Running a Refrigerator Compressor on 12V or 24V and more

[jonahmulski]

I am putting a new refrigerator into our galley. It'll be nice to have cold beer again.
The systems I am considering Sea Frost and Frigoboat both have the Danfoss BD50 compressor that can run on either 12v or 24v. We will be living dockside for a year before cruising the Americas and beyond. While living at the dock we will have shore power which will keep our battery system topped off via our inverter/battery charger.
Fridgoboat sells a separate device that will automatically switch between 110V and 12V and output 24V to the compressor (Coastal Climate Control: AC/DC Power Converter).
If I don't install this the fridge will run off the batteries while at the dock.
Questions:
Over time will it be hard on our batteries if the fridge is running off the battery bank that is being charged by the inverter/battery charger?
How much energy is lost in the conversion from 12V to 24V?
What are the pros and cons of running the compressor on 12V and 24V?

[djmarchand]

Your batteries will be fine at the dock on shore power.

I doubt if there is any significant loss running on 24 v vs 12 v. Both are converted to higher voltage AC to run the compressor.

David

[deckofficer]

Originally Posted by djmarchand

Your batteries will be fine at the dock on shore power.

I doubt if there is any significant loss running on 24 v vs 12 v. Both are converted to higher voltage AC to run the compressor.

David

I always thought Danfoss used DC motors in that size??

[Suijin]

Living aboard is hard on your batteries and shortens their lives considerably, and the biggest perpetrator is refrigeration. I would definitely go with the 110 option if you plan on keeping the boat and being on shore power for any length of time.

Basically, the more of your boat you can power directly with 110 the better. Otherwise your batteries are cycling day in and day out, pretty much needlessly (if you're on shore power), which shortens their service life.

[onestepcsy37]

what suijin said.

batteries have a 'cycle' life - a cycle being one discharge/recharge. the number of 'cycle's' a battery can go through depends a lot on the quality of the battery and the percent of discharge on each cycle.

so when you use your batteries, even though you're recharging them dockside, they're still going through 'cycles', and gradually wearing themselves out. it's probably cheaper to use dockside power when you can, rather than wear down those batteries, mostly because they'll probably wear out while you're in some tropical paradise where new batteries are either unobtainable or super expensive.

all of the above not withstanding, i never plug in to shore power. i've always been afraid of stray currents in marinas.....

[ranger58sb]

Originally Posted by jonahmulski

Fridgoboat sells a separate device that will automatically switch between 110V and 12V and output 24V to the compressor (Coastal Climate Control: AC/DC Power Converter).
If I don't install this the fridge will run off the batteries while at the dock.
Questions:
Over time will it be hard on our batteries if the fridge is running off the battery bank that is being charged by the inverter/battery charger?
How much energy is lost in the conversion from 12V to 24V?
What are the pros and cons of running the compressor on 12V and 24V?

Our NovaKool fridges use 12v DC compressors. The AC capability is just a converter, much like consumer electronics these days, so the compressor always runs on DC no matter whether actual input is AC or DC. The switching mechanism chooses AC when present, DC otherwise.

I suspect the AC capability costs an extra dollar and a half, IOW not much, in the grand scheme of things amortized over time.

If your batteries are low when you return dockside, running the fridges on AC while the batteries are charging may be an advantage. FWIW, we run ours on AC at the dock -- or when underway with genset running (usually July and August, for the aircons).

-Chris

[DotDun]

Originally Posted by onestepcsy37

what suijin said.

batteries have a 'cycle' life - a cycle being one discharge/recharge. the number of 'cycle's' a battery can go through depends a lot on the quality of the battery and the percent of discharge on each cycle.

so when you use your batteries, even though you're recharging them dockside, they're still going through 'cycles', and gradually wearing themselves out. it's probably cheaper to use dockside power when you can, rather than wear down those batteries, mostly because they'll probably wear out while you're in some tropical paradise where new batteries are either unobtainable or super expensive.

all of the above not withstanding, i never plug in to shore power. i've always been afraid of stray currents in marinas.....

Not true. While plugged in your shorepower charger will pick up the load of the 12v refrigerator, not the batteries. The charger will maintain the batteries at ~13.3v in float mode, when the refrigerator kicks on, the charger will increase it's output in response.

[djmarchand]

The motor on a Danfoss compressor is actually AC. The controller takes DC input and inverts it to variable frequency AC to run the compressor. That is why it is so efficient compared to a straight DC motor.

And DotDun is correct as to how your shore power charger works.

David

[Suijin]

Originally Posted by DotDun

Not true. While plugged in your shorepower charger will pick up the load of the 12v refrigerator, not the batteries. The charger will maintain the batteries at ~13.3v in float mode, when the refrigerator kicks on, the charger will increase it's output in response.

I stand corrected.

That said, most every liveaboard that I know complains about how hard living aboard is on their batteries. However, the decline in battery life may be a function of the charger's inability to know what part of its output current the battery is getting and what part is powering loads . Therefore there is no way for the charger to detect when the battery is fully charged, resulting in frequent overcharging and reduced battery life. Hmmm.... more research needed.

[skipmac]

Originally Posted by djmarchand

The motor on a Danfoss compressor is actually AC. The controller takes DC input and inverts it to variable frequency AC to run the compressor. That is why it is so efficient compared to a straight DC motor.

And DotDun is correct as to how your shore power charger works.

David

That's my understanding on both points.

[skipmac]

Originally Posted by Suijin

I suspect this is entirely a function of the features of your charger and how your boat is wired. Every liveaboard that I know complains about how hard living aboard is on their batteries. If their chargers were picking up all the DC load, then there would be no difference in battery life.

There is no doubt that living on board and frequently discharging and charging the house batteries uses up the batteries. But if you are plugged into shore power and have a charger running the batteries will not be discharging or at least very little under normal house loads.

If you have a 50 amp charger and you are pulling 10 amps for fridge, lights, pumps and such the charger will have plenty of output to supply that load unless you have a large house bank that is very discharged and sucking down all the output of the charger. Even then, the higher voltage of the charger should be supplying the house loads and just put less into the batteries. The exception would be if you are drawing a really large load that exceeds the capacity of the battery charger.

[ctl411]

Originally Posted by djmarchand

The motor on a Danfoss compressor is actually AC. The controller takes DC input and inverts it to variable frequency AC to run the compressor. That is why it is so efficient compared to a straight DC motor.

And DotDun is correct as to how your shore power charger works.

David

No Danfoss BD2 -DB50f are DC motors 12 or 24 volt. The converter is 110 to 12 or 24 volt DC. This is straight from the Kollmann refrigerator manual.
To the OP difference between 12 vs 24 is 24 uses half the amps. But I run on 12 because that is what the rest of the boat is.

[DotDun]

Originally Posted by Suijin

I stand corrected.

That said, most every liveaboard that I know complains about how hard living aboard is on their batteries. However, the decline in battery life may be a function of the charger's inability to know what part of its output current the battery is getting and what part is powering loads . Therefore there is no way for the charger to detect when the battery is fully charged, resulting in frequent overcharging and reduced battery life. Hmmm.... more research needed.

I would suggest you research how modern battery chargers work. Yes, a charger does know when a battery is fully charged. It's done via knowledge of how the battery works. Hold the voltage at a constant for a period of time, the battery will fully charge. The house loads in parallel with charging are only of consequence when they & the required current the battery wants exceed the rated maximum output of the charger. That's your fault for not designing it properly upfront. The downside is it may take a little longer to fully charge the battery, which I believe is where you assume the charger doesn't understand the state-of-charge. It really doesn't matter, as the charger will always hold the battery at a voltage higher than it's own voltage, hence charging to full will happen given enough time.

The chemical reaction inside a battery is well known and predictable as it ages. The decline in battery life is due to internal changes in a battery, the continual transfer of electronics forced by chemical reactions between acid and metal cause 'wear' - i.e. chemical/metal changes.

Liveaboards complain about life on batteries because they are a terrible way to store energy. Unfortunately, we don't have better technology in the consumer space. Come back in 50 years, it'll be different.

[Dockhead]

Originally Posted by skipmac

There is no doubt that living on board and frequently discharging and charging the house batteries uses up the batteries. But if you are plugged into shore power and have a charger running the batteries will not be discharging or at least very little under normal house loads.

If you have a 50 amp charger and you are pulling 10 amps for fridge, lights, pumps and such the charger will have plenty of output to supply that load unless you have a large house bank that is very discharged and sucking down all the output of the charger. Even then, the higher voltage of the charger should be supplying the house loads and just put less into the batteries. The exception would be if you are drawing a really large load that exceeds the capacity of the battery charger.

That's right. Your batteries will simply float while you're connected to shore power, unless the load exceeds charger capacity (unlikely unless tiny charger). Doing no work and not getting worn out.

So no reason not to use simple DC supply for refrigeration.

One caveat is that your charger needs to have proper voltage sensing and a good program. An el cheapo charger can hurt your batts whether you're living aboard or not.

Really good chargers will shift down to "storage mode" - a lower voltage than float, to reduce corrosion of the positive plates - and will give your batts an absorption charge once a week to blow out the sulphation.

[SV THIRD DAY]

Originally Posted by Dockhead

So no reason not to use simple DC supply for refrigeration..

Ah...but the reason for the suppliers of the more Costly 120V/12V refrigeration controllers love to hype the dual voltage nature of their more expensive contollers...ha ha ah

I'm the owner of www.Technauiticsinc.com and we have been making CoolBlue refrigeration units since 1968. If we had a dollar for every sales hype we have heard over the years....whew....I could buy a better boat to live aboard on his this old Hudson Force 50!

But seriously, simplicity rules here so just stick with a 12v unit and save the extra money for Tacos or Beer.