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Old 04-23-2019, 12:52 AM   #1
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Charging Batteries

So when my trailer is hooked up and Iím driving down the road are my batteries getting charged from my trucks charging system???
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Old 04-23-2019, 04:08 AM   #2
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most tow vehicles only provide a trickle charge, at best.
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Old 04-23-2019, 05:49 AM   #3
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The short answer is "maybe".

It depends on the make and model of your truck, whether the truck came with the tow package factory installed, whether ( as in the case of chevy/gmc) the dealer prep included the fuse box fuse and connection, and the type of connector your trailer has.

If your truck has a 7 pin standard or 6 pin trailer plug, with the engine running, check for power at the connector as shown below. If it has a 4 pin connector, there is no battery charge circuit or electric brakes (unless you added them with a separate connector). The 7 way is most common,
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Old 04-23-2019, 05:53 AM   #4
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Originally Posted by bikendan View Post
most tow vehicles only provide a trickle charge, at best.
The reason for this "top off charger" is in the article below. Running the truck to charge up a depleted battery will most likely take more than a full tank of fuel.
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Old 04-23-2019, 06:02 AM   #5
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With my last trailer (not a Forest River), pulling it with a 2004 gas Silverado 2500HD with factory tow package, every time I took it to the dealer for service and it was there for more than a few days, the battery would be dead. I would always have to back my truck up close enough to connect the 7-pin to operate the electric tongue jack to get it hooked-up. I would then head home, which was about 30 miles. By the time I got home, there was always enough battery power to disconnect from my truck and use the tongue jack to get it level. I never measured anything to check the state of the battery — all I know is, it worked.

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Old 04-23-2019, 06:58 AM   #6
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With my last trailer (not a Forest River), pulling it with a 2004 gas Silverado 2500HD with factory tow package, every time I took it to the dealer for service and it was there for more than a few days, the battery would be dead. I would always have to back my truck up close enough to connect the 7-pin to operate the electric tongue jack to get it hooked-up. I would then head home, which was about 30 miles. By the time I got home, there was always enough battery power to disconnect from my truck and use the tongue jack to get it level. I never measured anything to check the state of the battery — all I know is, it worked.

Bruce
If you read the article, it will explain why it worked. "Worked" is not the same as "fully charged".

At the normal charge rate associated with the two battery "system" created when you plugged in your trailer, the alternator will see a start discharged starting battery and your depleted trailer battery. The alternator will start fast charging both batteries.

The start battery is designed to take (and deliver) high current quickly but has limited capacity so it is fully charged quickly. A starting battery is rated in "CCA" or Cold Cranking Amps. A starting battery rated at 660CCA will output 660 amps at Zero Degrees Centigrade (32 Degrees F) for 30 SECONDS till dead.

The trailer battery is designed to be take and deliver a lower current (typical deep cycle batteries are rated at 5 amps for a number of hours - a 100AH battery will last 20 hours with a 5 amp constant load) and can not take or deliver a high charge rate for very long without overheating. The more depleted they are the faster they can take a charge, but the voltage recovers much faster than the capacity.

Since the truck's alternator uses the combined voltage of the starting battery and the trailer battery "as a system", it will cut back the charge rate once the start battery has been topped off.

The trailer battery may only be 25% or so filled when that happens.
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Old 04-23-2019, 08:10 AM   #7
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The reason for this "top off charger" is in the article below. Running the truck to charge up a depleted battery will most likely take more than a full tank of fuel.
Interesting article, thanks for posting. I have read of others who have modified their vehicle's charging system to eliminate a camp generator. I don't recall the details. Maybe it was an unsubstantiated boast? Or maybe done by adding a second alternator and custom regulator?
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Old 04-23-2019, 08:44 AM   #8
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Has anyone installed a jumper battery terminal to their set up? I'm considering it. My thought is for situations like nomad297 mentioned. If the battery is dead, you can just use jumper cables from tow vehicle to battery for a few minutes and charge direct with the engine running. That would be significantly more efficient than trying to trickle charge through the tow system. Is my theory correct or would that not work? Attaching a link to the jumper terminal. I'd run that straight to the battery. Then there is a direct link from the jumper terminal to the battery without needing to open the battery compartment and everything else, just hook the jumper cables to the truck battery on one end, and the jumper terminals on the other end, and presto, charged RV battery.

https://www.amazon.com/dp/B07P6D5QSY...detail_1?psc=1
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Old 04-23-2019, 09:18 AM   #9
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Yes, but the process is slow. The alternator on your truck produces some power to bring up your batteries, but I agree that it is at a very slow rate. Don't rely on it.
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Old 04-23-2019, 10:45 AM   #10
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Interesting article, thanks for posting. I have read of others who have modified their vehicle's charging system to eliminate a camp generator. I don't recall the details. Maybe it was an unsubstantiated boast? Or maybe done by adding a second alternator and custom regulator?
The BEST way (IMO) to do this is to install a dedicated truck inverter connected to a dedicated deep discharge battery in the truck. Plug a dedicated Smart Charger into the inverter when you need to charge the camper battery.

That way you can run the engine to power the inverter when charging the trailer battery and can run AC power tools when not.

For this application I would use this charger:

https://www.amazon.com/NOCO-GB40-Ult...-1-spons&psc=1

Charger Reviews - https://mozaw.com/best-deep-cycle-battery-charger/

It pulls a maximum of 1200 Watts when charging (1000 amps peak times 12 volts) so a 1500 watt inverter would power it just fine.

https://www.amazon.com/Cobra-1575-Pr...s%2C128&sr=8-2

You would obviously need the engine running to supplement the Truck's deep discharge battery if the Camper's battery was less than 50% of capacity and to manage the initial peak load.
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Old 04-23-2019, 11:24 AM   #11
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Originally Posted by Semperfi24 View Post
Has anyone installed a jumper battery terminal to their set up? I'm considering it. My thought is for situations like nomad297 mentioned. If the battery is dead, you can just use jumper cables from tow vehicle to battery for a few minutes and charge direct with the engine running. That would be significantly more efficient than trying to trickle charge through the tow system. Is my theory correct or would that not work? Attaching a link to the jumper terminal. I'd run that straight to the battery. Then there is a direct link from the jumper terminal to the battery without needing to open the battery compartment and everything else, just hook the jumper cables to the truck battery on one end, and the jumper terminals on the other end, and presto, charged RV battery.

https://www.amazon.com/dp/B07P6D5QSY...detail_1?psc=1
No "Presto" for many reasons. The low charge rate is to protect your camper battery from exploding at near capacity high charge rates. A smart charger is needed to modulate the charge rate to a speed that the battery can accept without overheating.

Hmm, (Engineers please forgive me):

Picture in your mind a thick sheet of lead, say an inch thick. It has an interior like a sponge (has electron sized holes in it). The electrons can ONLY enter the sponge from the electron sized holes on the flat sides of the metal.

The electrons are VERY happy to stay on the flat surface and have no desire to go into the sponge. (Call that "surface Charge"). Since there is a HUGE flat area to go to, the plate can accept electrons as fast as you can pump them in. Once the flat side of the plate is full of electrons, if you put a meter on the "battery" it would read "full charge" (12.7 volts in a 12 volt battery) since that is all the battery has the potential to deliver at one time. The CAPACITY of the plate to hold electrons would be near zero, however.

To continue to charge this battery, the PUSH (Voltage) needs to increase to encourage the surface electrons to move into the lead making room for new electrons to find a home on the surface. It takes more "push" (voltage) to move the electrons the deeper into the lead sponge you go. However, it takes "work" to push those electrons into the sponge and "time" to get them in there the deeper you go.

If you maintained the CURRENT at the same rate as when the lead sponge was "empty", the excess current will cause a heat buildup in the electrolyte. "Boiling" in the vernacular. Smart chargers monitor the charge state of the battery and reduce the rate commensurate with the ability of the battery to accept the electrons.

The upshot is that battery, when metered, will still say 12.7 volts (or more) as long as the surface of the plate is full AND THERE IS NO LOAD.

As capacity is filled, the ability of the electrons to migrate and refill the surface holes as they are used (and thus maintain battery voltage) increases, so voltage can be maintained under load.

This is why you NEED to reduce the rate of charge as the battery fills to avoid damaging your battery. Since you are trying to stuff LOTS of electrons into a deep cycle battery, it takes HOURS, even with a smart charger, to do so at a rate that will not kill the battery.

Plate design can effect the capacity of the battery and the plate surface area effects the ability to accept and release electrons.

Thin, Waffled plates can accept and deliver HUGE rates of current for a short period of time (starting batteries for example).

Thick Flat plates (of the same size) can deliver low current for LONG time (true deep cycle storage batteries).

Hybrid Plates (of the same size) are not as good as starting batteries to start an engine nor are they as good as a deep cycle for storage, but they are the ones we most likely have in our camper (Marine Batteries).

There is a relationship between capacity, current demand, and, the ability to deliver electrons to the surface quickly. The higher the rate you are sucking the electrons from the plate, (like sucking on a collapsed straw), the less able the battery is to deliver electrons to 100% of the plate surface. This causes a reduction in voltage and capacity the higher the amp demand on the battery. This is known as the Peukert Effect.
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Old 04-23-2019, 11:24 AM   #12
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The best way to use the vehicle to charge the trailer batteries is to use a DC to DC converter. That way the low voltage that appears at the 7 pin connector after all the voltage drops gets boosted to a voltage that will actually charge your battery.

No need to have an inverter and a charger.

This unit is a multi stage charger. This is just one example of some that are available. You mount it on the trailer side of things as close to your batteries as possible.

https://www.renogy.com/products/battery-chargers/
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Old 04-23-2019, 11:32 AM   #13
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The best way to use the vehicle to charge the trailer batteries is to use a DC to DC converter. That way the low voltage that appears at the 7 pin connector after all the voltage drops gets boosted to a voltage that will actually charge your battery.

No need to have an inverter and a charger.

This unit is a multi stage charger. This is just one example of some that are available. You mount it on the trailer side of things as close to your batteries as possible.

https://www.renogy.com/products/battery-chargers/
Voltage is not really the issue as the Alternator can easily supply all the voltage needed to charge the camper's battery. It is the proper current for the charge status of the battery to be charged that causes the problems.
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Old 04-23-2019, 11:37 AM   #14
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Voltage is not really the issue as the Alternator can easily supply all the voltage needed to charge the camper's battery. It is the proper current for the charge status of the battery to be charged that causes the problems.
Voltage is the issue and its caused by the small gauge wire between the alternator and the 7 pin connector. With the voltage drops, the 14+ volts that the alternator is putting out becomes low 13s at the 7 pin connector. In addition, since the alternator is local sensing the truck's battery, it looks fully charged to it. This DC to DC converter gets rid of this problem. It can actually source 14.7V at 20A even though the voltage at the trucks connector is lower than 13V. It is also a multi stage charger so it will change to float charge once the batteries are charged.

My truck can only supply 2A while I am driving, verified by my bluetooth power meter and solar panels turned off. If I didn't have 700W of solar already, I would definitely have one of these DC 2 DC supplies.

On our products we designed, we would use DC to DC converters all the time for the very same reason. Have a large single voltage power supply trying to feed many loads after many voltage drops. We ended up just putting small DC to DC converters at all the loads. Very common to do this these days in our industry.
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Old 04-23-2019, 12:25 PM   #15
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Thanks for the replies babock and Herk. I understand what you were saying aboutt he battery / sponge analogy. I possibly didnt properly convey my thought, I wasnt saying presto charge the battery (to full), more of a presto, now you have enough juice that you can use the power tongue jack to raise the trailer enough to hook up to the tow vehicle. That was my theory and figured this was as good a time as any to float the theory, since I havent done it yet, to see if the theory was plausible or a waste of time and money.

The backstory was exactly what Nomad mentioned. Picking up a trailer that had a dead battery. I attempted to manually crank the tongue jack, it got me most of the way but then the cheap wrench bent and I was still stuck in a situation where I needed to raise the tongue. Using the umbillical cord connected to the truck was working, but it was just so slowly that I would have been there all night. Fortunately I happened to have one of those portable car battery chargers (just by luck, it isnt typically something I carry). But to employ the charger I had to open up the battery box, and get the cables attached and all that. My mind immediately realized that a jumper batter terminal would make that whole process easier. Instead of opening the battery box, it would have been more convenient to just connect the charger at the terminals. If using the vehicle to jump the battery wont work, I can still use the portable charger like I did before. I will continue to research further.
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Old 04-23-2019, 12:36 PM   #16
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Thanks for way more information than I needed. LOL I should have mentioned that it is a Toy Hauler with an Onan Generator. I start the genny about every 2 weeks and let it run 30-45 minutes and so far that keeps my batteries charged. They are deep cycle batteries and about 1 yr old. I just wanted to know if while traveling my batteries would get a trickle charge. I do have a tow package with a 7 pin connector on my Ram 1500.
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Old 04-23-2019, 01:31 PM   #17
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Thanks for way more information than I needed. LOL I should have mentioned that it is a Toy Hauler with an Onan Generator. I start the genny about every 2 weeks and let it run 30-45 minutes and so far that keeps my batteries charged. They are deep cycle batteries and about 1 yr old. I just wanted to know if while traveling my batteries would get a trickle charge. I do have a tow package with a 7 pin connector on my Ram 1500.
I have never been accused of brevity Ö
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Old 04-23-2019, 06:30 PM   #18
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I'll read up on this DC to DC concept and the other info supplied in this thread, this is a new spin on things for me!

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Voltage is the issue and its caused by the small gauge wire between the alternator and the 7 pin connector. With the voltage drops, the 14+ volts that the alternator is putting out becomes low 13s at the 7 pin connector. In addition, since the alternator is local sensing the truck's battery, it looks fully charged to it.

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truck can only supply 2A while I am driving, verified by my bluetooth power meter and solar panels turned off.


What are the opinions on this:
Install a short, 8-10ft, very heavy gauge wire set on the tow vehicles primary battery, and to an added +/- port on the trailer's battery box. You've bypassed the small 7 pin wire. The alternator will see a joined battery bank that needs charging. Wouldn't this mostly charge the trailer's batteries in an hour or so? I'm not worried at all about the effects of running the TV engine at idle or elevated idle for an hour or so as it's less stress on the TV than prolonged idling experienced when off-roading. It is certainly quieter than any generator.

If this is illogical or even idiotic, I'd like feedback as to why. As I said in my earlier post, other have claimed to have done something similar and I want to learn if this is feasible or someone's pipe dream. Thanks in advance.
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Old 04-23-2019, 06:36 PM   #19
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I'll read up on this DC to DC concept and the other info supplied in this thread, this is a new spin on things for me!








What are the opinions on this:
Install a short, 8-10ft, very heavy gauge wire set on the tow vehicles primary battery, and to an added +/- port on the trailer's battery box. You've bypassed the small 7 pin wire. The alternator will see a joined battery bank that needs charging. Wouldn't this mostly charge the trailer's batteries in an hour or so? I'm not worried at all about the effects of running the TV engine at idle or elevated idle for an hour or so as it's less stress on the TV than prolonged idling experienced when off-roading. It is certainly quieter than any generator.

If this is illogical or even idiotic, I'd like feedback as to why. As I said in my earlier post, other have claimed to have done something similar and I want to learn if this is feasible or someone's pipe dream. Thanks in advance.
First of all, you will never charge a flooded lead acid battery in an hour. Just not going to happen.

If you get a large enough wire and have little voltage drop, that would help immensely but it is still WAY cheaper to run a generator since it is going to take you MANY hours to charge a discharged battery.
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Old 04-23-2019, 08:03 PM   #20
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First of all, you will never charge a flooded lead acid battery in an hour. Just not going to happen.

If you get a large enough wire and have little voltage drop, that would help immensely but it is still WAY cheaper to run a generator since it is going to take you MANY hours to charge a discharged battery.
Well, dang! Was hoping a 200 amp-hour bank could at least get 70-80% charged in a reasonable amount of time. Thanks for the input.
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