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Old 08-21-2012, 11:29 AM   #1
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AC/DC Fridge

If I have an invertor, is there any value to having a AC/DC Fridge or will AC work just fine. Amp hours always confuse me.....
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Old 08-21-2012, 11:47 AM   #2
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If I have an invertor, is there any value to having a AC/DC Fridge or will AC work just fine. Amp hours always confuse me.....
A DC fridge works just fine. If you have no need to run energy hungry things like A/C, you need no generator for AC current. A twelve volt boat is simple. Your charging system is more important that a generator. If you are hanging on a hook for long periods, and don't want to run your main engine, an AC generator may be in order.
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Old 08-21-2012, 11:50 AM   #3
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I been reading about AC fridges and they seem to use less amp then DC fridges, but I'm not sure how much the invertor uses to convert. The primary reason for this question is my Norcold is dead and If I can function properly with a $150 fridge as opposed to $900 unit I want to go that route.
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Old 08-21-2012, 12:44 PM   #4
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David: I had an AC/DC fridge with an auto-switching device built-in that ran the fridge on AC when available (eg: on shorepower or when inverter was on...which was always when we were off shorepower). This auto-switching device failed....and I have now learnt this is pretty common and that they have a life typically of only about 5 years in continuous use. Replacing them is expensive. When the auto-switch failed, the fridge switched itself to run only on DC....but wasn't staying as cold as normal. Investigating this problem was what caused us to find the auto-switch had failed. The fridge wasn't operating well because we had excessive voltage drop in a too-indirect DC cabling...which had been disguised by the shorepower/inverter operating properly & delivering full voltage until the auto-switch failed.

Rather than do the costly replacement of the auto-switch, the advice I got from 2 marine refrigeration techs was to direct-connect the fridge to the house battery bank buss (with appropriate fusing & cable sizes of course) to be sure to get the full 13+ volts to the fridge. The compressors of these fridges are VERY sensitive to voltage drop, despite indicating that they run at a wide voltage tolerance (mine said on its compressor label 10-14v or similar). They'll run at low voltage, just not very effectively.

The refrig techs also pointed out that modern inverter efficiency is 'good', but still only 85-93% (my Victron inverter, a quality unit, states that it is 87% efficient). So you are using up to 15% more battery power on the inverter vs a direct-connect to the House bank. That's not a small % for a big consumer like a fridge. So my advice would be to go the DC route but to do it properly.

Having said all that though, your 2nd post makes a good point: standard domestic fridges are much cheaper than 12v models. Often they are better insulated and more energy-efficient too...maybe enough to offset inverter losses, though that is a guess. But domestic fridges don't come with door locks and you will certainly need to do something about that. So if you have a quality inverter, a good size battery bank (800Ah+), external regulators to large (120A+) alternators & can fit locks to fridge and freezer compartment doors to stop them flying open in a seaway...then go the standard domestic AC route by all means.

Finally, AC or DC, you can help you fridge's efficiency by ensuring it is well-ventilated around the compressor/behind the fridge. In tight spaces, installing a brushless, super-low-draw computer fan adjacent the compressor and wired to run off the compressor circuit so it comes on when the compressor cycles, is a huge efficiency booster, particularly in hot weather. Suitable computer fans are less than $20 at any computer parts place. I installed one with a remote on/off switch so I can turn it on when days are hot: it reduced the fridge's cycling-on times noticeably.
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Old 08-21-2012, 12:48 PM   #5
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Originally Posted by patzfan4eva View Post
I been reading about AC fridges and they seem to use less amp then DC fridges, but I'm not sure how much the invertor uses to convert. The primary reason for this question is my Norcold is dead and If I can function properly with a $150 fridge as opposed to $900 unit I want to go that route.
You will lose at least 10%, but more like 12% of power in a inverter. A fridge will run better, and use a little less power with a true sine wave inverter. The compressor may run slower and run longer on a modified sine wave inverter. Also a modified sine wave can cause the start up amperage to be higher. However that only lasts a few seconds at most.

I run a 115 volt Sub Zero fridge/freezer from an inverter, but also have a generator.
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Old 08-21-2012, 01:21 PM   #6
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A well placed piece of Velcro about 4 to 6 inches long makes a fantastic fridge lock. Not the yachtiest looking thing but darned effective.
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Old 08-21-2012, 01:25 PM   #7
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Rather than do the costly replacement of the auto-switch, the advice I got from 2 marine refrigeration techs was to direct-connect the fridge to the house battery bank buss (with appropriate fusing & cable sizes of course) to be sure to get the full 13+ volts to the fridge.
I don't see how this is accurate. A voltage drop is a voltage drop. When a load is on the batteries and they are unable to stay at 13.5V, voltage is going to drop everywhere. Not just on that circuit.

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Old 08-21-2012, 01:40 PM   #8
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A lot depends on what is running on AC and or the inverter. If its just the refrigerator and a couple of items not problem. But if you have a lot of AC stuff, like heaters/water heater/batter chargers etc then they have to be turned off and on so only the refrigerator is running. On the Eagle its to much trouble to have just an AC refrig. It would be better to just have a DC refrig.
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Old 08-21-2012, 02:23 PM   #9
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The amp rating on the ac fridge look low because that is for 120volts when converting to 12 volts tru your inverter multiply by a factor of 10 plus the loss% tru the inverter I have had both 12 volt is better for me
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Old 08-21-2012, 04:25 PM   #10
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I asked my mechanic who said that in his opinion, with the money I'll save on an AC unit, I can purchase additional batteries to help power it thru the invertor.
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Old 08-21-2012, 08:42 PM   #11
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I asked my mechanic who said that in his opinion, with the money I'll save on an AC unit, I can purchase additional batteries to help power it thru the invertor.
Also add solar panels to your system to feed the batteries, via a good regulator. Your batteries will love you for it and live longer in better shape.180watts of panels gives enough charge run a 12v fridge on a sunny day with batteries staying full. BruceK
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Old 08-22-2012, 11:00 AM   #12
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I been reading about AC fridges and they seem to use less amp then DC fridges, but I'm not sure how much the invertor uses to convert. The primary reason for this question is my Norcold is dead and If I can function properly with a $150 fridge as opposed to $900 unit I want to go that route.
They seem to use less amps than a DC refrigerator because you're looking at the current (amps) draw at 120 volts for the AC refrigerator and at 12 volts for the DC refrigerator. Multiply the current draw of the AC reprigerator by ten, then add ten percent to that and you'll have the current drawn from your batteries by the inverter to run the refrigerator.

If you want the most efficient refrigerator for your boat (and it matters if you're using it away from shore power or while anchored), buy yourself a quality, marine, AC/DC refrigerator to replace the failed one. Nova Kool and Isotherm are well thought of brands to consider.
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Old 08-22-2012, 11:04 AM   #13
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A lot depends on what is running on AC and or the inverter. If its just the refrigerator and a couple of items not problem. But if you have a lot of AC stuff, like heaters/water heater/batter chargers etc then they have to be turned off and on so only the refrigerator is running.


Running a battery charger from an inverter?

That would be the elusive "perpetual motion machine".
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Old 08-22-2012, 11:08 AM   #14
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....... standard domestic fridges are much cheaper than 12v models. Often they are better insulated and more energy-efficient too..........
I think it's the other way around. Not the cost, but the insulation and efficiency. Plug something into a wall outlet in your home and efficiency isn't that important. There's a nationwide electrical grid available to power it.

On a boat, there's just a battery or two. Quality marine refrigerators are designed around efficiency.
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Old 08-22-2012, 11:58 AM   #15
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You know....you guys are supposed to help me by making it easy to buy a AC fridge!!! Thanks a lot!

I'll check into this further with some of the electrical engineers here at work and get there opinion as well. But $1300 (Nova Kool) $1150 (Norcold) etc versus $170 is hard to swallow without a significant argument for them.
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Old 08-22-2012, 02:05 PM   #16
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You know....you guys are supposed to help me by making it easy to buy a AC fridge!!! Thanks a lot!

I'll check into this further with some of the electrical engineers here at work and get there opinion as well. But $1300 (Nova Kool) $1150 (Norcold) etc versus $170 is hard to swallow without a significant argument for them.
Ask yourself this question - If a 120 volt AC "dorm" refrigerator powered by an inverter from 12 volt batteries was a reasonable and less expensive alternative than a "real" AC/DC refrigerator, wouldn't the entry level boat manufacturers like Bayliner and Glastron do this instead of installing an AC/DC refrigerator in their boats?

You didn't bother to post the capacity of your inverter, how it was installed, or your battery bank capacity. Is the inverter rated for continuous duty?

Remember, an AC/DC refrigerator runs and draws power until the thermostat determines that it's cold enough, then it shuts off. An inverter would have to run 24/7 to provide power even when the thermostat is satisfied.

Unless your electrical engineers are boaters and experienced with boat electrical systems, their advice is more apt to lead you astray than to set you straight.

BTW: You have what appears to be a pretty nice boat. Why screw it up with a dorm refrigerator? You'll have to put a real one in when it comes time to sell the boat anyway. Think of it as an investment.
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Old 08-22-2012, 02:09 PM   #17
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Ron,

All very good points, I'm just being cheap. LOL! I'll find out what my invertor is rated for before any decisions are made. Someone has to explain to me why the same size fridge differs by over $1000 from "Dorm" to "Marine" use!!
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Old 08-22-2012, 03:22 PM   #18
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Ron,

All very good points, I'm just being cheap. LOL! I'll find out what my invertor is rated for before any decisions are made. Someone has to explain to me why the same size fridge differs by over $1000 from "Dorm" to "Marine" use!!
The word "Marine" of course
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Old 08-22-2012, 03:45 PM   #19
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...I'll have 4 x 220 amp/hour batery nak. I will only use AC refrigeration. A freezer and a refrigerator with a 2500 watts inverter
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Old 08-22-2012, 03:52 PM   #20
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I ran a undercounter 120v fridge for 2 years Ran off a mod sine inverter no problem no problem underway did use more juice while at anchor I replaced it with a 12voly unit just seemed like the right way to go
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