehicle (TV) alternators have built-in regulators, so there is no need to worry about a TV boiling an Airstream's batteries dry. Otherwise, TVs would be ruining their starter batteries, too.

I thought about this but TV's have regulating circuitry between the alternator and the battery. The connection to the trailer is a dumb connection. I think some kind of device like those mentioned by other posters here, would need to be installed in the trailer to shut off the circuit. I could do it myself manually, but I don't think it would be simple enough for my wife to do, and she'd probably be mad at the increase is complexity.
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Old 03-10-2016, 08:23 AM   #12
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Charging AS batteries from tow vehicle connection?

Sounds kinda like another argument for archaic lead acid wet batteries to me.

Charging my house batteries while I drive without a lot of complication is a great convenience in my book.



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Old 03-10-2016, 09:35 AM   #13
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Perhaps faulty, but my recollection is that amperage is limited by the most resistive part of its run. Sending lots of amps through heavy gauge only to block them transitioning into light gauge leaves no appreciable increase in amps delivered to the intended destination. Electricians and electrical engineers, beam me some know how on this
No!
Resistance is the sum of all the parts. A short piece of 12 gauge wire does not negate the advantage of the larger 8 gauge running the length of the truck. It certainly adds to resistance but only for a short distance so the added resistance is quite small. This can most easily be measured by voltage drop. It I put the volt meter across the small bit of 14 guage wire on the fuse in my charge line, I get non-detectable voltage drop; so I have full voltage where I connect to the charge line. If I put it across the 10 gauge wire running through my truck I will see a volt or two drop. If I had used 8 or 6 gauge I would see less voltage drop.
Part of your recollection is based on fact. If I exceed the amperage capacity of the 14 gauge wire, then it will in fact overheat and probably burn it. But unless I'm somehow able to deliver more than 15 amps the small wire just adds a bit of resistance....probably equal to adding a foot or two of extra length to the heavier 10 gauge (in my truck) charge wire. So yes it is a choke, but not a very big one in the overall scheme of things.
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Old 03-10-2016, 09:53 AM   #14
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I thought about this but TV's have regulating circuitry between the alternator and the battery. The connection to the trailer is a dumb connection. I think some kind of device like those mentioned by other posters here, would need to be installed in the trailer to shut off the circuit. I could do it myself manually, but I don't think it would be simple enough for my wife to do, and she'd probably be mad at the increase is complexity.
The regulation is built into the Alternator, and is/should be about 13.8 volts max so protected TV battery = Protected House battery (assume both Lead Acid). Actual voltage from the Alternator will depend on TV usage. More load from things like radio and lights will shave the voltage as more amps are drawn, as will outside temperatures, which is why nominal 13.8 from the alternator does not usually boil the TV battery. If the Voltage in the TV charge line creeps up to 13.2 the House battery should be "fully" charged. If it reaches the 13.8 delivered by the Alternator, since the house battery has stopped accepting amps, it starts to boil off water. If you have not been losing water in your TV battery then I would not expect to lose it in the House battery.

That being said; the idea of an auto cut-off makes sense if your TV is or seems to be the reason for overcharging the House battery
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