Charging RV batteries with Tow Vehicle

[LivingRDream]

I am looking for what it would take to use my 2014 Dodge Ram Diesel to charge the batteries on my 2014 Landmark-Key Largo. I have a residential fridge and want to be able to boondock at night and recharge the batteries as I drive the next day.

1) Does anyone have information regarding the needed items?
2) If possible to do this, is the center auxiliary terminal available on the wire connector on the Key Largo?

Any information would be greatly appreciated!

 

[travelin2]

I don't know about your Ram but my SD charges the 2 batteries in the BH once the 7 pin is connected and engine is running
Other Ram owners can confirm but you shouldn't "need" anything


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[danemayer]

You shouldn't need to do anything. But the charge coming through the umbilical may not be sufficient to fully recharge the batteries. You should be able to get through 1 night, but it might be iffy getting through the 2nd night.

 

[Jesstruckn/Jesstalkn]

Ya I just did a plug in a 2014 Dodge truck and the top right power terminal was hot with the truck running.
Your truck and RV should both be ready to go.
You can check for power if you want.

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[travelin2]

Before we had a generator I had cause to use the truck to charge my batteries once when boondocking longer than planned during colder weather with increased furnace usage.
The SD at an idle for 30-45 minutes each day would bring my batteries to enough charge to make it to the next day. (No residential refer)


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[LBR]

FYI, on the label inside our resi frig, it states consumption is 2.8 amps......However, my PI HW-50 EMS shows a draw of 11 amps, and to back that up, the Power Command displays shows 10 amp draw for the frig. It draws waaay more than the model/serial tag states.

 

[LivingRDream]

Thank you for the suggestions, I will do some testing tomorrow and see what is up.

- - - Updated - - -

FYI, on the label inside our resi frig, it states consumption is 2.8 amps......However, my PI HW-50 EMS shows a draw of 11 amps, and to back that up, the Power Command displays shows 10 amp draw for the frig. It draws waaay more than the model/serial tag states.

My residential fridge is rated 8.5Amps@115volts.

 

[hoefler]

Rough rule of thumb for invertor use, the amp draw 12 volt is approximately 10x the amp draw on 120 volts.

 

[Todster]

I was just going to help clarify. The amp draw inside your fridge may be @115v AC. By the time you run that through the inverter. Your most likely 10-12 amps @12vdc.


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[LivingRDream]

I was just going to help clarify. The amp draw inside your fridge may be @115v AC. By the time you run that through the inverter. Your most likely 10-12 amps @12vdc.


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Thank you.

- - - Updated - - -

I was just going to help clarify. The amp draw inside your fridge may be @115v AC. By the time you run that through the inverter. Your most likely 10-12 amps @12vdc.


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I appreciate the response.

 

[LivingRDream]

I have tested with a Volt meter on the Positive terminal of the truck connector and show 12.38 volts, even when key is off. When starting the engine, I receive 14.42 volts.

I opened RV junction box at the hitch; with the RV connector plugged into the truck. I received the same results, an increase of aprox. >2.0

At the RV Battery, when testing, I only see at most 0.20 Volt increase when starting the truck. I ran a wire from the junction box + connection to the battery. When I tested the voltage on the wire prior to connecting it at the positive terminal it showed >2.0 volts. But when connecting to the battery terminal and testing, there is only an increase of at most 0.20 volts.

Trying to understand why the increase is evident when new wire is not connected to the battery, but very little present when connected to the battery.

FYI, shore-power was disconnected and the connections where made on the outside terminals of the battery box.

 

[danemayer]

I have tested with a Volt meter on the Positive terminal of the truck connector and show 12.38 volts, even when key is off. When starting the engine, I receive 14.42 volts.

I opened RV junction box at the hitch; with the RV connector plugged into the truck. I received the same results, an increase of aprox. >2.0

At the RV Battery, when testing, I only see at most 0.20 Volt increase when starting the truck. I ran a wire from the junction box + connection to the battery. When I tested the voltage on the wire prior to connecting it at the positive terminal it showed >2.0 volts. But when connecting to the battery terminal and testing, there is only an increase of at most 0.20 volts.

Trying to understand why the increase is evident when new wire is not connected to the battery, but very little present when connected to the battery.

FYI, shore-power was disconnected and the connections where made on the outside terminals of the battery box.

Turn the battery cutoff switch to OFF and see if there's any change. If yes, something is sucking power. If no, take the batteries to an auto parts store or shop and have them tested.

 

[LivingRDream]

Turn the battery cutoff switch to OFF and see if there's any change. If yes, something is sucking power. If no, take the batteries to an auto parts store or shop and have them tested.

Thank you Dan,I will do that.

 

[LivingRDream]

When checking the voltage at the battery using the trailer plug connection I only see about .02 volt increase when truck is running. I connected jumper cables from the truck to the batteries and got .5 volts increase. I am considering running a #10 wire/cable with a quick connector to the RV battery from the truck when traveling. Any thoughts on damage that could occur if I do that?

 

[danemayer]

When checking the voltage at the battery using the trailer plug connection I only see about .02 volt increase when truck is running. I connected jumper cables from the truck to the batteries and got .5 volts increase. I am considering running a #10 wire/cable with a quick connector to the RV battery from the truck when traveling. Any thoughts on damage that could occur if I do that?

I think since you have good readings at the junction box under the pin, you need to run down the power loss in the trailer.

Have you had the batteries tested?

 

[LivingRDream]

I think since you have good readings at the junction box under the pin, you need to run down the power loss in the trailer.

Have you had the batteries tested?

No, batteries have not yet been tested.

- - - Updated - - -

No, batteries have not yet been tested.

Shutting off the Battery Cut Off did not make a difference in the readings.

 

[mlpeloquin]

Charging current from the truck alternator has to run quite a distance. 16ga wire is 4.016 ohms/1000ft. 10ga is .9989 ohms/1000ft. Remember that the run is to and from, positive and ground. So the voltage drop across the wire will be V=I/R.

V is Voltage
I is current - which would be the charging current from the truck
R is resistance

So R for length is (XX ft * R per 1000ft)/1000 * 2

This will give you the resistance drop across the entire run to and from for the given gauge wire. Note that the contact resistance of the connectors is not included. It should be trivial, but at higher and higher currents it might not be and get hot.

If your truck is 13.5V and the fifth wheel batteries, without the converter connected is 12.5V then there is 1V difference. Assume the internal resistance of the batteries is 0 ohms then the charging current is 1V/R(length of wire) = charging current

Example: assume 18ft of 16ga (truck battery to RV battery)
V/R=I (in amps)
1V/.144576 = 6.916 amps. It will take a while for the RV battery(s) to come up to voltage, but as they do the current will go down as the voltage difference lessons.


So if your RV batteries were really discharged say 11.5V then:

2V/.144576 = 13.833 amps

 

[LivingRDream]

Charging current from the truck alternator has to run quite a distance. 16ga wire is 4.016 ohms/1000ft. 10ga is .9989 ohms/1000ft. Remember that the run is to and from, positive and ground. So the voltage drop across the wire will be V=I/R.

V is Voltage
I is current - which would be the charging current from the truck
R is resistance

So R for length is (XX ft * R per 1000ft)/1000 * 2

This will give you the resistance drop across the entire run to and from for the given gauge wire. Note that the contact resistance of the connectors is not included. It should be trivial, but at higher and higher currents it might not be and get hot.

If your truck is 13.5V and the fifth wheel batteries, without the converter connected is 12.5V then there is 1V difference. Assume the internal resistance of the batteries is 0 ohms then the charging current is 1V/R(length of wire) = charging current

Example: assume 18ft of 16ga (truck battery to RV battery)
V/R=I (in amps)
1V/.144576 = 6.916 amps. It will take a while for the RV battery(s) to come up to voltage, but as they do the current will go down as the voltage difference lessons.


So if your RV batteries were really discharged say 11.5V then:

2V/.144576 = 13.833 amps

Thank you for your response. That will help me decide what to do.

 

[caissiel]

I tried everything mentioned above for years. My batteries would never get charged properly on the road and after 3 days traveling 500 miles per day the RV batteries would be dead. The 3 batteries only lasted 2 years max. And I did not use an AC fridge. The only solution was​ to install 200 watt solar panels. After 5 years my 3 batteries are still great. While driving the truck alternator shows 13.3 volts which is idle no charge voltage. The solar panels while charging the RV batteries also charge the truck batteries.
I recommend for anyone with AC fridge needs at least 300 watts of solar. The only solution that have a return on investment. Mine saved the value of installation in batteries saved.
To me as a full timer, solar is the only solution after 12 years living full time in an RV.

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[mlpeloquin]

What most people do not under stand that charging batteries is not a one for one deal. It can take up to 50% more charge current or charge time to fully charge the batteries. 10 amp hours for five hours may take 20 amps for five hours to bring the batteries back up to full charge. It always take more power to recharge a battery than you get back out of it. So 13 amps for six hours of driving time will get you 13 amps for 3 hours back out. And do to the resistivity of the copper wire, the alternator's charging current will taper off as the batteries slowly go up in voltage. So it will take even longer to recharge and have less current to supply your rig. Remember when plugged in that your converter has heavy gauge wire connecting its output for a low voltage drop across the connections and its output in boot mode is 14.3V at up to 80 amps. That is why when plugged in the batteries get charged fairly quickly and when towing they do not.

300 watts of solar at its peak output is 22 amps at 13.5V. Running down the road or stationary so long as in the sun the panels are doing far better than a trucks alternator ever can through 16 gauge wire. If you dry camp for any length of time, solar is a cleaner way to go, but a generator will get the job done.