Marine Fridge 12V vs Regular 115V with inverter

[Yaho]

I discovered the fridge on my 38 convertiable was 115v only (must have been replaced by previous owner or maybe never was 12v) after my first year of use. I like to have my fridge working when im out and im not running the generator all the time to do it.
I can buy a bar fridge many places for cheap and put it on an inverter. The whole package might cost $450. When i looked at the dedicated marine 12v fridge they are like $1200 !!
Has anyone used a regular fridge on an inverter?? If so how does it work for you on battery power? Has anyone had both that they can compare. What are some of the more efficent models out there for in 12v.??
Cheers! Glen!

 

[Legend]

I recently replace my Isotherm OEM frig/freezer. My original was just a 12 volt. It was never an issue Bc I was on shore power unless I was away on a mooring. The new one. also ISotherm is dual power when on shore power it runs 120/115 and 3 minutes after disconnecting it converts to 12 volt. Hope I get 15 years out of this one too! Good Luck

 

[Fishtales]

You can certainly do this if it is that important to you. Sounds like the last guy installed a standard fridge versus a marine fridge. The boat comes with a dc fridge stock. I'd rather a dual voltage fridge so it runs off AC on the dock. If you are not on a slip, I don't think you will run it anyway. You'll drain the batteries. So your real advantage is having a fridge while running.
I usually use coolers and ice and rarely go into the fridge when cruising. If fishing with one to two people maybe, but we always have ice in the cooler so why not just use it?

 

[seasick]

You can certainly do this if it is that important to you. Sounds like the last guy installed a standard fridge versus a marine fridge. The boat comes with a dc fridge stock. I'd rather a dual voltage fridge so it runs off AC on the dock. If you are not on a slip, I don't think you will run it anyway. You'll drain the batteries. So your real advantage is having a fridge while running.
I usually use coolers and ice and rarely go into the fridge when cruising. If fishing with one to two people maybe, but we always have ice in the cooler so why not just use it?

If you have shore power and a battery charger, what's the down side of using the fridge at the dock on battery which is being topped off?
For me, given the choice of 12dc or 115ac, I would choose the dc version for use on the water. The inverter is not 100% efficient so it will use more battery charge than a 12 volt fridge. Up side is that maybe it could run something else on the water. I think the real issue her and it is a consideration is that a marine 12v or dual voltage fridge is a whole lot more expensive than a 'dorm' fridge even with an inverter. I don't know how well one of those inexpensive fridges would hold up with the moist air, maybe limited ventilation clearances and sometimes extreme shock and vibration.

 

[Fishtales]

nothing. the only gap is while running. true on dc and that is why that is what is used. ac is more efficinet but dc is just fine. Agree, a dorm fridge isn't built to marine standards, like will fail faster.

 

[jillyrubyjane]

Maybe Portable refrigerator 42 quart – Ezzicool can fit with you, compatible to both 110 volt and 12 volts power cord, 3 level battery protection, accurate temperature control, and LCD display panel. Not only marine you also can use it for cars, RVs and trucks. One thing I really like about this refrigerator is 30-minute fast cooling capacity (90 degrees F to 32 degrees F)

 

[seasick]

Marine fridges, when on 12 volts usually have voltage detection circuitry that shuts it off if the supply voltage goes below a certain value. That keeps your battery from being completely run down.
Regarding inverters, the required capacity can be greater than the wattage listed on the fridge since mechanical loads like compressors draw a lot more current when starting. That is called the in-rush current. Undersizing n inverter can lead to early failure. The VA rating ( volt-amps) is more important that the wattage rating. Inverters may tell you that but fridges usually don't. The two numbers can be quite different
AC non- marine fridges on shore power can be an OK less expensive way to go but if you want the DC power option, inverters and ac fridges are not a good idea.

 

[kq7v7u3s]

I have always wondered why more ac isn't used on boats. My thinking is as follows:
At a given wattage ac wiring is smaller because it doesn't have to carry as much current. For instance, 240-watt load at 12 volts would need a wire rated at 20 amps, but at 120 volts ac would only need a wire of 2 amps FISHING LURE (185MM / 43G & 150G) - MZFLHXX & MZFHXX. If u add up all the wiring on a boat wouldn't the smaller wire save a lot in weight?
I realize that batteries are all dc but the inverter technology has come a long way.
I also read that dc suffers wire loss that ac does not. Is this not a problem on the short runs in a boat?
Marine wire is stranded. Is this because of the vibration loads? It seems that stranded wire has much more surface area that can corrode than solid wire.

Any thoughts u have on ac vs dc on a boat would be greatly appreciated.

 

[DennisG01]

The weight of wires is essentially a non-issue. The weight savings are inconsequential.

Just like in a car, converting from the initial 12V system to everything being 120V appliances (fridges, pumps, lights, etc, etc... plus, not all systems are even available as AC-powered) is inefficient.

The other important thing is safety - 12V is MUCH safer to work around. If you mistakenly short DC with a wrench, you'll scare the crap out of yourself and probably make a crater in your wrench... do the same thing with AC and results can be much worse.

Marine wires are stranded because solid copper can break and arc/spark. The proper marine wiring is tinned to avoid corrosion.

 

[seasick]

Easy thoughts first. Marine wire is stranded because stranded wire is less likely to break from either mechanical vibration or mechanical abrasion ( The most common are nicks from shoddy wire stripping.)
It is true that for a given load, a higher voltage means a lower current and a lower current means lower voltage drop .
Voltage drop equals I x R where I is the current and R is the resistance. In general the resistance of a copper wire is directly proportional to its thickness (wire gauge).

Dennis is correct in that the higher the voltage, the more dangerous it is for humans. Putting that aside for now, the issues with inverters are many. Yes, inverter technology has come a long way but it isn't perfect. All inverters introduce some loss, that is they are not 100% efficient. So for a 100 watt load, you may need 110 watts of input.
Inductive loads and mechanical loads like a fridge compressor can draw many times their running current during startup. This is called Inrush Current. An inverter has to be sized for the in rush and that adds cost and complexity.
Many mechanical loads again like compressors have inductance that causes a situation where the demand curve of the current flow is not in sync with the voltage curve. This offset results in a situation where you may have a higher current draw with a lower effective power. All that means is that current will be drawn but not as much work is being done. Since a compressor needs a certain amount of mechanical power,

A more serious issue with AC powered devices like fridges is that as the voltage decreases which it will as the battery runs down, the current draw increases. That can cause overheating of the compressor and the wiring. Most electromechanical devices that have a specific mechanical load requirement will be sensitive to low voltage. That includes fridges and air conditioners.

There are inverters that could handle the loads you need but they are expensive and just not cost effective .

One interesting thing about tinned wire is that Grady historically did not use tinned wire and as far as I know still doesn't but don't hold me to that

 

[Hookup1]

Is your current refrigerator working? It sounds like you are going to replace it.

It's my understanding that all the small refrigerators run on something like 18 VAC so they all have transformers to drop the voltage down from 120 VAC to 18 VAC or inverters to convert 12 VDC to 18 VAC. I'll check this out more this weekend. A relay circuit detects shore power and switches.

I have a 12 VDC unit and would like to have it run on 120 VAC while on the dock and automatically switch to 12 VDC when shore power disconnected. My Norcold on a previous boat worked that way.

UPDATE: So I stand corrected...compressor runs in 15 ~ 25 VAC. If you look at page 11 of 16 in this document you see that at this Norcold compressor runs on 15 ~ 25 VAC internally with multiple inverters in the system. Note SPDT relay drawn incorrectly - contact should switch between battery/inverter. Coil is correct.

First stage is 12 to 32 VDC from batteries or 85 ~132 VAC converted into 24 VDC.
Second stage is DC/DC converter taking above and converting it to 44.5 ~ 55 VDC.
Third stage is an inverter to produce 15 VAC ~ 25 VAC across compressor (see page 5 of 16).

Now I have a 12 VDC only unit. I'm not sure if it can be converted or is it worth it to do.

This all backtracks to where the manufacturers get the compressor motors from. I'll bet my 12 VDC refrigerator uses a inverter to power a 24 VAC compressor.

This typical of where all the marine electronics are going. Not so much refrigerator technology but all the other digital components require "clean" power so they "make their own" with internal inverters and switching power supplies. Garmin units can run on anything from 10 VDC to 36 VDC without having to flip a switch. Common with other manufacturers. Problems with some radios being 12 VDC only.

This might explain why a 12 VDC / 120 VAC refrigerator is so expensive!

If your are really to determined to have a 12 VDC / 120 VAC automatic switch system you need a 120 VAC DPDT relay and a 120 VAC 12 VDC 10 amp or better power supply. When on shore power the relay is "picked up" and powers the 12 VDC power supply. When no shore power the battery powers the refrigerator.

These refrigerators draw draw 10 to 15 amps at 12 VDC!

LP Gas Refrigerators - Thetford North America

norcold.com

I don't have a generator on my boat. I never shut the engines down. The F-150's are my generator to keep the batteries up. If you are counting on your house battery alone to keep your refrigerator working for a long day its not going to happen!

For now I'll stick with my 12 VDC refrigerator. The only problem with that is when I'm on shore power my ProMariner charger is supplying 10 amps intermittently to run the refrigerator plus bait well pumps. I have had fuse issues with this charger that were.discussed in another thread. Quick look at Amazon and I can get a 120 VAC to 12 VDC 15 amp switching power for $20.

https://www.amazon.com/ALITOVE-Transformer-Switching-Converter-Security/dp/B078RZ6C3N/ref=sr_1_2_sspa?dchild=1&keywords=120vac+to+12v+dc+converter&qid=1605871009&sr=8-2-spons&psc=1&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUFFNEpEWk5YM05GNlImZW5jcnlwdGVkSWQ9QTA4OTE0NjUxTzUySk5XRU5XQUdHJmVuY3J5cHRlZEFkSWQ9QTA5MTE5MDAxNzI4MjJETDFaUDA3JndpZGdldE5hbWU9c3BfYXRmJmFjdGlvbj1jbGlja1JlZGlyZWN0JmRvTm90TG9nQ2xpY2s9dHJ1ZQ==

And a 120 VAC DPDT relay for $10.

https://www.amazon.com/110VAC-120VAC-Electromagnetic-Mechanical-Indicator/dp/B07SYJ4NCM/ref=sr_1_1_sspa?crid=1VA4CNXAFDGZ&dchild=1&keywords=dpdt+relay+120v+coil&qid=1605872913&sprefix=DPDT+relay+%2Caps%2C165&sr=8-1-spons&psc=1&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUEySklTQzNITDYzSVNNJmVuY3J5cHRlZElkPUEwMjkwMDI5MVZKWEoxREc1SkVMMiZlbmNyeXB0ZWRBZElkPUEwMTE4MTU4MURFR0lIMVJFTjEyRyZ3aWRnZXROYW1lPXNwX2F0ZiZhY3Rpb249Y2xpY2tSZWRpcmVjdCZkb05vdExvZ0NsaWNrPXRydWU=

So for $30 you can add dual power to your 12 VDC refrigerator. I may do this project but not this year.

If you have a 120 VAC refrigerator and decide to go dual power you can use the same relay and a 300 watt power inverter. That will power a 120 VAC 2.5 amp load and will draw 25 amps! Maybe you could get away with a 200 watt power inverter. That will power a 120 VAC 1.7 amp load and will draw 16.7 amps! That's a lot of amps in either case. You would be better off running your generator.

https://www.amazon.com/BESTEK-300W-Power-Inverter-Adapter/dp/B004MDXS0U/ref=sr_1_26?dchild=1&keywords=120vac+to+12v+dc+converter&qid=1605871009&sr=8-26

 

[Fishtales]

It kills me to buy marine priced items when there appears to be alternate solutions that could work. My advice is unless you 100% know, go the marine route. It is a brutal environment and things that are not designed for it will become intermittent and prematurely fail - not to mention that most don't have the safety requirements. Sux to drop $1200 but if a fridge is important to you, probably better to do this and cut corners or jury rig something. Just my 2 pennies.

 

[Hookup1]

I agree with Fishtails on the marine refrigerator for GW sized boats that are running on 12 VDC when they are out on the water. Given the inefficiency of the 12 VDC to 120 VAC inverters you will use too much battery power.

All the big rigs I'm involved with are dependent on 120/240 VAC while on the water. They have generators and in most cases dual generators for A/C, fish box refrigeration, ice makers, drink boxes, water makers, freezers, wash machines, dryers, dishwashers and refrigerators. The appliances are usually ordinary GE or Maytag units but sometimes they are commercial Sub Zero's or Kitchen Aid.

If you have a generator and run it whenever you are off the dock there is nothing wrong with buying a refrigerator at HD or Lowes and using it in you boat. Inside the cabin they are reasonably protected and will last a while. With the money you save on buying a marine refrigerator you can probably buy 2 or 3 regular ones. Good example is we use a college dorm mini-refrigerator on the flybridge that we can buy for under $100. They last a couple of years and we throw them away.

Not really a safety issue. You have to work at it to electrocute yourself with a refrigerator or microwave!

 

[Hookup1]

So here is an update on my stream of conciseness post above.

My refrigerator is a Norcold DC-340D. 12 VDC @ 3.7 amps. That is actually not a bad amperage.draw - half what I expected. The link above from Norcold had much higher amperage consumption for the newer units. I'm leaving everything the way it is.

 

[DennisG01]

The small fridges we use in smaller boats (30'-wish and under) are quite efficient on 12V power. As noted, they really don't draw all that much juice - and if stock it up full and limit how often you open the door... even better.

Many times, I've been away for 3 or 4 days at a time, running the fridge, toilet, lights, stereo, etc with very little engine run time - never once had a battery issue starting the V8 back up. This was with a cruiser-style boat.

To clarify what I meant about AC being dangerous, I didn't really mean at the appliance end. I was referring more to the KQ's thought of the entire boat being AC and having AC in the bilge or other places where you're regularly turning wrenches.