Tug Can'k keep up with DC fridge requirements?

[Kip in Ga.]

Well look at him!
He looks like a trouble maker.

Originally Posted by Raz

The voice of experience. I lost a battery that way. The DC switch got turned on for two days. I blamed it on the dog.

[Darral T.]

Yessir...while it's considerably more expensive, I also have a Battery Minder! I keep it on my Corolla that gets driven very little. BUT, it also has an 8A charge rate should we leave the lights or something on a vehicle and need a to charge it up quickly. Of course with these chargers, you dont have to worry about them over-charging at ALL. I also have 3 of the .75A Battery Tenders. Do I believe in battery maintenance???

[QUOTE=Kip in Ga.;633130]Agreed on keeping batteries topped off 24/7. I use "Battery Minder" brand which keeps the battery at full charge as well as de-sulfates them.

[RogerDat]

The manufacturer of a vehicle puts an alternator in that meets the anticipated needs of the vehicle. A new, known good alternator is only known good in respect to intended output. An RV refrigerator is probably not a load factored into intended output.

Yes lesser gauge of wire can drop voltage and the greater the voltage difference between line and battery the better the charge rate will be. But (you knew there was a but coming right?) If the amperage output of the alternator is not sufficient to run the heating element the 3 way fridge uses to replace a propane burner you will lose charge from the battery on that circuit.

If you think about it the way you would planning solar capacity, if heating element of fridge draws down more amps per hour than alternator feeds into battery eventually you end up with depleted battery.

[Jann Todd]

Originally Posted by Kip in Ga.

Normally I run the fridge on DC while towing. Hardly anything in the fridge during towing. When arriving at the campground, first thing to do is plug in to power. Same thing on arriving home. Never "Noticed" any problem with the fridge. But never had any cause to look for a problem.

This last trip when arriving home we cleaned out any fridge leftovers and turned it off.

Parked the camper under it's shelter but a few hours passed before I made the last walk thru. Daylight was fading so I turned on the interior lights and they were dim. Monitor showed the battery at 11.7 volts.

I plugged in to AC power and the voltage showed 13.5 charging volts within just a few minutes. Turned in the fridge and voltage remained at 13.5. Turned off the power supply and the voltage dropped as was expected.

Turned off the fridge and Plugged in the Tug with the engine running. Voltage
climbed to 13.2 quickly. Turned on the fridge and the voltage began to drop with the tug still running.

Apparently the tug had not been keeping up with the fridge requirements while traveling.

Turned fridge off and the AC power back on and let the camper power supply do it's thing. Next morning I turned off the Power supply and slowly over the course of several hours the battery settled in at 12.7 volts. That's good! That was last weekend 3/19-3/20. The battery is still showing 12.7 indicating it capable of holding the voltage.

FWIW the battery is about 7 yrs old.

All that to ask if anyone else has experienced the TUG not keeping up with the fridge requirements. ? ? ? And if anyone has any ideas?

Thanks,
Kip

I would venture to say your battery has about had it. Just because it is holding a charge without any drain doesn't mean it will hold up to running the fridge while driving. We tow with fridge on and on 12V so we don't have to worry about turning it off when fueling up. It is has a small freezer so it is a little bigger than some in fiberglass units. We've never had a problem with our battery dying unless it is really hot and the fridge has to overwork. And then it was just a little low and the battery was on its way out. If it always worked before and now it's not then I suspect the battery. Also keeping the fridge full of cold foods will help it stay cold and not have to work so hard. It's harder to keep an empty space cold than a full one. If you don't want to put food in it then full a couple of jugs with water and freeze them before you leave and put them in the fridge. The water is usable also for drinking or anything. Always chill the fridge at home on 120V before leaving. We've done this for 36 years in several units and no problems.

[Kip in Ga.]

Amazing the things that come to mind, early in the am.

Got up around 5:30, and the coffee was perked and ready to go. Poured a cup and sat on the deck to hear and see the world beginning to awaken. We are semi rural so not much people activity. Even when folks are leaving for work!

Then it hit me. I remember helping my neighbor run some heavier wire directly from his Tug battery's pos post to the rear, for the same reason being discussed here. I only helped get the wire to the back of the truck. Don't know how or where he went from there.

I remember asking him later if the new wire did any better. Seems he said "NO". But there again the problem could have been with his battery. As it could be with mine.

Ran some more test with mine yesterday and the tug easily keeps up with the voltage with light loads. But with the fridge (ONLY) running on DC, the tug can't keep up, PERIOD. Will try to find the time in the next day or so to swap the current battery with a 1 year old Deep cycle battery I have here.

I do appreciate all the advice and help.

Thx
Kip

[Raz]

A larger wire may solve your problem, it may not. There are 4 variables: alternator capacity, charge line size, state of charge of the tow battery, and any load on the tow battery. Everyone's situation is different.

Charging the battery with no load In this case everyone wins. The alternator should have enough capacity to charge the tow battery. Some will suggest that too small a wire will cause a voltage drop. Not true. Because there is a battery at each end of the charge line, the voltage drop is always the difference between the two batteries and is completely independent of wire size. What wire size does is determine the charging current and thus how long the battery will take to charge. A 16 ga charge line will charge a battery but it might take longer than a typical travel day. I use 10 ga. wire when I install a charge line. My truck, which came pre-wired has 12ga., though it might be 14ga. Charging just the battery has never been an issue.

Charging with a load. Now adding a load, like the refrigerator, complicates things by adding a constant drain on the battery to be charged. As the charge line size determines the charging current, bigger is better. The circuit needs to provide current to the fridge and have enough to also charge the battery. The alternative is warm food and a dead battery. Adding a larger charge line will solve the problem, only if the alternator has the additional capacity to handle the new load. With all the electrical demand many vehicles have, many alternators are not up to the task. Trucks and large SUV's tend to have larger alternators and so a larger charge line is worth a shot. I've never bothered adding a bigger line to my Frontier as I like to drive with my headlights on and suspect any excess capacity is taken there. Also, as the truck came pre-wired it may very well be that the designers intentionally used small wire to limit the load. Raz

[redbarron55]

I have a Norcold 704DC swing compressor 12 or 120 and it draws about 40.watts running and it cycles about 50 %.
Of course the tug has no problem with power to the trailer.
The fridge runs on 120 actually it it is there and 12 DC if not.
It automatically switches off when the battery voltage goes too low.
Old tech but it has worked for 20 + years so far.

[Kip in Ga.]

I don't know near what I should about this subject, so at best am making some assumptions! (gasp)

I do understand, or think I do, that draining a lead acid battery more than 80% or so can began to damage it. So I do all possible to put a full 100% charge on
all batteries, when they get down 50% or so. Preferably more often than that. During storage I try to keep the voltage at 90+% on anything rechargeable.

With that said, 2 questions arises.

>With no charge power going to the battery.
With a good fully charged 100 amp battery, shouldn't a fridge drawing 10 amps run continuously for 7 hours and still leave a 30% reserve in the battery? Maybe longer as it won't run continuously once the thermostat is satisfied?

>Why does battery voltage show lower with a load on the battery?

FWIW: Info from Trojan Batteries reads like this:

% of charge....6v.......12V.........36v

100..............6.37....12.73......38.20
90................6.31....12.63......37.85
80................6.25....12.50......37.49
70................6.19....12.37......37.12
60................6.12....12.24......36.72
50................6.06....12.10......36.31 [ Rec max discharge]
40................5.98....11.96......35.87 [ Rec max discharge]
30................5.91....11.81......35.44 [ Rec max discharge]
20................5.83....11.66......34.97.... [Possible Battery damage]

[Carl V]

Originally Posted by Kip in Ga.

>Why does battery voltage show lower with a load on the battery?

Voltage drop under load is why you can't actually run a 10A fridge for 7 hrs on a 100Ah battery and still have 30% capacity left.
After a few hours under a 10A load, the voltage will be around 10V or so, and at that voltage the fridge will not cool much and keep draining the battery for nothing.

[Orlen Wolf]

Re: your posted questions. Sorry for the long response but I do think this is a more complicated problem than is, at first seen. I've also seen incomplete and incorrect responses posted to this question.

2 questions arises.

>With no charge power going to the battery.
With a good fully charged 100 amp battery, shouldn't a fridge drawing 10 amps run continuously for 7 hours and still leave a 30% reserve in the battery? Maybe longer as it won't run continuously once the thermostat is satisfied?

The battery is 100 amp hour, not 100 amp and that rating is usually specified at a 20 hour rate i.e. 5 amps for 20 hours to essentially 0 charge. If you discharge it at grater than 5 amps you will get less total amp hours.

Another note - There is no thermostat in the DC line on any RV refrigerator I have seen so the 10 amp draw will be continuous. The DC resistance of all refrigerators I have checked over the years has been very close to 1 ohm so your current draw could be about 12 amps.

>Why does battery voltage show lower with a load on the battery?

A fully charged battery should read about 12.7V when it has been setting idle for a few hours. There is also a so called surface charge on a battery right after it has been fully charged because the charge voltage must be higher than the battery voltage to force a current to flow into the battery to charge it. When you put a load on the system that surface charge quickly disappears and you are back to 12.7V.

As a battery discharges the terminal voltage drops as you can see from the Trojan Battery state of charge chart you provided below.

Analysis of a camper electrical system is actually fairly complex but it can be reduced for mathematical calculations. Consider the central point in the system as the power convertor. There is normally one point in the camper convertor where the coach battery, charge line and 12V circuits all come together. I electrical engineering this is called a node.

The sum of currents going into and out of this node is zero and there are resistances associated with each leg. The charge line is only a source (i.e. provides a current to the node), the coach battery is a source/sink (i.e. it can either provide a current or accept a current depending on its state of charge) and the refrigerator is only a sink (i.e. it can accept a current based on the resistance of the heating element). For discussion I will designate these currents as I1 for the charge line, I2 for the coach battery and I3 for the current hungry refrigerator.

With the refrigerator disconnected from the circuit (I3=0), the tow vehicle engine running and the battery fully charged I1 and I2 are zero and the voltage measured at the node is 13.8. As soon as you connect the refrigerator I3 becomes about 12A and, since the sum of currents into and out of the node must be zero the coach battery I2 and/or the charge line I1 must provide the required current. The charge line voltage is held st 13.8V at the source (the front of the tow vehicle) and the coach battery voltage is variable based on its state of charge so I1 and I2 will both contribute to the the current into the node.

The charge line has about 0.06 ohm resistance (based on about 20' of #14 wire plus the wire from the trailer connector on the TV to the converter) and the line from the coach battery to the node has about 0.005 ohms resistance (based on 5' of #10 wire). The coach battery output will quickly drop toward 12.7V as the surface charge is dissipated. The coach battery will begin to discharge and its terminal voltage will slowly drop causing a drop in the node voltage since the resistance in the coach battery lead is much much less than the resistance of the charge line. As the battery voltage drops the charge line will pick up more and more of the current load until a point is reached where the charge line is providing almost all of the current and the battery is almost completely discharged.

The calculations needed to determine the coach battery discharge curve are somewhat complex involving a lot of mathematical hand waving and gefinger poken on a calculator (or a lot of slide rule slipping if you are old school) but suffice to say that at the end of an eight hour drive your battery charge will be down to about 25% or less. That is way lower than the recommended 50% minimum battery charge level and approaches the "possible battery damage" level shown in the Trojan Batteries chart you posted.

Increasing the size of your charge line to #10 will reduce the resistance by 2/3 which will increase the equilibrium point somewhat. I use a #6 wire on my tow vehicles just because I had a lot of silicon insulated #6 wire purchased at a going out of business sale at at the local A-bomb trigger plant. But that doesn't completely eliminate the problem.

There is a true solution but it involves another piece of technology, a voltage boost module. This module is wired in series with the charge line at the node. Cost is in the $100 range for a unit rated at 40A max. These units are frequently used by ham radio operators who run their radios from 12V backup power systems for portable operations or when commercial power fails during disasters. A web search will quickly find sources for these devices. Some companies that manufacture/sell them are Hellroaring, West mountain Radio, DX Engineering and MFJ Enterprises.

FWIW: Info from Trojan Batteries reads like this:

% of charge....6v.......12V.........36v

100..............6.37....12.73......38.20
90................6.31....12.63......37.85
80................6.25....12.50......37.49
70................6.19....12.37......37.12
60................6.12....12.24......36.72
50................6.06....12.10......36.31 [ Rec max discharge]
40................5.98....11.96......35.87 [ Rec max discharge]
30................5.91....11.81......35.44 [ Rec max discharge]
20................5.83....11.66......34.97.... [Possible Battery damage]

[Kip in Ga.]

Obviously this stuff is a lot more complicated than I had imagined.

FWIW I found the owners manual on the fridge.
It is the 4.0 cu ft model RM2453.

2 things of interest.

1. The fridge pulls 15 amps

2. It also suggest not running it on DC "while parked"!
According to that manual, it will run down the battery quickly.

So due to some good advice from you guys about not running it on DC even when the camper is plugged to 120ac, because an interruption in AC power could cause the fridge to literally destroy the battery.

Obvious that my tug isn't going to keep up with the fridge without some wiring changes Including some type of relay that would separate the tug from the trailer when the truck isn't running. I am seriously not qualified or comfortable attempting that.

Also controversy concerning running the fridge on LP while towing. Although I've done that successfully several times, I prefer to error on the side of caution now.

I didn't see it in my manual, but others have said the RV fridge will drop less than 10 degrees in 5 or so hours provided the door is not opened. Assuming they are talking about a fairly full one.

Recon we will just get everything cold like we have been doing, and go ahead and put it in the fridge instead of a separate ice cooler. Was trying to keep weight off that curb side tire. Will still carry water in the tug but, all other food + a couple bottles of frozen water will go in the fridge.

I do appreciate the time and effort you folks donated to this. Didn't get the answer that was hoped for but got the answerI needed.

k

[Glenn Baglo]

Drove five hours up from Oregon with a pint of ice cream in the freezer of my 5 cu. ft. Dometic. Fridge was turned off and door was not opened.
When we got home, the ice cream was no longer frozen solid, but perfect for scooping.
We generally fill the freezer with frozen meats prior to leaving from home.

[Kip in Ga.]

Ran a test yesterday on the battery, by running the Fantastic Fan on high for 4 hours.The voltage dropped from 12.7V to 12.2V.

Ran the same test last year and the voltage dropped from 12.7V to 12.5 volts.

That is quite a difference. So as some of you pointed out, the battery is INDEED beginning to fail. So replacement is coming.

Thanks,
k

[redbarron55]

I installed a factory wiring kit on my 2016 Chrysler Town and Country and I found that they ran the power directly from the battery to the trailer jack at the rear.
What I found interesting was that they also ran the ground for the trailer jack from a grounding stud at the battery all the way back.
This means that except for incidental grounds the effective length of the charging circuit is twice as long. So...twice as much voltage drop.
You might want to make sure you have a good ground near the 7 pin jack to the chassis.
In my experience many people have trouble finding problems because they are not looking at the ground returns.