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Old 09-13-2012, 06:56 PM   #1
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Refrigeration questions; Very confused

Ok, I have read a multitude of posts on the subject of AC/DC refrigeration and I am more confused now than ever. I have a 48ft. motoryacht, we all know MY's are power hogs. I am looking for a solution to be able to stay on the hook without running my genny (phasor 15KW) 24/7. I have a 14.4 cu.ft. refrigerator that is 120v. Due to the size and location it is impractical to remove it and replace it. FF I have looked at the Sun Frost products and $3200.00 is more than I want to spend, also I have looked at other manufacturers and none have a frig for the dimentions I require. I do not want something smaller nor do I want to become a cabinet maker to make a smaller unit fit and look right.
I have 4-8D flooded batteries. I want to add an inverter JUST to run the refrigerator. Would I need to add a couple of more batteries or could I use what I have already? I have determined that the frig uses approx. 30 watts of power or 3 amps ( I do not know if that is accurate I found it on a website) and I assume that is per hour? I would run the generator a couple of hours a day if needed to top off the charge. I do not have room for solar panels (maybe later if I add a hardtop on the bridge). This is the only need, to keep the frig up and running without 24/7 use of the genny. All other house power needs are taken care of.

Thanks.
Chris
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Old 09-13-2012, 08:11 PM   #2
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FF will say: propane.
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Old 09-13-2012, 09:15 PM   #3
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Retro fit the existing refrigerator cabinet with eutectic tanks, installing a separate refrigeration unit to power them? After initial pull-down, run genset 1 hour twice a day to maintain cold.
Or is that the "Sun Frost" system ruled out on cost?
People I know using LPG (propane) like it but use smaller refrigerators. BruceK
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Old 09-13-2012, 09:49 PM   #4
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You've already said what the answer is. If you want to run the fridge, get an inverter. No big challenge there. Make it an inverter/charger so you have a high capacity way to recharge your batteries.

Your 4X8D batteries, if they are your house bank represent something around 800 amp hours of capacity. Thats a decent house bank.

I have a similar sized boat and a similar size house bank. My batteries last overnight and are close to being at 50% in the morning. We run the generator for between 2 and 3 hours in the morning, and the same in the evening. Since the stove, hot water heater and washer/dryer are electric we generally run the generator in conjunction with some of these other activities. Out inverter/charger outputs up to 150 amps, which represents a pretty good recharge rate.

No way I'd EVER put a propane fridge on my boat. Not a chance.
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Old 09-13-2012, 10:28 PM   #5
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If your drawing 3 amps on a 120 VAC applience then power "watts" would be 3 amps * 120 VAC or 360 watts when the refrig is running. Inverters efficiency is around 90 to 95%. Thus, total power needed from the batteries would be 360 + (360*.05) or 378watts. Assuming your total battery capacity is 800 amp/hrs and your don't want to discharge them more then 50% for long life then..... 800/2 or 400 amp/hrs can be used before recharging.

Converting amp/hrs to watts/hrs would be 400*12 or 4800watt/hr

As the refrigerator is using 378 watts we can now find out how long your battery would last....

4800/378 = 12.78 hours. But wait, the refrigerator isn't on all the time. How many times it cycles on depends on how well its made, how well insulated, etc. Can more insulation be installed on the sides??

Given all that you should easily go two days without having to recharge.

Make sure that who ever installs the inverter KNOWS what they are doing.

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Old 09-13-2012, 11:37 PM   #6
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Thanks for all the quick replys. Sooo 4-8d's and a 1000w inverter, just for the refrigerator, would be sufficent? or use a larger inverter. I hate to sound dumb but with electrical I am. Now, what is the trick to wiring up the inverter WINGSPAR? From what I have read it looks fairly straight forward and why should I use a inverter/charger KSANDERS if I plan on running the genny for a couple of hours because like your boat the galley and such is all electric also. Like I said earlier, mechanical stuff I'm good, electrical I'm sitting in a corner with drool running down my shirt.

Thanks again.
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Old 09-14-2012, 12:23 AM   #7
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Originally Posted by knotheadcharters View Post
Thanks for all the quick replys. Sooo 4-8d's and a 1000w inverter, just for the refrigerator, would be sufficent? or use a larger inverter. I hate to sound dumb but with electrical I am. Now, what is the trick to wiring up the inverter WINGSPAR? From what I have read it looks fairly straight forward and why should I use a inverter/charger KSANDERS if I plan on running the genny for a couple of hours because like your boat the galley and such is all electric also. Like I said earlier, mechanical stuff I'm good, electrical I'm sitting in a corner with drool running down my shirt.

Thanks again.
The inverter/charger would probably give you a quicker charge rate, which would reduce your generator run time requirements.

Unless you already have a gigantic charger on board.
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Old 09-14-2012, 02:26 AM   #8
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KSANDERS, I have a 60 amp Sentry charger already on the boat.
Once again I feel the need to apoligize. Normally I would have asked the PO all of these questions regarding the set-up currently on the boat but I bought the boat as a bank repo and have no history and no way to contact the PO. I have owned the boat since last December and have had no problems with it as I had a thorough survey both on boat and engines done and have put about 40 hours on it in its current configuration. I am now back overseas working so this is when I start figuring how to optimize systems for spending considerable more time on the boat when I am home.
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Old 09-14-2012, 06:32 AM   #9
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A modern DC fridge will give the best bang for the buck.

If you cant afford the Sun Frost the next bet will still be $1500+ .Tho in the size you seem to require two would not save much.

Since it might be smaller than the existing box , you get the option to glue a few inches of added insulation to most surfaces.

Not that much wood work to shrink the fridge hole on a built in.

The modern multi speed Danfloss or similar is far more efficient than a house fridge with an inverter.

One inverter hassle is their efficiency varies with loading.

Although they might only cost 10% at best , it can easily run doubble.

That's why AC/DC fridges include their own inverters, to attempt higher inverter efficiency.

While operating any DC item lowers batt voltage , the DC compressors are less bothered by lowered voltage than an inverter that simply will gobble more amps.

The energy price you pay for any refrigeration is determined by the outside surface area , as well as the quality of the insulation.A huge box will be really difficult , even with unlimited bucks.



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Old 09-14-2012, 09:40 AM   #10
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KSANDERS, I have a 60 amp Sentry charger already on the boat.
Once again I feel the need to apoligize. Normally I would have asked the PO all of these questions regarding the set-up currently on the boat but I bought the boat as a bank repo and have no history and no way to contact the PO. I have owned the boat since last December and have had no problems with it as I had a thorough survey both on boat and engines done and have put about 40 hours on it in its current configuration. I am now back overseas working so this is when I start figuring how to optimize systems for spending considerable more time on the boat when I am home.
Charger size directly affects the recharge rate of the batteries, which directly affects the generators required run time.

I do not know what your real life DC loads are going to be. Nobody here knows that. People here can tell you the draw of a fridge but the actual draw on your boat is really based on how you operate the boat.

I can tell you the math of charging batteries though. If you run your house bank down to 50% (a good rule of thumb for battery life) then you will have depleted 400 amp hours. You need to replace those 400 amp hours. Without getting into the charge curves for multistate chargers the simple math is 400 amp hours divided by 60 amp charging rate equals approx 6 hours 40 minutes to recharge.

If you purchased a larger charger you can see that recharge rates and generator run times decrease. A 150 amp charger will replenish 400 amp hours in 2 hours 40 minutes.

Be forewarned that those calculations are not exact. They are a little low because chargers do not put out their full rated current for the entire charging time, but they are great for comparison sake.

If you like your fridge I'd say to keep it and get a good quality, high capacity inverter/charger. Once you have AC power on the hook the world of functionality opens up beyond just the fridge. You can always replace the fridge later. I have a full sized vitrifrigo fridge on my boat that I just put in this spring as a replacement. The cost was $1350 and the fridge works great.

Also, If you've never been on the hook for any extended time, you will quickly learn that refrigeration is only one portion of the loads on your batteries. The bigger the boat, the bigger those loads. For example one of my Furuno navnet displays takes 5-8 amps of DC power all the time, and my satcom system takes around 15 amps DC all the time. a single 10 watt light bulb takes 0.8 amps. That stuff adds up quickly.
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Old 09-14-2012, 10:16 AM   #11
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Installing an Inverter isn't hard. The problem that I've seen so many times is that average (Do it your selfer) possible due to lack of electrical knowledge or limit time simply doesn't follow the manufactures installation instructions. I've worked out many issues with inverters over the years where the input cable guage is way to small, very poor crimp connections, outputs connected to incoming power,etc,etc and etc. The comman factor I'm told before I look at it is that the Inverter is JUNK.

For your install I would consider running the output of the inverter to a seperate wall plug behind the refrig. Plug the refrig into the new outlet. Thus, none of your existing house wiring would need to be redone or modified.

You have stated that the refrig is a 120 volt unit. I'm assuming for a boat your size you already have an inverter on board. Is that correct? Possibly larger house batteries? What do you have for house batteries now???
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Old 09-14-2012, 10:26 AM   #12
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For your install I would consider running the output of the inverter to a seperate wall plug behind the refrig. Plug the refrig into the new outlet. Thus, none of your existing house wiring would need to be redone or modified.

I would run the output of the inverter to a buss bar located behind the main AC panel. That way you could with some panel modification have several loads on the inverter. Then you just mark the front of the panel with the loads that are on inverter.
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Old 09-14-2012, 10:34 AM   #13
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manual setting, so I know what is happening. Instead of spending big bucks on inverter charger might want to just put a timer on the refrigerator as they do not have to run 24/7. I have timers on all high amps items. The nut to crack is how to charge back the amps used, not using the? Luckily with DC NOT AC, you can gang charge, more than one source of charge.
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Old 09-14-2012, 01:47 PM   #14
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I don't know your boat, but if you have any built-in ice box/cooler, you could install a 12 volt Sea Frost unit. We did that on our sail boat and it is extremely efficient. Install was easy, and the owner will do semi-custom for the lenght of the lines, etc. Another option would be one of the Engle or Waeco 12volt cooler fridge/freezer units. They are also very efficient and owners rave about them. One of the Camano's we looked at a few weeks ago had one in addition to the built-in fridge in the galley. I saw a NT 32 that had one in the saloon with a custom cusion top to use it for extra seating and to make it blend with the interior. Anyway, I know this isn't the exact solution you want, but much less expensive than replacing the entire 120 unit.
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Old 09-14-2012, 02:31 PM   #15
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"charger size directly affects the recharge rate of the batteries, which directly affects the generators required run time.

Not really.

The rechaege rate is limited by the battery construction, AGM will take the fastest recharge , but all slow way! down after being about 85% full.

The most rapid way to recharge a large batt set is with a smart 3 or 4 stage regulator. This will take you from 50%soc to 85% soc the fastest , esp as alts can be 150-400amp , and have the power to charge hard when the batt is low or high. .Mant batt chargers can not make their rated amps on a large batt set , even on line current, never mind the usual puny power a noisemaker produces.

The "kill a watt" and other current measuring devices are dirt cheap now , start by MEASURING the fridge use over 3 -5 days to decide if you need a better more efficient fridge , or are just going for a bigger hammer.

An SOC meter will be required while cruising.
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Old 09-14-2012, 05:26 PM   #16
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"charger size directly affects the recharge rate of the batteries, which directly affects the generators required run time.

Not really.

The rechaege rate is limited by the battery construction, AGM will take the fastest recharge , but all slow way! down after being about 85% full.

The most rapid way to recharge a large batt set is with a smart 3 or 4 stage regulator. This will take you from 50%soc to 85% soc the fastest , esp as alts can be 150-400amp , and have the power to charge hard when the batt is low or high. .Mant batt chargers can not make their rated amps on a large batt set , even on line current, never mind the usual puny power a noisemaker produces.

The "kill a watt" and other current measuring devices are dirt cheap now , start by MEASURING the fridge use over 3 -5 days to decide if you need a better more efficient fridge , or are just going for a bigger hammer.

An SOC meter will be required while cruising.
OK, let me clairify

Corrected statement:

charger size up to approx 25% of the battery capacity in amp hours directly affects the recharge rate of the batteries, which directly affects the generators required run time.

That statement is now electrically correct. In all practically with an 800 amp hour battery bank like the OP has, that statement is correct almost 100% of the time because a 200 amp charger in a recreational battery system is a pretty rare thing indeed.

The higher acceptable recharge currents that are offered by AGM batteries only come into play with smaller battery banks because of the large capacity chargers necessary to take advantage of them.
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Old 09-14-2012, 07:17 PM   #17
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Thanks for all the inputs, it seems that the easiest route would be bite the bullet and buy the Sunfrost. I don't mind spending the 3200 for the unit I was just looking at an easier way, which it seems doesn't exist without a whole lot of refit and retrograde. I am going to wait and see if the existing fridge gives up the ghost in the next year or so then I will replace it and add 2 more 8d's to the house bank. The Sunfrost will work as the Admiral likes the size of the existing and will not go to a smaller fridge and the Sunfrost is an exact fit. Again thanks for all the tech help, I need to study up on all things electrical. I do know that all my lighting is going LED. Thanks again as I know I will be asking more questions.
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Old 09-15-2012, 06:56 AM   #18
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To use LESS energy is always the preferred solution ,

and will save big bucks compared to creating , storing then using energy.

The up front price is always higher , but in the long term far lower.

Contemplate that hard working (50% SOC to 85% SOC) battsets are limited in their number of charge , discharge cycles.

A look at the Trojan or Surette website will give an idea.

Replacing dozens of batts fairly often is not cheap, and can be heavy work.

Installing and driving a substantial alternator is also not cheap, even with rebuilt commercial gear.

The Eutetic concept is also not cheap , and the plates required take up lots of interior space in the reefer.
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