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Old 09-21-2019, 10:58 AM   #1
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Question Can TV really charge trailer batteries??

I'm not the greatest regarding 12v knowledge so need some help..

OK so I have a battery in my TV and 2 batteries in my Trailer. The trailer batteries are connected to the positive post of the TV battery. I have one alternator that is supposed to look after charging all 3 batteries when needed. How does it work??

When the TV vehicle battery is discharged, it is supposed to charge that battery only??
When the trailer batteries need a charge it is supposed to charge them only.??
Is it a case of water flows to the lowest point? ie; senses that the trailer batteries are somewhat discharged and send the charge to them? Is this what they call the lowest point of resistance??

Assuming that the TV batteries can hold a charge and that they were fully charged at the start of a trip, will the alternator be able to at least maintain the trailer batteries charge assuming that there is almost no draw on them when travelling down the road?? (ie; Battery "Kill Switch" is off so only draw would be propane detector.)

Thanks for your help.
Woody
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Old 09-21-2019, 11:08 AM   #2
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The voltage regulation in the vehicle charging system is only concerned with maintaining the charge in the starting battery. It doesn’t know the trailer exists per se.

Since the trailer batteries ARE connected to the TV battery, some current will flow to replenish the trailer, but it’s limited due to the long length of relatively small wiring which results in voltage drop.

Net result is the connection is adequate for maintaining the trailer batteries, in other words they won’t discharge from parasitic loads such as the detector or fridge.

If discharged, they’ll pick up some charge during a drive, but nothing near what the charger / converter can do.

There ARE DC to DC chargers made to fully charge secondary batteries in transit. RedArc is one brand.
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Old 09-21-2019, 11:10 AM   #3
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Quote:
Originally Posted by woodytrwo View Post
I'm not the greatest regarding 12v knowledge so need some help..

OK so I have a battery in my TV and 2 batteries in my Trailer. The trailer batteries are connected to the positive post of the TV battery. I have one alternator that is supposed to look after charging all 3 batteries when needed. How does it work??

When the TV vehicle battery is discharged, it is supposed to charge that battery only??
When the trailer batteries need a charge it is supposed to charge them only.??
Is it a case of water flows to the lowest point? ie; senses that the trailer batteries are somewhat discharged and send the charge to them? Is this what they call the lowest point of resistance??

Assuming that the TV batteries can hold a charge and that they were fully charged at the start of a trip, will the alternator be able to at least maintain the trailer batteries charge assuming that there is almost no draw on them when travelling down the road?? (ie; Battery "Kill Switch" is off so only draw would be propane detector.)

Thanks for your help.
Woody
Your tow vehicle alternator doesn't care how many batteries are charging. It will supply xx amount of maximum amps at a set voltage, usually around 14.4V but that varies a lot on modern trucks.

If your trailer batteries are at 12.1V and your tow vehicle's batt is at 13.6 the trailer batteries will be sucking more electrons than the tow vehicle batt. Wire size from truck alt to trailer batteries will limit the available amps and that is usually a good thing. Running tow vehicle alternator hard with dead trailer batteries is hard on it and will eventually kill it if you had, say a 4/0 wire connecting it all instead of a 10 ga.

I'm not sure running the trailer with battery disconnect on the disconnected state is wise. Your electric trailer brakes might be shut off and depending how it's all wired you may not be charging the trailer batts.
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Old 09-21-2019, 11:28 AM   #4
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Its like you said " water runs to the low point "

When you connect low voltage batteries to higer voltage batteries, the voltage is going to try to equalize whether the alternator is running or not.
That will cause a drop in the chassis battery that the alternator will try to raise back to its set point.

Depending in wire gauge, including the tow to trailer ground wire, and the amp rating of the alternator, you will see some charging.

Motor homes don't use special alternators, just higher output models and heavier gauge wire between the battery banks.

You will get some charging to low batteries while driving but how much depends on the charge and ground wires.
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Old 09-21-2019, 12:22 PM   #5
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If your trailer batteries are dead the trailer brakes won't work until the batteries are charged, That being said, I get 8 to 10 amps charge current into my trailer from the TV charging system through the 7 wire plug. It will charge and it will keep the charge when done but it can take a long time.

If you have 6 volt 250 amp/hr batteries it might take 24 hrs of driving if they are dead dead dead.
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Old 09-21-2019, 12:44 PM   #6
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Leave Ttrailer Battery Disconnect ON......so batteries can get a charge
(Use for storage not when camping or traveling)


Umbilical cord between TV and Trailer has a 'charge' line' It is adequate to MAINTAIN trailer battery charge while driving.
Can do a little charging if trailer batteries are low.


Umbilical cord also provides Brake Voltage via TV Brake Controller
BUT should trailer separate from TV the trailer batteries supply the trailer brakes with voltage via the Emergency Breakaway Switch.
Trailer batteries by LAW must maintain FULL Voltage on trailer brakes for 15 minutes


So plug in that umbilical cord.
Leave disconnect switch ON (allowing voltage to/from trailer battery)
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Old 09-21-2019, 07:04 PM   #7
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Hi folks. Thanks for your input.
For point of clarification, when the "kill switch" is set to "off" in the trailer, all it does is stop all 12v current going to the trailer accessories such as lights, water pump, etc. from the trailer batteries. It has no affect on the batteries. This "kill switch" is located on the 12v line between the trailers batteries and the 12v distribution panel.
The brake away switch when applied gets its energy from the trailer batteries and not from the TV.
I posted this item, because I was wondering how that poor alternator in the TV could determine that if the trailer batteries were not up to full charge and it should put out a charge to these trailer batteries but not charge (ie: overcharge) the TV battery at the same time. I know that when the breakaway switch is activated, that it will use the 12v power in the trailer batteries to activate the trailer brakes.
So my concern, was just if and how the alternator was able to (1) service the need of the tow vehicle battery and (2) also service the need of the trailer batteries without one need affecting the other.
I know that the charge line going into the TV battery is far heavier than the line going to the trailer batteries. I would assume therefore that if those trailer batteries were run down, the alternator could not bring the trailer batteries up to anything like a full charge.
Some people that I have talked to tell me that the alternator in the TV can only "maintain" at best the charge in the trailer batteries but say if you had a phantom leak in the trailer batteries then the alternator could not handle this and therefore make the braking power from the trailer batteries in the case of a breakaway situation next to useless. But would this not violate federal, state requirements?
Woodytwo
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Old 09-21-2019, 07:17 PM   #8
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I just reread all of your replies to my original questions. I believe that you have answered my original concerns. Yes the alternator will do its damnest to get the trailer batteries up to charge while not overcharging the TV batteries. BUT there are limitations depending on how much the trailer batteries were discharged.
I can live with that.
Woodytrwo
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Old 10-01-2019, 04:07 AM   #9
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Wire size severely limits the amount of charge. Now if you run a 4 guage wire back there,
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