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Old 04-03-2017, 03:11 PM   #1
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Charging batteries while towing

Ok... So I'm sure someone has already asked this question before but I searched and couldn't find it or an answer so here goes...

If your camper is hooked to your truck (towing vehicle) and your towing vehicle is running is there any voltage going to the batteries on your camper to keep them charged while you drive down the road? If so does it matter or make a difference if the master battery switch is on or off?
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Old 04-03-2017, 03:19 PM   #2
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as long as you have the round 7 connector plug there should be power, on my truck I needed to plug a relay in to get it. it may be switched with the ignition or not, if not it could drain the truck battery when towing.

it should be connected to the trailer battery & the breakaway switch. past that point it doesn't really matter but I would leave it connected to the coach to run the fridge, convenience for quick stops etc
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Old 04-03-2017, 03:22 PM   #3
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The short answer is yes, provided you have a 7 pin connector, your tow vehicle is correctly wired and has the fuse/relay installed. (not always installed from the factory)

Remember the wires from the alternator to the battery in the trailer are small so you'll get a trickle charge at best but it will provide some charge.

To your question about the disconnect switch... depends.
We read reports of disconnect switches being wired 12 different ways to Sunday.
A voltmeter test on the trailer battery (engine on/engine off) is the only way to tell for sure.
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Old 04-03-2017, 03:27 PM   #4
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Without knowing how your vehicle or trailer is wired it is hard to answer your question. That being said, (in most cases)yes you will have 12v to the trailer from the TV (with a 7pin connector) but not enough amperage to do much in the way of charging due to the small wire size.

Battery shut off is another thing altogether, hard to say without knowing how it is wired. Some testing is needed here.
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Old 04-03-2017, 03:48 PM   #5
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most tow vehicles only supply a trickle charge at best.
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Old 04-04-2017, 01:03 PM   #6
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Generally, most vehicles are not wired to provide this charge. You can check a wiring diagram for your vehicle or check with a voltmeter at the plug. As stated before, you must have the "Barger" 7 wire plug to generally have this capability. If it is a 4 wire connection, then you don't.

Here is the wiring diagram

https://www.etrailer.com/faq-wiring.aspx

Personally, I wire a new line running from the tow vehicle battery back to the tow plug. Use at least 10 gauge, preferably 8 gauge, with an auto-reset 10 Amp circuit breaker right up by the battery. You will also need to verify that the trailer side also has wiring. Typically, you will if you have a trailer with a battery for the brake system. That should also be protected with a circuit breaker near the battery of the tow vehicle, since you don't want a short in that line to fry your trailer. You can also add an in-line charger for that purpose such as

https://rvibrake.com/products/towed-...FY2Lswod5uMPUQ

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Old 04-04-2017, 01:50 PM   #7
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OK.... SO HERE IS WHAT I FOUND SO FAR:

1) The 7 pin plug on my 2017 GMC Sierra does have the ability to provide 12+ volts to the camper.

2) The pins on the 7 pin plug that provide the 12V power are not switched with the key and remain hot all the time even if the vehicle is not running.

3) The two pins of the 7 pin plug have a dedicate (30 amp) fuse under the hood just for those two pins.

I can't imagine that connector and the small wire associated with it providing 30 amps but that's what the fuse is rated for.
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Old 04-04-2017, 02:03 PM   #8
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There is an easy to check if your are getting charging voltage from your tow vehicle.. if you have a 12v plug in inside your RV, you need to buy one of these:

https://www.amazon.com/Intelligent-C...+digital&psc=1

With the tow vehicle disconnected from the RV, you should read only RV battery voltage.. Once you plug into the tow vehicle, with the engine running, it should give you a great reading, around 13 + volts..
Also you can keep an eye on the RV battery voltage when dry camping..and charge your phone etc with this meter..
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Old 04-04-2017, 02:18 PM   #9
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On newer F150s 2016 they don't supply voltage until they detect current draw. The truck throws an error if I have the battery cutoff turned off saying it cannot charge and to check the wiring. The way the trailer battery is wired would determine if this is the case for your rv.

You cannot just test the pins to know with this setup as you actually need to check it with the trailer connected.

Clintbonnie73 is right. I have a similar gadget I put in the lighter socket inside the trailer to know how the battery is doing and if I am getting a charge from TV. Of course any volt meter would work.
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Old 04-04-2017, 02:23 PM   #10
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By my understanding if you have the trailer towing button on the gear shift lever pushed then the computer in the 2011+ GMC/Cheverolet will boost the output of the alternator. The owners manual will explain it all for you.

My 2011 Silverado has this feature.
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Old 04-04-2017, 04:53 PM   #11
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I tow with a minivan, not a truck. The dealer who sold me the A-frame installed the tow wiring. I had always wondered if the fridge running on DC (13.5 amps) was draining the camper batteries while towing, or whether the wiring and alternator was keeping up (especially with running lights on).

So I hooked up, turned on the running lights and the fridge (on DC) and measured voltage at the camper batteries (dual 6V). I did this on several different occasions. I was pleasantly surprised to see at least 13.2V, and sometimes 13.8V.

This means the alternator is keeping up with the demand. But if I stop for an hour and leave the fridge running on DC, it may take several hours or more driving to recharge those 13.5AH used. I ran into that exact situation - we were checking into a Texas State Park and I had to wait about 1.5 hours in line and then getting checked in. It was a 40 minute drive to the campsite - and my batteries were not close to fully charged upon arrival. I further depleted them by waiting for a thunderstorm to pass before I hooked up the electric and switched the fridge to AC.

I have converted the tail lights and a couple of clearance lights to LED to reduce the current draw and voltage losses to the camper while towing, and that has helped. I'm just thankful the dealer did a good job of wiring the minivan in the 1st place. I never dreamed the fridge would draw that much on DC.

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Old 04-04-2017, 05:05 PM   #12
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Quote:
Originally Posted by bikendan View Post
most tow vehicles only supply a trickle charge at best.
Says who?
I saw (victron bmv-700) 12.6 amps recently. If the battery was more dead, I assume it would have been higher. The line is fused at 30A.
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Old 04-04-2017, 05:29 PM   #13
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Originally Posted by bikendan View Post
most tow vehicles only supply a trickle charge at best.
What happens depends on the both the wiring and the programming of the alternator processor.

If the wiring is only 14-18 gauge from engine to connector at the vehicle rear, you will be limited by voltage drops regardless of alternator rating or processor programming. With good wiring (10 or 12 gauge), the processor programming will determine voltage and resulting charge current to the RV batteries and load (generally more conservative programming - slower recharging - than your RV converter).

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Old 04-04-2017, 08:53 PM   #14
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My converter went out one time, didn't have enough power to power the pump to put the slides in. I hooked the 7 pin cable up and ran the truck about 20 minutes, built up enough power to put slides in and get truck hooked up and by the end of the day batteries had built up pretty good.
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Old 04-05-2017, 11:29 AM   #15
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There are chargers used in the over-the-road (OTR) trucking industry called "trail chargers" that are used for this specific purpose. They take in "about" +12 volts (sometimes as low as +9), and convert it to the very-specific voltages necessary for a battery to accept the charging currents necessary for effective charge maintenance. There are many manufacturers for these, and different maximum currents available.

Tekonsha makes a product called a "tow charger" which essentially does the same thing, but has far less current capabilities, and is used primarily for maintaining "break-away" batteries on smaller trailers. It might be sufficient to compensate for any parasitic losses to an already-charged battery incurred "in transit" while being towed (propane detector perhaps), but would likely not recharge a depleted battery.

The in-line charger that Charlie mentions above in Post #6 appears to be a heavier-duty unit that may provide up to 15 amps of current (through 16-gauge wire?!?!), but it appears as though to really do this right, about $300 in equipment, and some wiring, may be necessary.

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Old 05-23-2018, 09:31 PM   #16
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Red face Towed Vehicle Charger Info

Well guys, I've been doing quite a bit of interesting reading on this subject. I'm not an electrician, and unfortunately don't have a digital voltmeter (now on order), so my own checking is just approximate. I was interested in what the "Towed Vehicle Charger" did beyond the 12V current coming from my 7 pin connector. I saw some comments on a U-tube Video that said that the charger "provides isolation and a fuse protected power source. Isolation is important because should the towed vehicle battery/s be bad, it could damage the towing vehicle battery", and "it has diode protection which acts as a valve against reverse flow from the trailer into the towing vehicle chassis". Don't know how expert these guys are, but sounds reasonable.


Now I wonder if my 2017 Jeep Grand Cherokee wiring provides the same "isolation" and fuse protection. I wonder who really knows the answer to that?


I also determined that my 7 pin plug 12V pin is always hot, and appears to put out more voltage when the car is running (hard to tell with my old voltmeter)?


Anybody else done any research on this? I feel nobody truly understands all the facets of charging a towed vehicle's batteries.
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Old 05-23-2018, 11:02 PM   #17
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All that seems like an over statement. It's a 12ga fused (30a) connection from the battery/fuse block.

Isolated? Dunno how.
Protected? Yeah, a fuse.

Some mfg (Ford) have a relay and it's only on with ignition. Most are always hot.

I've seen 13a current going into my campers batteries when driving.
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Old 05-23-2018, 11:16 PM   #18
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Quote:
Originally Posted by aeblank View Post
I've seen 13a current going into my campers batteries when driving.
What vehicle do you have and what gauge wire is there?

On my 2018 Ford F-150, the wire is...get this...14 gauge!!!

You might get 10A of current flowing with major voltage drop if there are loads in your trailer drawing it but if it's just the battery as a load, the voltage drop will be so much that there will be little actual battery charging going on.

If you really want to charge the battery, you need to run a substantially much bigger cable from the battery to the 7 pin connector or just have a separate high current conector/pigtail to charge the trailer battery if you are really going to rely on that.

Assuming the ground/frame of the TV contributes minimal voltage drop and you want to limit the voltage drop of the positive feed to 3%, you would need a 6 gauge wire for 20 feet @20A.

If you want 10A, you need 8 gauge wire for a 3% voltage drop.
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Old 05-24-2018, 05:54 AM   #19
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What vehicle do you have and what gauge wire is there?
It's a GMC Sierra (diesel, not that it matters other than maybe alternator size). I'd say the wire is 12ga. Didn't really feel like 10, and certainly not 14!

Current measured with the victron battery monitor (Bluetooth into the truck, sweet).

I've always thought of the wire as a natural current limiter. Big draw due to dead camper batteries and the voltage drop in the wire would limit the current to not blow the fuse. As the battery comes up in voltage, it's not wanting as much juice, so the drop isn't as bad, so the battery can be topped off more/better.

I dunno how long battery charge at 13a takes, but it's more than a trickle, at least in my book.

Here it is accepting almost 12a...

https://i.imgur.com/akPmF2a.png
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Old 05-24-2018, 06:05 AM   #20
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A couple of things to note:

Isolation diodes are a disaster - they drop 0.7 volts in the forward direction. Which is why relays are used instead of big diodes.

Automotive and light truck alternators are programmed to recharge and carry the loads of the car or truck. They are not programmed (with the exception of some trucks designed to tow) to recharge your RV or boat or trailer batteries. The regulators usually have 2 modes, continuous voltage (13.7 - 13.8V), and a trickle mode (13.2V) for long days of driving where the battery has been fully charged. Very few (perhaps none) will have a bulk mode (14.4V) because you have already destroyed your starting battery if it can use bulk mode to recharge.

The max amps that a Group 24 battery is going to take at 13.7V is around 10. If we still had ammeters in cars and trucks, you could see this. To get 20 amps recharging current into the RV battery, you need 14.4V pushing it in, and even then it's going to taper down pretty quickly to 12-15 amps. The size of alternator is really immaterial except for carrying loads with the vehicle running.

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