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Old 02-22-2021, 01:00 PM   #1
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Does your tow vehicle charge your trailer's battery?

Simple question: When your travel trailer is connected to the tow vehicle, does it charge the trailer's battery? And if so, will that drain the battery of the tow vehicle if it remains connected while the vehicle is off, or does it pause the charging?
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Old 02-22-2021, 01:10 PM   #2
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Technically the tv does charge the tt battery.

Actually, not much. Due to the length of the wire from the to the tv is so long the line loss is significant. Mine measures at 12.9 volts from the tv. Virtually nothing. My converter minimum voltage is 13.1 volts. A trickle.

In the old days the tt battery and tv were simply connected. You could discharge the tv too much. More modern vehicles there is a relay that turns the circuit off. Thus not discharging your battery.

Class Aís and other motor homes are different.
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Old 02-22-2021, 01:25 PM   #3
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As mentioned the tow vehicle does put some charge in the R/Vs battery when connected via the 7 pin plug. How much has been greatly discussed on this forum and opinions vary as does the tow vehicle's make. Some newer vehicle have settings in the on-board computers that increase the alternator output when the 7 pin is connected or the lights are on and so forth.

My current tow vehicle puts out 12.9v(+-)@ about 4 or 5 amps when measured at the battery. Not a lot but not the dismal charge many say they get.

As for removing the plug when the tow vehicle is off... as also mentioned, some vehicle makes have relays that disconnect the tow vehicle battery from the R/V battery but others do not as well as, some years did, some did not. You will need to test YOURS to be sure.

There are DC to DC chargers that increase charging rates.
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Old 02-22-2021, 01:30 PM   #4
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Originally Posted by dm6 View Post
Simple question: When your travel trailer is connected to the tow vehicle, does it charge the trailer's battery? And if so, will that drain the battery of the tow vehicle if it remains connected while the vehicle is off, or does it pause the charging?
You don’t mention your tv? I have an F-350, 2018 and before that an F-250, 2013 and they botched charged my battery that was also operating my residential refrigerator. I have empirical data and have shared that on this forum several times where I used my Victron battery monitor to see the state of charge of my battery both before I hooked up and drove and once I arrived at my destination. The charge level increased but it was not significant but it did not decrease and the batteries powered my fridge too.. I’ll see if I can find my post.

See post 16.. https://www.forestriverforums.com/fo...ac-208760.html
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Old 02-22-2021, 01:53 PM   #5
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Thanks for the info. I don't have a trailer yet but just curious about it for the future. Like for example, if you had to stop to camp out overnight for a night with only battery, and then hit the road the next day to drive for several hours, if that would top the battery back off. Sounds like it will give it a trickle charge in most cases.
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Old 02-22-2021, 02:40 PM   #6
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Thanks for the info. I don't have a trailer yet but just curious about it for the future. Like for example, if you had to stop to camp out overnight for a night with only battery, and then hit the road the next day to drive for several hours, if that would top the battery back off. Sounds like it will give it a trickle charge in most cases.
A lot depends on the tow vehicle and the amount of "juice" you took out of the battery during your stop.

A DC-DC charger like the Renogy DCC 1212-20 will really help increase the amount of charge from tow vehicle to battery even if you don't increase wire size from truck battery to trailer battery. Costs $127 which is a very reasonable price considering the improvement in charge rate.

I have one and with a larger wire from my truck battery to the DC-DC charger I can recharge my "Lithium" batteries from half empty (100 ah) to full by the time I make it to my next stop.

Worth considering if you find your truck isn't keeping up with your needs. Like I said, it all depends.
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Old 02-22-2021, 03:01 PM   #7
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The rv battery has loads on it virtually all the time if the batteries are not disconnected. Clocks, am fm Chanelís. Radio, alarmes etc. The fridge also draws power.

A good rv battery holds 50 amps of power. So, if dead, at 4 amps charging it will take a long time. Keep in mind the parasitic loads deduct from that.

We have 4 batteries, thus we could need 200 amps.

So recharging is sort of, sort of.

You recharge the rv with shore power. Typically 40 amps. Or a lot of solar. The tv is usually not a choice for most. Motor homes are different.
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Old 02-22-2021, 03:11 PM   #8
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I would not count on the TV to add any significant charge to your TT battery. Even at 12.9 V drawing 5 or 6 amps you are not at a high enough voltage to charge. The power available from the TV will power your loads in the TT without depleting your battery as long as they are less than the 5 ir 6 amps.

The DC to DC chargers do not deliver any more power to the TT battery. They just trade amps for volts so the voltage is high enough to charge the battery. So, you will have less amps available but enough voltage to push some power into the battery.

As ll of this assumes your TV has the basic factory wiring that is usually too small a gauge. You can run larger wires and a beefier alternator if you want towing to be a significant source of power for charging your TT batteries.
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Old 02-22-2021, 05:35 PM   #9
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Just a quick question.....I think one of the reasons the TT battery doesn't get a lot of charge is the TV alternator uses the voltage from the TV battery to "regulate" the alternator.....so if the TV battery is fully charged you don't get much charge to the TT. I had considered the DC to DC charger but thought they were a bit pricey. Why not use a invertor (I have a 300 watt) to power a charger and hook it to the TT battery? I realize there is some loss in conversion but at least the charger would be reading the TT battery
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Old 02-22-2021, 09:35 PM   #10
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The power to your TT, via the 7 pin connection, is of a very low amperage and will not charge your TT battery. If the lights are out, the TT battery remains fully charged and there is little or no drainage while connected via the 7 pin connector.

I have a 50AMP Renogy DC-DC charger with MPPT controller. I connected a plus and minus 4 gauge wire from the TV battery to the DC-DC controller in the TT and then to the TT lithium battery.

So, I will be topping off my TT battery as I am towing. I can also simply use my TT as a stand alone battery charger while boondocking.
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Old 02-22-2021, 09:47 PM   #11
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My difficulty with the DC to DC charger is exactly what you were saying with the hook up....running 4 guage wire from TV battery to the TT, kinda difficult with my TV. Plus the cost of the Renogy. Why wouldn't an inverter and charger work? I have hooked up an old 10 amp charger thru my 300 watt inverter and it charged my travel trailer......just trying to think outside the box.
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Old 02-22-2021, 10:07 PM   #12
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My difficulty with the DC to DC charger is exactly what you were saying with the hook up....running 4 guage wire from TV battery to the TT, kinda difficult with my TV. Plus the cost of the Renogy. Why wouldn't an inverter and charger work? I have hooked up an old 10 amp charger thru my 300 watt inverter and it charged my travel trailer......just trying to think outside the box.
That sounds okay to me, except that you are only charging 10amps. Today I was looking at a 25 amp Victron battery charger and they wanted 173 dollars for it. I think the Renogy 50 amp MPPT cost me about 230 dollars and it also acts as a solar panel MPPT controller for my 400 watts of panels.
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Old 02-22-2021, 10:39 PM   #13
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Thanks for the info. I don't have a trailer yet but just curious about it for the future. Like for example, if you had to stop to camp out overnight for a night with only battery, and then hit the road the next day to drive for several hours, if that would top the battery back off. Sounds like it will give it a trickle charge in most cases.
This is probably correct dependent on the tow rig. Definitely true of my current truck. My prior chevy 250 would give a healthy charge to the TT battery while hooked up, running or not. If needed you could hook up your TT battery to your TV battery with jumper cables (or similar) and run the TV for a while to charge up the TT battery.
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Old 02-23-2021, 01:04 AM   #14
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A proper tow package should include a higher output alternator. The 4 gauge wire from TV battery to TT battery should cause the alternator to 'see' a lower voltage because the batteries will naturally equalize voltage.
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Old 02-23-2021, 08:11 AM   #15
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Something that I did not see mentioned is that at least in the past, the hot wire of the 7 pin harness was not always connected in the factory installation of the wire harness. My 2007 Silverado 2500 was that way. There was a taped up red wire on the wiring harness that needed to be connected to the terminal on the side of the fuse box. The owner's manual said to check that there was a fuse in a specific slot or one would have to be installed. So, the first thing I would do is check with a meter or test light to see if there is power to the hot wire.
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Old 02-23-2021, 02:34 PM   #16
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I use a 12V socket adapter on my 7 pin trailer plug and can run a heavy-duty 12 air compressor with no issue. My 20 RAM has a 30 amp fuse for that circuit so I'm sure it's fairly heavy gauge wire. So voltage drop in the wiring isn't the issue, unless the trailer wiring is too small, like 20 AWG or smaller. Even then it would probably still charge. It really boils down to the fact you've got two batteries in the mix connected in parallel, which keeps the voltage high enough so the regulator won't allow the voltage output from the charging system to get high enough to charge the trailer battery quickly. But it will charge it. The DC to DC charging system is the trick to independently monitor the trailer battery only so it can get a bit more voltage to charge. Most of the electronics in the trailer are a very low load on the trailer battery unless you are trying to run the refrigerator while towing. Otherwise, it will charge albeit slowly. Just check the voltage on the trailer battery with the tow vehicle engine running. If you're seeing 13 or so volts it's getting charged.
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Old 02-23-2021, 02:48 PM   #17
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My F250 with "Extra HD Alternator" and tow and plow package does indeed charge my battery while towing with no issues. I have never discharged the battery to a level that it did not "recharge" while towing.
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Old 02-23-2021, 03:00 PM   #18
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i am certainly not an expert. but i have wondered how the dc to dc charger can charge the battery better when nothing changes on the tow vehicle. i understand the concept of trading amps for volts. but i wonder if there is more to it.

in a stock setup the tow vehicle alternator / battery are directly connected to the trailer battery. there is resistance in the circuit composed of the cabling itself and the internal resistance of the trailer battery. that combined resistance will limit the amps that will flow on the circuit.

now what happens if you place a dc to dc charger into the circuit? is it just that the battery is now getting charged at a higher voltage with less amps (trade amps for voltage). or does the dc to dc charger have a lower resistance in the charging circuit from the town vehicle as compared to directly connecting to the battery? if so, would this now lower resistance in the charge circuit allow more amps to flow through it? if this is the case could the dc to dc charger actually provide more amps at a higher voltage as compared wiring direct to the trailer battery?

i truly don't know. it's just something i have been wondering about.
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Old 02-23-2021, 03:07 PM   #19
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V x A = Power. The same amount of power is at your plug. The issue the battery has is it needs a certain amount of voltage to charge. Think of voltage like water pressure. It has to force it's way in there. Think of Amperage like the amount of water available/able to travel through the pipe. So, if you need more voltage you can trade some of the amperage (flow) to get more voltage (pressure). The amperage drops. You now have less current available, but it is able to "force" it's way into the battery. Ignoring some minor losses in the DC-DC the amount of power remains the same, just used in a different way.
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Old 02-23-2021, 03:11 PM   #20
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A DC to DC power supply as they're called essentially takes the available 12V and produces a higher DC voltage. In its use for charging a 12V battery, this higher voltage would be regulated to be 13V to 18V or so depending on the condition of the battery being charged. The circuitry used to do this is too elaborate to explain here. But you can do a youtube search and get plenty of details there.
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