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Old 04-17-2016, 10:44 PM   #15
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If you are using 175 watts when operating your fridge on 12v, this sounds like the power requirement of operating the fridge heating element. When on 12v the heating should be by propane and not electricity. 12v should only be used for fridge control electronics. Heating of the chimney should be electrical when plugged into 120v or by propane when 120v is not available.

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Old 04-17-2016, 11:04 PM   #16
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Voltage Drop when Fridge on DC

I believe they call it a 3 way because it can cool with propane, 120 volt AC or 12 volt DC.
175 watts on 12 volts is almost 15 amps. Probably more when you figure in the losses. If the element were powered continuously it would reduce a battery's capacity in short order.
It would be relatively easy to duplicate the load of the refer by turning the refer off or putting it in another mode.
The start the furnace, the water pump ceiling fan; stove exhaust fan and every 12 volt light in the coach. Furnace= 6-8 amps; water pump 3-4 amps; fans combined 4-6 amps.
Try this to see if you get a similar voltage drop.
I still say it is a bad connection or battery or something is not wired correctly. The #6 wire you are using should be more than adequate to power the refer.
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Old 04-20-2016, 10:23 PM   #17
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The results are in

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Originally Posted by TG Twinkie View Post
I believe they call it a 3 way because it can cool with propane, 120 volt AC or 12 volt DC.
That's correct, this fridge can cool using either 120v, propane or 12v as a power source.

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It would be relatively easy to duplicate the load of the refer by turning the refer off or putting it in another mode. Start the furnace, the water pump, ceiling fan, stove exhaust fan and every 12 volt light in the coach. Furnace= 6-8 amps; water pump 3-4 amps; fans combined 4-6 amps. Try this to see if you get a similar voltage drop.
Ok, started with 12.9 volts then had 3 ceiling fans, 1 hood vent exhaust, 1 bathroom ceiling exhaust, 14 LED lights, 2 USB wall chargers all going at the same time: 194 watts, 17 amps (fridge only was 175 watts, 15 amps). Voltage dropped to 11.5v (lights dimmed a little, you could hear the pitch of the fans change).

So, in summary, to answer your question, I think this qualifies as a similar voltage drop to the fridge. Does this point to culprit? I guess this means it's not a problem specific to the fridge or its' wiring per se...
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Old 04-21-2016, 08:36 AM   #18
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Again; bad battery(s) or bad connection.
Have you checked the ground/common connection at the point where it connects to the frame or skin.
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Old 04-21-2016, 10:10 AM   #19
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Do the lights ever dim or flicker?
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Old 04-21-2016, 10:26 AM   #20
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I don't see a problem here. Any time you put a load on a battery you will get a voltage drop. The bigger the load, the higher the voltage drop.

Dan
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Old 04-21-2016, 10:56 AM   #21
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I don't see a problem here. Any time you put a load on a battery you will get a voltage drop. The bigger the load, the higher the voltage drop.

Dan
Good point Dan.

The voltage read under load is not indicative of battery charge level, at least not accurately. To test the batteries, run the load for a specific amount of time related to the specified capacity of the batteries and then take the load off. An hour or so later, measure the voltage and compare it to a state of charge chart. This test will tell you if there is a battery capacity problem.

That said, I don't know what a good number for the internal resistance of a battery is. His is exhibiting (12.9-11.5)/17 or about 0.08 ohms. Maybe someone else has that data.

Al
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Old 04-21-2016, 12:34 PM   #22
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Voltage Drop when Fridge on DC

I would not think that a bank of 4 6 volt batteries would drop 1.4 volts with only a 17 amp load. I can see .5 volt drop but 1.4 seems like too much.
Are the readings being taken at the battery posts?
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Old 04-21-2016, 01:05 PM   #23
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Quote:
Originally Posted by TG Twinkie View Post
Again; bad battery(s) or bad connection.
Have you checked the ground/common connection at the point where it connects to the frame or skin.
Yup, checked the ground connection and seems to be just fine. Bolted directly to the frame.

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Do the lights ever dim or flicker?
They don't flicker, but they will dim if I turn everything on at the same time.

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Originally Posted by TG Twinkie View Post
I would not think that a bank of 4 6 volt batteries would drop 1.4 volts with only a 17 amp load. I can see .5 volt drop but 1.4 seems like too much. Are the readings being taken at the battery posts?
The "seems like too much" is how I felt too, which is why I started this thread, but I'm not sure how to calculate what would be an acceptable drop. My fridge is a Norcold NX841 by the way.

The readings are from the Victron Battery Monitor, which is connected directly to the positive and negative battery posts (via a shunt for the negative).

Maybe a better question is this: has anyone else tried to run their fridge on DC only? If so, what sort of voltage drop are you getting from a cold start? Amp draw? Wattage?
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Old 04-21-2016, 02:50 PM   #24
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Good point Dan.

The voltage read under load is not indicative of battery charge level, at least not accurately. To test the batteries, run the load for a specific amount of time related to the specified capacity of the batteries and then take the load off. An hour or so later, measure the voltage and compare it to a state of charge chart. This test will tell you if there is a battery capacity problem.

That said, I don't know what a good number for the internal resistance of a battery is. His is exhibiting (12.9-11.5)/17 or about 0.08 ohms. Maybe someone else has that data.

Al
I did a little digging.
This reference talks about the use of golf cart batteries in RV's
http://yarchive.net/car/rv/golf_cart_batteries.html

This reference gives a real world measurement of internal resistance.
https://www.solarpaneltalk.com/forum...nal-resistance

Based on the second reference it seems possible that trekkerboy's problem is the batteries. The internal resistance I calculated above is 5 times higher than that measured in the second reference.

Al
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Old 04-26-2016, 08:27 AM   #25
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Quote:
Originally Posted by Al and Missy View Post
I did a little digging.
This reference talks about the use of golf cart batteries in RV's
http://yarchive.net/car/rv/golf_cart_batteries.html

This reference gives a real world measurement of internal resistance.
https://www.solarpaneltalk.com/forum...nal-resistance

Based on the second reference it seems possible that trekkerboy's problem is the batteries. The internal resistance I calculated above is 5 times higher than that measured in the second reference.

Al
Thanks for digging into this for me Al! Just ordered a battery hyrdrometer to determine which battery(ies) are the problem. Thanks again for all your wisdom and help!
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Old 04-26-2016, 09:58 AM   #26
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This should be a very easy troubleshoot.
175 Watts / 12 Volts DC = 14.6 amps (Volts X Amps = Watts).
I would grab my handy dandy multi meter with clamp on DC Amp reading;
(found one on ebay item 222032055360)
This should give a true and realistic Amp reading at the refer (providing adequate voltage is there (V X A = W).
If the voltage is low and the amp reading is relatively OK then go straight to the batteries. Most good battery sales will measure them for you, I just go to my Napa and get the guys to test them.
If the batteries are good then there is a wiring issue. You could try and wire them direct using another set of wires (but safety is always an issue!)
Good luck, 15 Amps X 8 Hours = 120 Amps! this would drain batteries rapidly
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Old 04-27-2016, 01:36 PM   #27
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Quote:
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This should be a very easy troubleshoot.
175 Watts / 12 Volts DC = 14.6 amps (Volts X Amps = Watts).
I would grab my handy dandy multi meter with clamp on DC Amp reading;
(found one on ebay item 222032055360)
This should give a true and realistic Amp reading at the refer (providing adequate voltage is there (V X A = W).
If the voltage is low and the amp reading is relatively OK then go straight to the batteries. Most good battery sales will measure them for you, I just go to my Napa and get the guys to test them.
If the batteries are good then there is a wiring issue. You could try and wire them direct using another set of wires (but safety is always an issue!)
Good luck, 15 Amps X 8 Hours = 120 Amps! this would drain batteries rapidly
I got in touch with Thetford (which owns Norcold) and got an answer to this question...

The voltage drop I experienced is normal. Evidently, the 12v power mode of this fridge was only intended to maintain cold temps, not cool the fridge from warm. In my case, I was testing the 12v mode by trying to cool the fridge from the start, not just maintaining the existing cool temps.

So, there you go.
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Old 04-28-2016, 08:58 AM   #28
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Very interesting, Im planning on getting a 12V only or a 12/120v fridge. Possibly from dometic, i will have to investigate the capabilities of 12v only
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