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Old 06-18-2018, 05:54 PM   #1
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Question Charging RV Batteries While Towing

I know that my truck is supposed to keep the RV batteries charged while towing. The truck has a 250A alternator,so that is not an issue.

If I hook my truck to my RV while parked, shore power off, truck, running, & truck to RV cable connected, how can I verify my truck is properly maintaining the RV batteries?

The controller for my Magnum 2800 shows the DC input to the inverter from the batteries (-25A), probably mainly for the refrigerator. I want to see how much DC the truck is sending to the RV which would indicate the RV battery charge is maintained. How can I verify this?

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Old 06-18-2018, 06:19 PM   #2
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Doesn't your inverter also show battery voltage? Hook up the truck, start it and measure the voltage at the RV batteries. Since the truck alternator is probably going to max out at 14.5 VDC and you will have voltage drop all the way back to the RV batteries you may only see about 13.0 to 13.5 VDC which is enough to maintain the RV battery, but probably not enough to increase the state of charge by much. The voltage drop is why most vehicles have a direct (short) cable from the alternator to the battery.

The higher the current from the truck to the RV the higher the voltage drop will be.
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Old 06-18-2018, 06:48 PM   #3
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If you have a multi-meter handy you could touch the #4 (12 volt Charging) and the #1 blade (Ground) on your trucks 7 way plug with the truck running and see if you get a reading. I did this with my truck and was getting a reading of 13.5 volts. I know we have made a couple of trips of 8 hours on the road and had no problem running the fridge. When we stopped and got setup I checked the batteries in the control panel before plugging in and they were still above 12 volts. Hope this helps.
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Old 06-18-2018, 07:12 PM   #4
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As others have said, measure the RV battery voltage.


The wire from the truck to the RV is relatively small, may have several connectors in it and is relatively long. The ground (return) wire is also long and relatively small. You cannot expect The truck to recharge a discharged RV battery but it should run the refrig and keep the battery from being discharged. Don't leave any power hungry devices on in the RV.
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Old 06-18-2018, 07:35 PM   #5
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Quote:
Originally Posted by Hunter11 View Post
I know we have made a couple of trips of 8 hours on the road and had no problem running the fridge. When we stopped and got setup I checked the batteries in the control panel before plugging in and they were still above 12 volts.
We FT and generally find this to be the case also. If for some reason our travel day is longer or the drain is greater. We run the generator.
BTW, for the most part a resting state 12 VDC. is about 50% SOC and about where the Magnum shuts down the inverter. I don't feel you can ask your vehicles charging system to serve two masters without cooking your vehicles primary starting batteries. Perhaps with some sophisticated isolation circuitry. Our boat's large frame true high output alternator had massive heat sinks and such an isolation controller to protect the starting batteries.
Your deep cycle house bank really requires the bulk, and absorb, cycles the Magnum provides. Your TV basically supplies a float cycle while traveling.


Quote:
Originally Posted by ;
How can I verify my truck is properly maintaining the RV batteries?
The best indication of your house banks SOC (State of Charge) will be a shunt type watt meter like solar folks use. Similar to a Trimetric etc. measuring watts in watts out.
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Old 06-18-2018, 08:16 PM   #6
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Originally Posted by Hunter11 View Post
I know we have made a couple of trips of 8 hours on the road and had no problem running the fridge. When we stopped and got setup I checked the batteries in the control panel before plugging in and they were still above 12 volts.
We have found this to be the same with us. That is until I installed the solar on the roof. After the solar instal we have had PLENTY of battery left after arriving to our destination.

Dan
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Old 06-19-2018, 02:29 PM   #7
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Originally Posted by David & Diane View Post
If I hook my truck to my RV while parked, shore power off, truck, running, & truck to RV cable connected, how can I verify my truck is properly maintaining the RV batteries?

The controller for my Magnum 2800 shows the DC input to the inverter from the batteries (-25A), probably mainly for the refrigerator. I want to see how much DC the truck is sending to the RV which would indicate the RV battery charge is maintained. How can I verify this?
As an earlier poster noted, the only way to accurately answer your question is to install a shunt-type battery meter which will measure both current (Amps) and Voltage. While voltage only can provide some information, it can be misleading due to varying loads on the battery, and the memory effect of the battery when the voltage is taken after a charging cycle (surface-charge).

Here is an article which describes the situation from a well-know marine electrical expert. Monitoring battery charge

This article explains that the standard approach is to use a shunt-type meter to accurately measure current. The current measurement is accurate (if the shunt is installed properly, and will answer the OP’s question (how to determine if the truck’s alternator is supplying charging current to the trailer’s batteries).

Note that a newer measurement approach is mentioned in the article also and involves a simpler approach — the meter estimates the capacitance of the battery by looking at how the voltage changes over time. I have no experience with this type of meter (which is more expensive) so I won’t recommend it.

I had the same question as the OP’s when I purchased a 2017 Ford Super Duty. I installed a Victron Energy BMV700 Battery Monitor. The monitor (including the required shunt) is about $150. It can be ordered online from Amazon, but I purchased it from Peter Kenedy’s web site since he is the go-to guy for 12 volt power https://shop.pkys.com/Victron-Energy...or_p_2810.html

Installing it took about 1/2 day as it required that the shunt be connected to the battery’s negative post, and all previous connections to that negative post, be connected to the other side of the shunt. In this way, the shunt sees and can accurately measure all the current going into and out of the battery. Doing the hookup required adding a new negative bus bar in the tight battery compartment. This installation should only be attempted by someone who has experience in heavy duty 12v power systems. My trailer uses two six-volt batteries, so it was essential to select the proper battery’s ground post. I had to fabricate a couple of new battery cables out of heavy 00 welding cable, which required the use of a hydraulic terminal crimper which I had previously purchased for a van customization I did a few years ago. A connection is also needed to the positive side of the battery to power the meter, and to enable accurate voltage measurements of the battery’s voltage.

I mounted the meter display in the pass-through compartment near the inverter on a small piece of 1/8 inch thick maple. One cool thing about this meter is that supports an optional blue-tooth dongle (which costs about $50). Adding this provides the ability to get all the battery information on my iPhone. So, I can monitor the battery’s state of charge, voltage, and loads from either the truck cab, on while setting in the living room of the trailer.

I hooked up my trailer to the truck, and killed the truck’s engine, and used the iPhone to look at the voltage, and current being used by the trailer at rest. Then I started the truck, and watched the voltage increase, and the current being used by the trailer go from negative (batteries being discharged) to positive (batteries being charged). I determined that the truck’s alternator did a decent job of charging the batteries. During a hot day in May which moving the trailer from Arizona to Southern Colorado, I left the iPhone display on the Victron app for several hours which driving, and could see when the refrigerator compressor started running, and was a able to see how the truck’s alternator adjusted the current output accordingly.

After a fuel / rest stop, I could see the battery’s state of charge go from below 100% back to 100% full charge after driving awhile.

Since I started the journey with the battery’s fully charged, I can not comment on how many hours of driving would be required to bring a battery at half-charge back to full charge. I suspect it would take much longer, than using the generator in my trailer, and the high-performance charger my inverter has.

If there is interest, I can post some pictures of my installation. Let me know if anyone would find pictures useful.

Again, the install is not complicated, but is best done by someone comfortable with working with high-amperage battery wiring and 12volt power systems. I would guess that very few RV mechanics would understand how to properly install a shunt-based battery meter, as I don’t think they are commonly installed on RVs.

Good luck.
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Old 06-19-2018, 03:17 PM   #8
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We drove to DFW, TX from SoCal only stops were for fuel and 3 two hour naps, never shut off truck. Refrigerator was 0 degrees freezer and 38 frig upon arrival. Gen was silent the whole trip. Just completed a 14 hour tow from NoCal to NW WA same results 14 hour trip.
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