Charging trailer battery while driving

[Raspy]

Originally Posted by ThomasC

Do you want to charge or maintain the battery. If maintaining the battery is all you need all of the advice you have gotten is great. If however you want to truly charge your trailer battery while driving you will need a battery to battery charger and heavier gauge wire.

Thomas,

Now plug a more realistic number into your calculator, the voltage of a running vehicle that you want to charge from, at its battery terminals. That voltage is between 14.1 and 14.5, which is the standard voltage of a vehicle charging system.

Then plug in a more proper wire size, such as used with the Anderson plug, of #6

Then plug in 40 amps.


The voltage at the end is 13.47 (using 14.1 as the source voltage), with only a .63 volt drop. That is a good voltage for charging the trailer battery, and a good amount of amperage, not just a maintenance charge. It would be better if it was 14.1 at the trailer, but 13.47 will charge very affectively, and is higher than float voltage. If the trailer battery is at 50% charge, it is only at about 12.2 volts as a starting point.

Then, when the trailer and the tow batteries reach full charge, the voltage at the tow can drop to a float voltage of about 13.2, and the amps can go down to about 5 or less. Not all vehicles have this feature, but Rams do, and many others must too. This still results in a float charge at the trailer of 13.12, which is above full charge and a reasonable float voltage. The battery is fully charged at that point.

This is why I recommend an Anderson plug and not just adding a #12, or #10 wire in the seven pin plug. It is a very simple upgrade and does not require a DC-DC charger. An additional advantage is that while parked, a suitcase solar system can be plugged into the trailer Anderson plug, which means no added trailer wiring for the solar. Or, the solar can be plugged into the tow vehicle's Anderson plug to charge its battery if it becomes discharged. Also, if the batteries are not on the trailer tongue, they will have longer than necessary wires going to the seven pin, in the trailer. This is because they probably have a ground bus somewhere, and a frame ground to combine the brake with the other trailer ground. With the dedicated #6 wires, you get a more direct, shorter path to the batteries with fewer connections along the way.

If someone is going to go to the trouble of adding a switched charging wire to the seven pin, and change to a seven pin from a 4 pin, and add a possibly needed ground wire to complete the circuit, and still have the added resistance of #12 wire on the trailer side, why not just upgrade to shorter #6 wires from the tow battery and add an Anderson plug? The Australian trailers come with this setup from the factory and it works very well.

[mlequo]

Originally Posted by Lynn Eberhardt

If you don't want to mess with your existing wiring, the easiest way would probably be to add a second connector between car and trailer. To make sure you have enough capacity, run a 10 gauge wire (black or red) from your car battery positive to the back of the car. Connect another (white) wire to a convenient chassis point near the rear of the car (or run all the way from the battery) for the negative. Run another black (or red) and white pair from the trailer battery long enough to reach the car. The positive wire should have a 30 amp fuse at the trailer battery. Affix an Anderson or similar polarized pair of plugs at the connection point.

To be sure you don't discharge the car battery while camping, either unplug your new charging wire when camped or use a relay between the car battery and the trailer. If you're electrically knowledgeable and have easy access to to a signal voltage to turn the relay on only when the car is running, you can wire it that way. Or you can spend more and use an easy to wire inline voltage sensing relay. Good luck with your project.

I have settled on Anderson plug and 4GA wire from a single jump cable I found in a back alley. Added 50 Amp breaker and voila! I may add relay in the future. Thank you all for your help!

[Eggcentric]

Solved that problem when we went full time in a new 5th wheel in 1999.
Run a single wire from the positive terminal of the battery back to the rear bumper, put a fuse on it near the battery.
Buy a two prong connector at any auto parts store, connect the negative terminal on the frame of the truck, the positive terminal to the wire from the battery, connect the other end of the negative to the trailer chassis, then connect the other positive to the trailer battery, again with a fuse, making sure that you have enough slack to turn tight corners.
Advantage is you can connect the trailer directly to the trailer and either steal power from the truck, (Careful not to kill the truck batt.) or run the engine to charge the trailer.
Occasionally on a long dry camp such as months long stays on the beach in Mexico I connected the two batteries to make SWMBO's morning coffee, with the invertor.
20 odd years of doing it thatta way and see no reason to get more complicated.

[wvorih]

Some of these later posts discuss the reason why it is almost impossible to charge the trailer battery while towing.

The voltage drop from the tow battery to the trailer battery is going to be more than 0.5 volts.
That's enough that the trailer battery will never properly charge up. After wrestling with this problem for years I found the following solution.

In principle I run the 12 volts from the tow vehicle to an inverter in the trailer.
Now I have 120 volts ac in the trailer while driving.
I put an ordinary battery charger in the trailer and plug it into the inverter and connect it to the trailer battery.

Note that the 12 trailer power must not connect directly to the 12 volt car power. Only to your charger and the trailer load.

Every time I arrive at my next stop, the trailer battery is fully charged! Even if it was pretty far run down at the start.

This is a relatively cheap solution since a 300 watt inverter and a12 amp charger are all you need and Walmart has good ones.

As some one suggested, I put a relay on the wire from the car battery to cut the current when the engine is stopped. This solution does not require a heavy gauge from the tow battery since the voltage drop is not critical. The 12 charger is also handy to keep the trailer battery charged from shore power while camped.

Good luck.

[Eggcentric]

I agree, in theory the trailer batt never gets fully charged, and the difference between battery types does not help.
In practice back in 1999, before we got our first solar pv panels we were able to do very well. We also used to connect up the truck to the trailer after returning from a drive, letting the batteries equalize which they do slowly. Also in the morning I would start the truck and connect the batteries while the diesel warmed up.
Must admit that sometimes I forgot to disconnect the truck overnight, but the diesel had two batts and started very easily.
In short there are better ways to bridge the battery gap, but I found my method to be the simplest, and effective. Did it once again 3 years ago on our way to Mexico. Did it in the rain, in Oregon and it took about 1/2 an hour.
Lots of compromises when you go full time in an RV, this one worked well.

[Jon Vermilye]

When I switched to lithium, I added a DC to DC converter to the charge line set to 14.4V (the charging voltage for lithium). I get 9 amps (what I wanted) at the trailer when ever towing.

[computerspook]

When I look at what I can find about the wiring for your trailer it is supposed to have a 7 pin rv plug anyway. I think I would find out what kind of wiring it really is supposed to have an try and replace it right.

[ThomasC]

Originally Posted by Raspy

Thomas,

Now plug a more realistic number into your calculator, the voltage of a running vehicle that you want to charge from, at its battery terminals. That voltage is between 14.1 and 14.5, which is the standard voltage of a vehicle charging system.

Then plug in a more proper wire size, such as used with the Anderson plug, of #6

Then plug in 40 amps.


The voltage at the end is 13.47 (using 14.1 as the source voltage), with only a .63 volt drop. That is a good voltage for charging the trailer battery, and a good amount of amperage, not just a maintenance charge. It would be better if it was 14.1 at the trailer, but 13.47 will charge very affectively, and is higher than float voltage. If the trailer battery is at 50% charge, it is only at about 12.2 volts as a starting point.

Then, when the trailer and the tow batteries reach full charge, the voltage at the tow can drop to a float voltage of about 13.2, and the amps can go down to about 5 or less. Not all vehicles have this feature, but Rams do, and many others must too. This still results in a float charge at the trailer of 13.12, which is above full charge and a reasonable float voltage. The battery is fully charged at that point.

This is why I recommend an Anderson plug and not just adding a #12, or #10 wire in the seven pin plug. It is a very simple upgrade and does not require a DC-DC charger. An additional advantage is that while parked, a suitcase solar system can be plugged into the trailer Anderson plug, which means no added trailer wiring for the solar. Or, the solar can be plugged into the tow vehicle's Anderson plug to charge its battery if it becomes discharged. Also, if the batteries are not on the trailer tongue, they will have longer than necessary wires going to the seven pin, in the trailer. This is because they probably have a ground bus somewhere, and a frame ground to combine the brake with the other trailer ground. With the dedicated #6 wires, you get a more direct, shorter path to the batteries with fewer connections along the way.

If someone is going to go to the trouble of adding a switched charging wire to the seven pin, and change to a seven pin from a 4 pin, and add a possibly needed ground wire to complete the circuit, and still have the added resistance of #12 wire on the trailer side, why not just upgrade to shorter #6 wires from the tow battery and add an Anderson plug? The Australian trailers come with this setup from the factory and it works very well.

Well that is just it if your tow vehicle is in good shape a few minutes after starting the voltage will drop into the thirteens. That is why I picked that number. No matter what you do you can't "properly" charge the house battery without some extra help.

[gordon2]

Boosts input voltages as low as 9 volts up to 13.8 volts at up to 30 amps peak...
https://mfjenterprises.com/products/mfj-4416c

[Raz]

Your trailer battery is not a power tool.
We're all familiar with what can happen when we use a large extension cord to power a table saw. A voltage drop due to the resistance of the extension cord can reduce the voltage to the saw resulting in a reduction in power delivered to the saw. People have a tendency to apply the same logic to charging a battery. What they forget is that unlike the table saw, the battery contains a chemical reaction that produces it's own voltage. The idea that a large charging current can cause a voltage drop thus providing insufficient voltage to fully charge the battery is incorrect. The voltage drop idea would require the battery terminals to be two different voltages at the same time; one due to voltage drop and the second due to the chemical reaction. Obviously that can't happen. In posts 15 and 16 I gave the mathematical proof. I suspect most ignored it . Perhaps too much like going back to school. . Basically the voltage drop on a charge line is the difference between the two battery voltages and is completely independent of the wire size. The wire size determines the charging current.

My truck came factory wired with a charge line that appears to be 12 ga. wire. My trailer has a group 24 battery that is seldon discharged more that 25% (75% state of charge). It has never failed to fully charge in a few hours drive. Using a larger wire, say 10 ga., would simply shorten that time. If your battery is not charging, the problem is not due to voltage drop.

First, turn off all loads in the trailer. Trying to run a refrigerator, or charging computers, tablets, cell phones may stop your battery from charging. These, as well as trying to charge large capacity batteries require larger wire and an adequate alternator and is a completely different situation than simply trying to charge the trailer battery on the average fiberglass trailer.

The resistance of the charge line determines the charging current.
Make sure all connections are good as they are part of the charge line circuit. I carry a chemical spray (called tuner spray in the old days) to clean the 7 pin connector. Refrain from using abrasives as they remove the conductive plating on the pins. Contrary to popular belief dielectric grease is the wrong stuff to use on any of your contacts. It has the conductivity of glass and will definitely interfere with charging. Also, unless you live in the arid south west, using your frame as a conductor is asking for trouble. Any exposed connections will quickly corrode.

Assuming your battery is in good shape, you eliminate internal trailer loads, and you minimize the resistance at the connection points, there is no reason that your battery won't fully charge in a few hours of driving without the need for very large wires, special connectors, voltage to voltage converters, inverters, etc.

I'm a retired EE who spent over 25 years teaching this stuff at the college level. While I'm happy to answer questions I have no interest in arguing. If you think you know more than I do, good for you. As I said, my battery charges fine.


.

[dleverton]

For some years I have been using a module available on ebay to charge the house battery on my Trillium. The well thought out posts above reveal the problem that your alternator is a voltage regulated device which is a long way from your house battery. The voltage drop from alternator to house battery (IR drop), is significant and wire and connectors all contribute to the drop. The charge current the battery accepts is very voltage sensitive.
The 5 amp buck boost module below takes whatever voltage that arrives at its input and converts it to a voltage ( higher or lower) that you choose. It has a display to tell you the voltage you present to the house battery and a potentiometer to set that voltage. It limits at 5A (higher current units are available). If you drive 6 hours you put 30 amp hours into your house battery which has worked well for me. I put one on the house battery in my VW van and another on the house battery in the Trillium.

The modules input power requirement (watts) equals its output power plus 5% to 10%, so 5A X 13.6V = 68W + 6.8 = 74.8W in. If the voltage reaching the module is say 12V then the module requires 74.8W / 12V = 6.23A. Power in is power out + 10%.
Here is the one on my van house battery.

As I mentioned, I have used this for years and a number of road trips, and it has worked well.

[dleverton]

Opps, the ebay add I included was the wrong module. Let me look to see if I can find the one I used.
By the way, you don’t need a buck boost because it will always operate in boost mode, so a 5A boost module will do just fine.

[dleverton]

Here is the one I’ve been using.

You might be put off by the low price, but this module is on a proper fr4 circuit board and uses the industry standard components. I cannot source the parts for that price. It’s like when Mom looked at the sale item and said “I can’t make it for that”.

[kipper]

Lots of good info here to much for me to digest.

I am in the process of what Raspy has done except included a DC-DC converter (Renogy) so that I can charge properly at the trailer battery and I also installed a blocking diode so that the Renogy doesnt backfeed to the TV battery.

Once complete and tested will post my circuit diagram

[Jon Vermilye]

Originally Posted by kipper

Lots of good info here to much for me to digest.

I am in the process of what Raspy has done except included a DC-DC converter (Renogy) so that I can charge properly at the trailer battery and I also installed a blocking diode so that the Renogy doesnt backfeed to the TV battery.

Once complete and tested will post my circuit diagram

Check to see if the Renogy already provides back feeding between the trailer & tow vehicle batteries. You may not need the diode. My Victron DC to DC converter does not require a diode...

[computerspook]

I have actually never needed to charge from my TV. I generally am able to leave fully charged. Of course I have the frig and at times it pulls more power than the TV can provide. Which can be an issue.



Generally I am going where I have easy access to power, or I take my generator and then top things off. If I am running on the generator I only run it long enough to charge the battery and then shut down and wait to discharge it. Do have a voltage meter in the camper for the battery.


One thing I have is an over kill 60A converter. The reason I went with that is an added feature. I had to do a bunch of extra wiring to make ti work, but it has a seperate battery connection. If there is no AC then the battery is connected to the camper electric to power it. When there is AC the battery is not connected to the camper 12 volt wiring. Instead it is connected to a float charge battery maintainer and the converter powers the camper electric. The change over is seamless.

Decided that was what I wanted to do so did a lot of web research to find this. But it does work fabulously.

If I was pulling power from the TV I would be thinking about making sure I was limiting how much. and then use a boost converter to actually charge the battery.