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Old 04-16-2016, 07:20 AM   #1
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Voltage Drop when Fridge on DC

We have a brand new Norcold NX841 3-way refrigerator in our AS and I was experimenting with running it purely off of DC. When I switched it over to just DC I noticed that the battery monitor went from 12.8v down to 11.2 right away (also noted fridge was drawing 15 amps and about 175 watts). I have 4 x 6v batteries (Duracell EGC2 golf cart batteries) wired in series parallel (460 amp-hours capacity), and 6 gauge wire running about 10 feet from the battery to the refrigerator.

My question: is it normal for the battery bank voltage to drop that much under a big load (like refrigerator)? If so, what's the physics behind this behavior?

At any rate, the refrigerator flashes a "DC LO" message and won't function on DC alone due to low voltage, hence my question.

Thanks for any input!
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Old 04-16-2016, 08:26 AM   #2
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Voltage Drop when Fridge on DC

I would check all of the connections. Sounds like you have a serious voltage drop.
Where is the monitor connected in the system?
Does the reading go back to 12.8 volts when you switch to propane?
11.2 volts is a dead battery in my book. But again it could be a bad connection.
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Old 04-16-2016, 08:33 AM   #3
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Unless there is indeed a bad connection, which increases the resistance in the circuit, Ohm's Law says your voltage drop is normal. Gas absorption fridges running on DC draw a heck of a lot of current.

I think that DC fridge function is intended for when you're on the road with the TV's alternator supporting the current draw of the fridge. Even then, make sure it doesn't overload your alternator.
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Old 04-16-2016, 01:15 PM   #4
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If you lose 1.6 volts. 11.2 down from 12.8 drawing only 175 watts. It's a bad connection or bad battery(s). Or perhaps the battery bank is not wired correctly and you don't have the full 4 battery capacity you think you have.
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Old 04-16-2016, 01:31 PM   #5
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Check the batteries with a hygrometer. You may have a bad cell.
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Old 04-16-2016, 01:56 PM   #6
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Originally Posted by TG Twinkie View Post
I would check all of the connections. Sounds like you have a serious voltage drop.
Where is the monitor connected in the system?
Does the reading go back to 12.8 volts when you switch to propane?
11.2 volts is a dead battery in my book. But again it could be a bad connection.

Yup, goes back to 12.8 when the fridge isn't using DC.

Battery Monitor is connected to positive terminal of the battery bank and a shunt which is connected to the negative terminal of the battery bank.

Haven't found any bad connections...
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Old 04-16-2016, 04:09 PM   #7
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Yup, goes back to 12.8 when the fridge isn't using DC.

Battery Monitor is connected to positive terminal of the battery bank and a shunt which is connected to the negative terminal of the battery bank.

Haven't found any bad connections...
Measure the voltage between the plus and minus battery terminals. Sounds like you are getting too much drop across the shunt, or the shunt is wired wrong.

I think there should be two wires connected to the shunt, one on each side. The monitor should be measuring voltage between the positive and negative posts and calculating current from the shunt resistance and the voltage across it.

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Old 04-16-2016, 04:22 PM   #8
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Was the original refrigerator a 3 way one, that is could it operate on 120 volts, 12 volts, or propane? 12 volt operation of absorption refrigerators takes a lot of power, 15 or more amps as you have noted.

I suspect your original refrigerator only used 12 volts for powering the electronics or even just the light in the refrigerator, and thus has only very small wires running to it, and they are not designed for the 15 + amp load you have put on them.

12 volt operation of the cooling portion of a refrigerator is a huge load on the trailer system, and will kill any battery quickly in any event. It has primarily been used in motorhomes where there is an engine driven alternator system to keep power to the refrigerator coming, without relying on the batteries. The system in my 1983 AS 310 had a 3 way refrigerator system, but had about #10 or #12 wires to the refrigerator, not the little ones typically found on trailer installations.

But, as noted in posts above it could be a poor connection, however, I think you are just overloading the wires that are already in place.
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Old 04-16-2016, 07:12 PM   #9
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Originally Posted by Al and Missy View Post
Measure the voltage between the plus and minus battery terminals. Sounds like you are getting too much drop across the shunt, or the shunt is wired wrong.

I think there should be two wires connected to the shunt, one on each side. The monitor should be measuring voltage between the positive and negative posts and calculating current from the shunt resistance and the voltage across it.

Al

Voltage at the positive and negative terminals of the battery bank read the same as they do from the battery monitor.

Yes, there are two wires connected to the shunt (Victron Energy Battery Monitor system by the way), well technically 3:

1. Incoming negative bus wire
2. Outgoing negative bus wire (to negative terminal on battery bank)
3. Comm wire (to battery monitor display)
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Old 04-16-2016, 07:17 PM   #10
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Was the original refrigerator a 3 way one, that is could it operate on 120 volts, 12 volts, or propane? 12 volt operation of absorption refrigerators takes a lot of power, 15 or more amps as you have noted.

I suspect your original refrigerator only used 12 volts for powering the electronics or even just the light in the refrigerator, and thus has only very small wires running to it, and they are not designed for the 15 + amp load you have put on them.

12 volt operation of the cooling portion of a refrigerator is a huge load on the trailer system, and will kill any battery quickly in any event. It has primarily been used in motorhomes where there is an engine driven alternator system to keep power to the refrigerator coming, without relying on the batteries. The system in my 1983 AS 310 had a 3 way refrigerator system, but had about #10 or #12 wires to the refrigerator, not the little ones typically found on trailer installations.

But, as noted in posts above it could be a poor connection, however, I think you are just overloading the wires that are already in place.

The original fridge was also a 3 way unit, yes.

My AS is a complete renovation; I ran 6 gauge wire from the 12v panel to the fridge knowing that it'd suck a lot of juice.

I'm wondering if it's not something with one of batteries in the 4 x 6v bank... Even with just one fantastic fan on and 1 phone plugged in to charger, the voltage drops from 12.8 to 12.26 at full charge... Seems like the bank should be able to sustain these small things without that much drop, right?
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Old 04-16-2016, 07:36 PM   #11
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The original fridge was also a 3 way unit, yes.

My AS is a complete renovation; I ran 6 gauge wire from the 12v panel to the fridge knowing that it'd suck a lot of juice.

I'm wondering if it's not something with one of batteries in the 4 x 6v bank... Even with just one fantastic fan on and 1 phone plugged in to charger, the voltage drops from 12.8 to 12.26 at full charge... Seems like the bank should be able to sustain these small things without that much drop, right?
OK, you do understand the power requirements of the 12 volt part of the refrigerator, and had one in the original installation.

So, it does now sound like the batteries are not doing what they should. I would measure each 6 volt battery under a load, and see how close they are to each other. Most likely you will find one of the batteries in each pair to have a lower voltage across it, compared to the other. Possibly you will only find one of the four with a lower voltage, and it is dragging on the other set. Measure the voltage at the battery terminals, not someplace else in the system.

How old are the batteries, and what charging system have you been using to keep them up during the renovations?
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Old 04-16-2016, 08:26 PM   #12
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Voltage at the positive and negative terminals of the battery bank read the same as they do from the battery monitor.

Yes, there are two wires connected to the shunt (Victron Energy Battery Monitor system by the way), well technically 3:

1. Incoming negative bus wire
2. Outgoing negative bus wire (to negative terminal on battery bank)
3. Comm wire (to battery monitor display)
I think there should be another small gauge red wire connected to the positive battery terminal to power the monitor. At least mine has it.
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Old 04-16-2016, 08:34 PM   #13
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I think there should be another small gauge red wire connected to the positive battery terminal to power the monitor. At least mine has it.

Doh! You're right, there is. So 4 wires total connected to the shunt.
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Old 04-16-2016, 08:38 PM   #14
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OK, you do understand the power requirements of the 12 volt part of the refrigerator, and had one in the original installation.



So, it does now sound like the batteries are not doing what they should. I would measure each 6 volt battery under a load, and see how close they are to each other. Most likely you will find one of the batteries in each pair to have a lower voltage across it, compared to the other. Possibly you will only find one of the four with a lower voltage, and it is dragging on the other set. Measure the voltage at the battery terminals, not someplace else in the system.



How old are the batteries, and what charging system have you been using to keep them up during the renovations?
Batteries are almost 2 years old. Been using the integrated charger/inverter/transfer-switch combo unit (AIMS Power PICOGLF20W12V120VR 2000W Pure Sine Inverter Charger) to keep batteries topped up.
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