Fridge on 12V Questions

[bensinc]

The three-way fridge in my A122BH stopped working off AC, and I plan to fix it but haven't gotten around to it yet. In the meantime, I've let it cool down in the driveway on DC, which has worked great.

I usually don't power the fridge while driving, as generally I don't go too far and it seems to hold temperature pretty well if I have it packed up, and I usually bring some bags of ice in there too for drinks and to help keep it cold. I often end up boondocking so I run on propane when I get to camp.

Just for fun I decided to put it on DC for a long drive home the other day, and when I stopped the fridge wasn't very cold, and I when disconnecting the battery for storage I noticed the terminals were very hot to the touch.

Any idea why that would be? I wonder if it's something to do with my trailer wiring not being up to snuff (which I hear is common)?

[Flybob]

The 12V side of a 3-way fridge is usually used while traveling using the TV aux power. If you do not have your aux (12VDC) pin in the 7 pin connector active, the fridge using the battery. The fridge has a fairly heavy draw on battery. Check the aux pin on the TV.

[TrailerTexan]

+1

[gbaldwin]

my fridge is 2 way... But I travel with it ON....on propane.

[Ed Picardi]

I would definitely not blow off for too long taking a look at the back of your fridge. If there is a short in your electrical heating coil behind the fridge you can be burning up a lot of juice in your line instead of heating the coils. That would explain your hot wires and lack of cooling. Hot wires in the walls is not safe.
The fridge, in general, pulls a lot of current in 12 volts which I use only while actually driving. When stopped I switch immediately to propane to avoid draining my batteries.
When connected to 120v you probably have a converter so you can test to see if the fridge can work on the 12v side of the converter or the 120v line voltage in front of the converter.
I learned a lot about my fridge by pulling off the access covers when the battery and gas were off and studied the wiring, the switching mechanism, and the propane lines.

[TrailerTexan]

The fridge draws approximately 10A on 12 Vdc. Your battery is probably rated around 50 to 60 AH. (Some are as high as 230 but that's uncommon and expensive). This means that your battery can supply power to the fridge for about 5 to 6 hours ( 60 AH / 10A = 6 Hours). And the terminals on the battery will likely be warm if not hot as the voltage drops because the heater in the fridge is purely resistive to DC power. This means as the battery Voltage drops the current will increase until the battery is discharged. For example, the fridge will draw 12A if the battery drops to 10V.

Also a very good way to ruin a battery

Propane is really the best way to go when traveling if possible. If using 12 Vdc because you are uncomfortable then make sure your TV is connected and charging your RV battery. Note also that most trucks with towing packages have high-output alternators just for this reason while many SUVs (or vehicles "converted" to trailer use) do not. Be aware of that because that 10A load will need to be available from the TV continuously. Might mean the difference between using your TV lights or not, for example. Not meant to scare you just to make you aware.

As to the original problem, the fridge uses the same heater for both AC and DC operation. You can plug the trailer into AC power and see if it works; if so then the problem is either in the switching logic of the fridge or the battery caused the problem. Based on your description I suspect a drained battery however you can check it easily enough.

Locate the wiring to the heater with power OFF! Disconnect one or both of those wires and use a DMM on Ohms to check the resistance of the heater. It should be
somewhere around 12 Ohms but NOT 0 or close to 0. Too low a resistance indicates a shorted heater and should be replaced. Anything from 10 to 20 Ohms is probably OK. If that is OK then reconnect the wires.

Use a DMM on AC to check those wires when the fridge is powered on AC. You should detect approximately 120 Vac when the fridge is warm. If not then something is wrong on the fridge side. If it is OK, then the heater is OK.
At that point switch the fridge to 12 Vdc from the converter and measure for 12 Vdc on the DMM at the same wires while fridge is still warm. It should be close to 12 Vdc (or close to the charging voltage from the converter which could be as high as 13.8 but is usually closer to 12.5 under load). If that works then turn off the fridge, charge the battery until fully charged and then remove AC power. Perform the 12Vdc check with the fridge set to 12 Vdc and warm. The voltage at the heater should be close to 12 Vdc (or whatever voltage is on the battery). If not, then either the battery or trailer wiring is the problem.

[gbaldwin]

Texan..... and am I right in saying this 5-6 hours is actually 2-3 hours as I have learned on here that you dont really want to let your battery fall below 50%. Am I understanding this right?

[5picker]

Maybe the connections that got hot were loose or corroded?

Hot wires/hot connections come from just a few things like...
Loose connections
Corroded connections.
Too light of gauge wiring
Too big of current draw.

[5picker]

Originally Posted by TrailerTexan

The fridge draws approximately 10A on 12 Vdc. Your battery is probably rated around 50 to 60 AH. (Some are as high as 230 but that's uncommon and expensive). This means that your battery can supply power to the fridge for about 5 to 6 hours ( 60 AH / 10A = 6 Hours). And the terminals on the battery will likely be warm if not hot as the voltage drops because the heater in the fridge is purely resistive to DC power. This means as the battery Voltage drops the current will increase until the battery is discharged. For example, the fridge will draw 12A if the battery drops to 10V.

Also a very good way to ruin a battery

Propane is really the best way to go when traveling if possible. If using 12 Vdc because you are uncomfortable then make sure your TV is connected and charging your RV battery. Note also that most trucks with towing packages have high-output alternators just for this reason while many SUVs (or vehicles "converted" to trailer use) do not. Be aware of that because that 10A load will need to be available from the TV continuously. Might mean the difference between using your TV lights or not, for example. Not meant to scare you just to make you aware.

As to the original problem, the fridge uses the same heater for both AC and DC operation. You can plug the trailer into AC power and see if it works; if so then the problem is either in the switching logic of the fridge or the battery caused the problem. Based on your description I suspect a drained battery however you can check it easily enough.

Locate the wiring to the heater with power OFF! Use a DMM on AC to check those wires when the fridge is powered on AC. You should detect approximately 120 Vac when the fridge is warm. If not then something is wrong on the fridge side. If it is OK, then unplug the 120 Vac to the trailer and switch the fridge to 12 Vdc (with a GOOD battery charge!) and measure for 12 Vdc on the DMM at the same wires while fridge is still warm. It should be close to 12 Vdc (or close to the battery voltage which could be as high as 13.8 but is usually closer to 12.5 under load). If that works then the problem is elsewhere, such as the battery.

Correct.
Many folks don't realize newer tow vehicles equipped from the factory with H/D tow packages have higher amp alternators and they get triggered to output more when in tow/haul or with the lights on. (depending on make/model)

[pgandw]

Originally Posted by bensinc

The three-way fridge in my A122BH stopped working off AC, and I plan to fix it but haven't gotten around to it yet. In the meantime, I've let it cool down in the driveway on DC, which has worked great.

Just for fun I decided to put it on DC for a long drive home the other day, and when I stopped the fridge wasn't very cold, and I when disconnecting the battery for storage I noticed the terminals were very hot to the touch.

Any idea why that would be? I wonder if it's something to do with my trailer wiring not being up to snuff (which I hear is common)?

The only reason I can think of for the battery terminals being very hot is a poor connection of the terminals to the posts. Remover the terminals, clean the posts thoroughly (I typically use a wire brush to get clean metal to clean metal), replace the terminals, and tighten the terminals to the posts. You should not be able to remove them by hand.

If the terminals were hot, a lot of battery was wasted heating up the terminals instead of heating the DC coil in the fridge.

There could be other problems with the fridge, as well. My A122 fridge was astonishingly inconsistent until I went through a series of fixes I detailed in a thread a couple of years ago, I'm liking my A-frame fridge. If you are suffering the same inconsistency I did, it may explain the failure on AC as well as DC.

Fred W
2019 Flagstaff T21TBHW A-frame
2008 Hyundai Entourage minivan
camping Colorado and adjacent states one weekend at a time

[jflarin]

Originally Posted by TrailerTexan

The fridge draws approximately 10A on 12 Vdc. Your battery is probably rated around 50 to 60 AH. (Some are as high as 230 but that's uncommon and expensive). This means that your battery can supply power to the fridge for about 5 to 6 hours ( 60 AH / 10A = 6 Hours). And the terminals on the battery will likely be warm if not hot as the voltage drops because the heater in the fridge is purely resistive to DC power. This means as the battery Voltage drops the current will increase until the battery is discharged. For example, the fridge will draw 12A if the battery drops to 10V.

Could you please explain how the current will increase when the voltage of the battery drops. That goes against Ohms law...

Best Regards,

jf

[TrailerTexan]

Not really. Ohm's law assumes a non-dynamic situation and a heating coil is definitely dynamic even at DC. As the coil heats up the resistance goes UP. (Semiconductors actually do the opposite until certain conditions are met based on the construction of the semiconductor). The 120 W is based on the heated condition. If the voltage is reduced the temperature is reduced which results in a reduction in resistance and an increase in current. This will continue until the internal resistance of the battery increases to the point that the current isn't available. The battery will also increase in temperature resulting in an even higher internal resistance until current ceases.

The numbers I quoted are approximations because I don't have the actual characteristics of the heating element. Those numbers were to illustrate the problem.

You are correct that Ohm's law would result in less current for a given voltage on a FIXED resistance but heating elements aren't fixed and their resistance varies widely with temperature. That's also why a 120 W heating element designed to operate at 120 V typically does not read 10 Ohms when cold.

Don't get me started on loudspeakers. The physics and math to explain how a speaker is at a constant impedance over a large frequency range is daunting to most people and even some engineers I can explain it...but I don't want to. I'm retired and don't have to do that any more

Michael

[TrailerTexan]

As to operating time, batteries are rated in AH to the point of fully discharged. This means you are safe to operate the battery until that point. A 60 AH battery can operate safely with a 10A load for 6 hours. I don't know where the "50%" number came from but I'm assuming that's a "rule of thumb". If the battery is fully charged and you know your actual load then you can calculate how long you can operate.

A lead acid battery normally referred to as "12Vdc" is actually approximately 13.8Vdc when fully charged without a load. With a load it is typically anywhere from 12.3 to 13 Vdc based on the actual load.

A lead acid battery is "fully discharged" yet recoverable by charging at a voltage level of 11.9 V. I don't know where the "50%" came from but that isn't technically accurate.

Allowing the voltage to drop below 11.9 Vdc is the point where permanent damage may occur to the battery. Other battery chemistry types have different values. It is prudent to use an accurate, calibrated voltmeter to monitor the battery when operating without AC power and stop using equipment when the voltage drops to 11.9. The suggestion of using two batteries when boondocking is smart.

[jflarin]

Originally Posted by TrailerTexan

Not really. Ohm's law assumes a non-dynamic situation and a heating coil is definitely dynamic even at DC. As the coil heats up the resistance goes UP.
...

The numbers I quoted are approximations because I don't have the actual characteristics of the heating element. Those numbers were to illustrate the problem.

You are correct that Ohm's law would result in less current for a given voltage on a FIXED resistance but heating elements aren't fixed and their resistance varies widely with temperature.
Michael

Hi Michael,

First, thank you for answering. Being an engineer myself, I could not agree with your initial post and I had to note it. As far as I know, the phenomenon you described happens on AC/DC power supply (if you take a 90 to 240V AC input power supply regulating 24V DC for example). It also happens with a motor with a constant mechanical load. I'll also agree that tungsten filalement in light bulb have a resistance that varies very widely (easily by a factor of 7 if my memory is right) between cold and at normal operating temperature. But a constant wattage element, I've never heard or that or encountered that. I'd guess that somebody who can manufacture that could be rich very fast...

As you nicely described it, the situation is more complex because nobody has 'actual characteristics of the heating element'. You got me curious about heating element characteristics. I'll go research that...

Thanks and best regards,

jf

[bensinc]

Thanks for all of the replies!

I'm thinking my issue is the battery, possibly combined with some less-than-tight connections.

I've noticed the battery hasn't been keeping up as well in general, and I'm also pretty sure my TV doesn't supply enough power while driving. The first and only other time I ran the fridge on 12V while driving was with a different TV, so that may be part of it!

As for the fridge itself, it cools great when on 12V via the converter on shore power, so I don't think there's any issue with the fridge itself (at least on 12V).

[babock]

The fridge will not be drawing 12A @10V for a number of reasons. First off, the heating element is resistive as stated earlier but the resistance isn't going to change very much over temp/voltage. Second, good luck getting 12A out of a battery that is at 10V. A battery at 10V will have ZERO output. Even at 11V, a battery is pretty much dead.


For the OP, if you really want the tow vehicle to power the fridge, buy a DC to DC converter. Will actually charge your trailer battery too. It will be mounted in your trailer close to your battery.



https://smile.amazon.com/Renogy-Batt...s%2C226&sr=8-3

Originally Posted by TrailerTexan

Don't get me started on loudspeakers. The physics and math to explain how a speaker is at a constant impedance over a large frequency range is daunting to most people and even some engineers I can explain it...but I don't want to. I'm retired and don't have to do that any more

Michael

They aren't at all flat impedance...far from it...but back to the main topic.

[bikendan]

Originally Posted by bensinc

As for the fridge itself, it cools great when on 12V via the converter on shore power, so I don't think there's any issue with the fridge itself (at least on 12V).

If youve got shore power, why in the world would you have the fridge in 12v mode and not 110v mode?
12v is the least efficient way of running the fridge. Propane is the most efficient and 110v is second.

[TrailerTexan]

Agree that a battery is dead as a doornail at 11.9 V. My point was to point out that the dynamic situation with the heater can easily make it worse as the battery drains. No need to get snarky.

I also worked as an engineer developing amplifiers and speakers and the impedance is not perfectly flat but it does not vary widely due to the physics involved regarding distance of travel of the coil, magnetic properties of the magnet, etc. The reported impedance is usually at 1KHz or 100Hz for woofers but a lot of science goes into maintaining an impedance as close to that as possible for efficiency across the audio bandwidth. No need to get snarky there, either.

[babock]

Originally Posted by TrailerTexan

Agree that a battery is dead as a doornail at 11.9 V. My point was to point out that the dynamic situation with the heater can easily make it worse as the battery drains. No need to get snarky.

Its not dead as a doornail at 11.9V either. The fact is that at 120W, the coil has a certain resistance. It would have that resistance no matter what voltage and current was being used. In your example of 10V(assuming a battery could do that) @120W, you would have the same resistance as you did using 12V. Of course, you wouldn't be flowing 12A anymore. You would be flowing much less. The heating element would have a somewhat linear reduction in power output as voltage dropped. Of course the heating element would have slightly lower resistance at lower power output but not much. Bottom line, as voltage drops, current drops and power of the heating coil drops.

[TrailerTexan]

Virtually all lead acid battery manufacturers use 11.9V as the "depleted" point of the battery. Since you just want to argue, ok, fine, batteries will produce current until no potential exists. If you want to run it down to 1V and apply a 30 MOhm load then it will probably still produce a trickle of current. Battery manufacturers have more research on their side and even though the battery will produce some current at 11.9V it is a bad idea and shortens the life of the battery by causing cell damage. The amount of current varies on the condition.

Resistance of wire is linear when you ignore temperature coefficients (T sub C) which many do when temperature isn't an issue. Heating elements aren't necessarily linear because of temperature and the type of construction. Nichrome wire for example is used often and the resistance varies widely with heating and not completely linear because of T sub C and the turns/twists of the wire. I had a project years ago where I saw the very phenomenon I described and a very stringent customer wanted an explanation because it would affect our power margin on a system operating from limited battery power. We had to contact the vendor who provided a detailed white paper that explained the phenomenon. So it does exist whether you believe it or not.

Since you just want to argue, fine. I bow to your extremely superior intellect and promise to never argue any points with you in the future. Just consider yourself correct at all times and that no one else has a useful opinion. I will try not to disgrace your presence with my lowly opinion.