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Old 06-09-2014, 16:16   #1
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Using a Combiner for the Fridge

Our boat is docked and hooked up to shore power year-round. I visit the boat on most weekends. The batteries are left on and other than a couple small computer fans for ventilation (draw less than half an amp) I leave the fridge on (DC powered). My concern is protecting the batteries in the event of a power loss. My marina is very old and the electrical hookups are a bit unreliable. Once a large rainstorm shorted out the dock power, and another time construction on the dock popped the breakers. The computer fans would run for weeks without shore power, but the fridge killed the batteries in a couple of days. I just replaced the battery bank and would like to protect them in the event the boat lost shore power again.

I read about the possibility of wiring a combiner between the panel and the fridge. The idea being that when hooked up to shore power the voltage would be over the threshold and the fridge would operate. Disconnecting shore power would drop the voltage and the fridge would not operate. I assume I could also wire a switch to bypass this setup if I wanted to fridge to operate when out sailing?

Assuming that leaving the fridge on is important (wife and kids like cold drinks when we get there and I like always having a cold beer in the fridge) is this solution reasonable? Interested in getting the feedback from people who are far more knowledgeable than myself!
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Old 06-09-2014, 16:58   #2
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re: Using a Combiner for the Fridge

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Our boat is docked and hooked up to shore power year-round. I visit the boat on most weekends. The batteries are left on and other than a couple small computer fans for ventilation (draw less than half an amp) I leave the fridge on (DC powered). My concern is protecting the batteries in the event of a power loss. My marina is very old and the electrical hookups are a bit unreliable. Once a large rainstorm shorted out the dock power, and another time construction on the dock popped the breakers. The computer fans would run for weeks without shore power, but the fridge killed the batteries in a couple of days. I just replaced the battery bank and would like to protect them in the event the boat lost shore power again.

I read about the possibility of wiring a combiner between the panel and the fridge. The idea being that when hooked up to shore power the voltage would be over the threshold and the fridge would operate. Disconnecting shore power would drop the voltage and the fridge would not operate. I assume I could also wire a switch to bypass this setup if I wanted to fridge to operate when out sailing?

Assuming that leaving the fridge on is important (wife and kids like cold drinks when we get there and I like always having a cold beer in the fridge) is this solution reasonable? Interested in getting the feedback from people who are far more knowledgeable than myself!
Most fridge compressors in boats let you set the voltage at which the compressor "cuts out" to protect the batteries.

For example, in Danfoss BD 50 compressor that I am familiar with you can install an 82-ohm resistor between the C (common) and P (protection) terminals of the control unit if you want the compressor to cut-out at 11.3V (measured at the terminals of the control unit) and then cut back in (ie restart) when voltage goes back to 12.5V.

Once you factor voltage drop between battery and compressor and voltage "sag" due to the compressorīs load (say 0.6V total) then the 11.3V may become high enough that will somewhat protect your batteries from falling below a reasonable state of charge level. Say resting voltage equal to 11.3V+0.6V=11.9V, which is equivalent to a 40% state of charge level for the flooded Trojans that I am familiar with.

Cheers

C
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Old 06-09-2014, 17:04   #3
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re: Using a Combiner for the Fridge

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Assuming that leaving the fridge on is important (wife and kids like cold drinks when we get there and I like always having a cold beer in the fridge)
The concept of leaving the fridge on when you're not there is NOT a new one, but the reasons for this have forever been beyond me. If you want a cold one when you get there, stop at 7-11 on the way to the boat or buy a cooler and bring it with you in the car. I've heard of more boats being damaged by being plugged into unreliable shorepower than I care to admit. It's just dangerous.

Other than that, instead of trying the combiner, get a solar panel sized to make up for your 100 ah/day fridge draw. It'll be a much better long term investment.
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Old 06-09-2014, 17:28   #4
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re: Using a Combiner for the Fridge

What stu says. I never hook up to shore power even though I have 30 amp service and all the cables/plugs to do it. My solar panel/battery bank will keep the fridge running for weeks, maybe months.
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Old 06-09-2014, 19:19   #5
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re: Using a Combiner for the Fridge

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Originally Posted by Stu Jackson View Post
The concept of leaving the fridge on when you're not there is NOT a new one, but the reasons for this have forever been beyond me. If you want a cold one when you get there, stop at 7-11 on the way to the boat or buy a cooler and bring it with you in the car. I've heard of more boats being damaged by being plugged into unreliable shorepower than I care to admit. It's just dangerous.

Other than that, instead of trying the combiner, get a solar panel sized to make up for your 100 ah/day fridge draw. It'll be a much better long term investment.

It seems like driving to the boat with a cooler full of cold stuff kind of defeats the purpose of the having the convenience of an onboard fridge. Turning off the fridge when you leave means you have repack all the cold stuff back into a cooler to go home. Might as well stick with a cooler and forget the fridge altogether.

+1 on the solar comments though.
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Old 06-09-2014, 19:48   #6
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re: Using a Combiner for the Fridge

You could put the fridge on a separate bank with isolation or I don't see why an ACR in the fridge circuit would not work.

Blue Seas makes an ACR with a manual combine feature.

In fact having a separate "non-essential" bus with an ACR might be a good idea.
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Old 06-09-2014, 20:16   #7
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re: Using a Combiner for the Fridge

Just google "low voltage disconnect"
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Old 06-09-2014, 20:22   #8
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Thanks for the input. I agree that adding solar would be the ideal solution, and it's definitely on the "list." The Blue Sea ACR with the override switch is a good suggestion, and seems like a relatively cheap and easy solution.

Obviously, the cheapest and simplest solution is a $3 styrofoam cooler..... But loading it up and lugging it to and from the boat every Friday kind of wipes out the benefit of having the fridge in the first place.
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Old 07-09-2014, 00:28   #9
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re: Using a Combiner for the Fridge

My prior post to "Just google "low voltage disconnect" was a bit too terse. However it is the answer to your original question. LVD circuits are included in many solar controllers, but can be purchased as 'stand alone' for under $100. (some solar controllers that have it are also in that price range. You'll want one that's adjustable, and let's you set the low voltage disconnect level, and the low voltage reconnect level.

My boat is "off the grid" and I use one that's built into my solar controller to assure my fridge doesn't flatten the house batteries after too many cloudy days on her mooring. I don't think you should apologize to other people who want to use their boats differently. I (like you) enjoy having cold beer aboard and other cold food/drinks, and I'm happy to be able to pay for that luxury/convenience.
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Old 07-09-2014, 00:33   #10
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re: Using a Combiner for the Fridge

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Originally Posted by Stu Jackson View Post
The concept of leaving the fridge on when you're not there is NOT a new one, but the reasons for this have forever been beyond me. If you want a cold one when you get there, stop at 7-11 on the way to the boat or buy a cooler and bring it with you in the car. I've heard of more boats being damaged by being plugged into unreliable shorepower than I care to admit. It's just dangerous.

Other than that, instead of trying the combiner, get a solar panel sized to make up for your 100 ah/day fridge draw. It'll be a much better long term investment.
This was a very judgmental and unhelpful answer and didn't address the OP's question.
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Old 07-09-2014, 00:50   #11
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re: Using a Combiner for the Fridge

If you leave the mains connected permanently, do you also find the anodes needs replacing each year? having ditched mains and gone over to solar I now find the anodes last much longer.

I agree with Stu, why not just switch the fridge on when you get there, they don't take long to cool stuff down especially if its transported cold in a cooler or cool bag.



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Old 07-09-2014, 00:51   #12
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re: Using a Combiner for the Fridge

I'm with Stu J.
Why leave the frig running when your not there? Do you know how much trouble/cost those things are to repair/replace?

The wear on the compressor seems senseless if your not going to be there full time. I just hope to have lots of pocket change to spare!

If you put cold drinks in a freshly started frig, they'll stay cold. I bring along a couple frozen 2qt water bottles just to get things started.

"A full freezer, once frozen, is more efficient then an empty one" Same goes for frig's!
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Old 07-09-2014, 00:55   #13
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re: Using a Combiner for the Fridge

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This was a very judgmental and unhelpful answer and didn't address the OP's question.
I disagree! It's a waste of materials and energy to produce electricity!
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Old 07-09-2014, 04:20   #14
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re: Using a Combiner for the Fridge

It's amazing how fast BS polarizes here.

OP may have other reasons for leaving the fridge on. Condiments for sandwiches perhaps, milk, cheese, cold cuts.

I am sure he could figure out hauling cold stuff back and forth from the house every week in a cooler.

The question was simple -

Is there a way to automatically isolate the fridge from the battery system in case shore power goes out?

Forget the reason and the eco-critic judgments. Forget the Anode question.

The answer is, yes. There are any number of ways to automatically shed DC loads on the battery bank.

What if he wasn't plugged into shore power and was meeting his fridge needs with solar 90% of the time and in case of 4 days of overcast he wanted to protect his $1,000 battery bank vs. $12 bucks worth of mustard, mayo and cheese.

Makes the eco-critic and the anode guy redundant.
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Old 07-09-2014, 04:37   #15
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re: Using a Combiner for the Fridge

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Just google "low voltage disconnect"
Good idea -

Blue Seas PN 7635

m-LVDC with remote manual override - $69.95 on eBay. $79.99 on Amazon.

Probably a better choice than an ACR.
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