How to set trailer battery?

wooper · July 2019

Hi all...new owner of T@B 320 with many questions. When traveling and stopping for a period of time but not at a campsite, is it best to unplug so it won't run down my car battery? I have read the fridge with drain it..or is there a better option? Thanks again...I apologize about all of these questions..:}

Sharon_is_SAM · July 2019

If you have an S model - best not to tow with the frig on DC or it will deplete your TaB battery as most alternators can’t keep up with the demand. Some tow with the frig on propane, others carry frozen bottles of water and alternate on and off periods on DC.

Some tow vehicles turn off the charge line when the ignition turns off (isolation relay). If yours does not, best to turn off the frig if on DC.

The clamshell Norcold on DC is more efficient - 4 amps/hr vs 11 in the S. I think short breaks it would be okay to leave the frig on.

Bayliss · July 2019

@wooper, I have a 320S and do run our Norcold 3-way refrigerator on battery when towing, but my truck alternator can handle it. As @Sharon_is_SAM noted, if the tow vehicle alternator cannot keep up with the trailer battery demand, it will very likely be depleted by the time you get to your destination. Although I use the battery while towing, I do shut the refrigerator off when stopped for more than a few minutes (e.g., stopping to take a power nap or eat.) I leave it on during brief stops at a rest area or to fuel up. Although I admit that I have towed in the past with the propane to the refrigerator on, after researching the dangers of doing that and considering the opinions of others (which vary widely), I concluded that is a very dangerous practice. With my previous trailer (5th wheel), which had a refrigerator that only ran on propane or electric, I left the refrigerator "off" while towing. I just made sure it was cold inside before heading out and re-started it while stopped for longer periods (e.g., sleeping at a rest area overnight), or when I arrived at my destination. If you decide to leave the refrigerator "off" while towing, it is best to minimize opening the door as much as possible to help maintain the cold temperature inside.

wooper · July 2019

Bayliss...thanks for your comments...I'm pulling with a 2011 2wd Honda Pilot...how would I know if my alternator is strong enough to handle it?

Bayliss · July 2019

@wooper, I do not have enough working knowledge about alternators to directly answer your question, but there are a lot of very bright folks on this forum. Hopefully, one of them will chime in and enlighten us all.

My 2017 Toyota Tundra has a 150 amp alternator, which came with the towing package (rated to tow 10,300 lbs., so way more than I need for my T@B.) I did a quick Google search on your Honda Pilot and it looks like it may have a 130 amp alternator. I think I read somewhere on this forum that someone had a Toyota Tacoma that had that size alternator as part of the tow package for that model, so yours may be sufficient. HOWEVER, I do not know what the recommended size alternator is for keeping up with the power needed to charge a trailer battery. I just know that mine has been sufficient.

If you don't get an answer here, it might be worth checking with a Honda dealer and/or do a more extensive Google search to see what you find. A good RV dealership, towing equipment installer, or battery shop are also good sources to check with. Sorry I can't be of more help.

A COUPLE MORE POINTS TO CONSIDER: It would be helpful to others if you can identify the year and model T@B you are pulling, plus the size of your Honda Pilot engine. It is a good idea to add that info to your signature settings for this forum so that it appears on all your posts, but also include that info in your question heading so people know your specific situation. Additionally, you should find out how your 7-way wiring is set up. You want to make sure that it has a charge line (sends power from your tow vehicle to the trailer) and will in fact charge your trailer battery while towing. Also, confirm that there is an isolation relay switch included in the wiring connection so that when your tow vehicle is not running, but still connected to the trailer, the trailer battery is not pulling power from your tow vehicle battery and running that one down. (Hope this info helps.......Greg)

Cbusguy · July 2019

There are dc to DC chargers that will protect the tow vehicles starting battery and they work very similar to a mppt solar controler. It will take my 13.8 volt alternator output and boost it to 14.4 for my lithium batterys with no issues.

https://www.amazon.com/Renogy-Battery-Batteries-Multi-stage-Charging/dp/B07Q4SVX3M/ref=mp_s_a_1_3?keywords=renogy+dc+to+dc+charger&qid=1563936866&s=gateway&sprefix=renogy+sc+to&sr=8-3&th=1&psc=1

Of course you could travel with the fridge on propane, controversial as it is, it works just fine.

klenger · July 2019

The limiting factor for the available charge current from a tow vehicle has little to do with the alternator size in the tow vehicle. The length and wire gauge of the charge wiring is the limiting factor. I expect that in most cases, the actual charge current is less than 10 amps.

ScottG · July 2019

klenger said:

The limiting factor for the available charge current from a tow vehicle has little to do with the alternator size in the tow vehicle. The length and wire gauge of the charge wiring is the limiting factor. I expect that in most cases, the actual charge current is less than 10 amps.

While the size of the charge line may be an influence, I'm not sure I agree it's the limiting factor. Modern alternators (technically, I think we have gone back to calling them "generators" now...) have smart circuitry that decreases voltage output depending on the demands of the vehicle. Additionally, how that variable voltage output is controlled seems to differ among vehicles.

My understanding of battery charging is that you need a voltage differential to "push" power into the battery. While 14.4V will usually do the trick, 13.2V may not get the job done completely. Factor in an undersized charge line and you may not even be getting that.

Now add in a fridge running on DC and your battery may even be losing charge while you drive. This is what many of us have observed with our own campers. I've monitored my T/V generator output on trips and noticed recently that when it stays above 14V (for whatever reason) it keeps up with both the fridge and the battery charger.

I'm certainly not claiming to be an expert here, just suggesting that there may be multiple confounding factors at play for any given configuration.

The DC to DC charger recommended by cbusguy looks like a good solution, albeit spendy. Fortunately, the T@B fridge is also a pretty good cooler, so pre-chilling and stocking it with a bin of happy-hour ice is usually sufficient to keep it cold during transit. :-)

Bayliss · July 2019

As I alluded to earlier, how an alternator works and what size you need for your particular needs is above my pay grade. Based on the last couple of interesting and thought-provoking posts, I Googled "does size matter with an alternator." Found the below link which may be of interest to some...........it links to other discussions, so you may want to look at those as well. (If you do the same search, you will find several links to articles on alternators. When I have some free time, I plan to go back to them and try to educate myself further on this topic.)

https://www.onallcylinders.com/2015/03/27/alt-text-5-things-you-should-know-before-using-a-high-output-alternator/

wooper · July 2019

Thanks all...I think my alternator is closer to 115 amps, so probably with not run the fridge when driving.

davel4wa · July 2019

Alternators and DC generators, if you are old enough to remember those, all employ a voltage regulator. The old DC generators usually employed an external regulator mounted on the firewall and were purely mechanical devices. When alternators became the default in automobiles the voltage regulator was typically built into the alternator and was electronic. The voltage regulator's job is to maintain a constant voltage to the battery and automotive electrical systems.
That being said, voltage and resistance are what determines the current (amps) sent to any connected device. The charging voltage for your TV battery and your trailer battery are the same for the same battery chemistry. If your TV alternator voltage is not high enough to charge your trailer battery then it's not high enough to charge your TV battery. If there is so much current load on the TV alternator that it can't keep up, then the voltage will decrease which is something you will be able to see on your TV voltmeter which most vehicles have these days. Assuming that is not the case unless you are driving an ear damaging 500 watt audio system in the TV or have some other power hog running, the only thing to impede your trailer battery charging is the resistance presented in the charge line wiring and connector. In the article presented earlier in this thread you will see a chart for reccomended wire gauges/line length. Using that chart you would require an 8 awg line to your tow harness which seems unlikely and I am unsure if the 7 pole trailer connector can accommodate that size wire. The trailer wiring would have to be the same gauge as well particularly on the Tab 400 with the batteries in the rear of the trailer. With enough inline resistance the voltage drop could possibly fall below the required level to provide charging to the battery. It is absolutely necessary to clean the trailer connectors, both sides, on a regular basis to be sure that is not exacerbating the charging problem. Stranded copper wire in 12 AWG has a resistance on order of 1.6 ohms per 1000 feet. For 8 AWG it's 0.64 ohm per 1000 ft. Assuming a 10 amp charging current, the voltage drop for 12 AWG wire is 10*1.6/1000 or 16 millivolts per foot. Assuming a total wire length of 35 feet, TV length plus trailer length, would result in a voltage drop of 560 millivolts or just over half a volt. If the TV alternator is outputting 14.6 volts, fairly typical, then your trailer battery would be seeing about 14 volts which is probably adequate. Using 8 AWG
wire would reduce the voltage drop to about 0.25 volts and the battery would see approximately 14.35 volts. Again, every wire connection in the path from alternator to trailer battery will present some resistance. It is very important to make sure all of those connections are clean and tight. Measure the voltage across the trailer battery with your TV connected and running (you need to raise the TV rpms above idle as alternators typically output lower voltage at idle) and if it's less than 14 volts, you need to check voltage at each connection to determine where the voltage drop is occurring.
With other loads running, like the 12V refrigerator, that current needs to be added to the battery charging current to arrive at the potential voltage drops. Measure the voltage, it will tell all.

Cbusguy · July 2019

There are many ways to tackle the situation, Mine is successful with an aftermarket alternator, 4 gage run to the trailer with power supplied to the trailer through appropriately sized anderson connectors into the renogy dc to dc converter. Spendy as Scott mentioned, but effective.

my battery is fully charged when we get to the campsite and my fridge is cold which is the goal. correct?

there is no one way. find what works for you.

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