Regulating Voltage to a Fridge

[ramblinrod]

Originally Posted by witzgall

I am concerned a bit about the wording above, specifically the call to only take batteries below 50% in an emergency. The rest of the statement is true, but we have found that occasional dips below 50% are not very detrimental to the overall longevity of the batteries. What I have seen, is that boaters become stressed out if they ever see the % drop below 50% The duration of time the batteries are left below 50% is a much more important factor, in our (Balmar's) experience. A few hours is not a big deal in these situations. Leaving SoC below for longer than that does start to sulfate batteries more so. All of this is predicated on accurate SoC values, based on the actual battery capacity as it ages.

Chris

Don't be concerned. It ain't worth stressing over. ;-)

But I agree in principle, there is no magic SOC number where slightly above is safe and slightly below is dangerous.

This is why I take issue with claims that when one switches to LFP they can use half what they had in FLA.

First of all, many consider that one can only use 30% of FLA total capacity.

This isn't true.

It is a rule of thumb that some came up with for optimum battery life with reasonable generator or alternator run time to recharge.

Firstly, one can go below 50%, it will just reduce FLA life expectancy.

If one chooses to go to 40% and that means your batteries last 2 years instead of 2.5 years, they may be OK with that.

Now on the charging side, if they can get them up to 100% every day with solar, instead of 80%, that will improve life expectancy EVEN MORE than the 10% deeper discharge takes away.

So mount adequate solar, so the batteries are brought up to 100% most days, and you may increase life expectancy to 4 years.

So now you have a capacity of 60% that may last 4 years.

Some will try to refute this if it interferes with their personal (or business) LFP justification, but while the numbers may be a little different in each case, it is correct in principle.

There are all kinds of myths on the internet about batteries.

One of my favourites is that you cannot add old to new.

Of course you can.

If the entire bank is at EOF (end of life) capacity (< 80% of new), then it is pretty much done, and should be all replaced, as batteries may start failing.

But if one has a 2 year old bank, that is well maintained, going strong and near 100% capacity, and wishes to add capacity to the bank, just add some new batteries.

The additional bank size, will help avoid the older batteries being so deeply discharged, so they will last even longer.

It is true, that the new batteries may be exercised (cycled) a bit deeper than the old ones. With each cycle, they get closer, until they are all pretty even. Run the bank to EOL capacity and replace the entire bank.

Throwing away perfectly good batteries, to increase bank size, because one can't supposedly add new to old, is a total waste.

[Mike Banks]

Use a Calcium deep cycle battery system of at least 50 ampere hours for just your refrigerator and have a dedicated solar charger and panel of about 100 watts solely used for priority charging it. Only when the refrigerator battery is fully charged can the solar charger switch to another battery.

[Mike Banks]

There is no reason why one can not mix old and new batteries--I did it all of the time with no problems as long as they are batteries connected in parallel. The same is not true for six or two volts cells wired in series--the old cell forms a weak link and can drag down the whole system. The only reason NOT to mix is for convenience and, I suppose, a slightly longer ampere-hourage capacity.

If you do add old ones to new, be aware that your maintenance and inspection regime has to increase. Old cells will as like as not have to be replaced sooner than the others, and excess gassing and more frequent top-ups with distilled water than the others is a sign that they are about ready for replacement.

I simply used twin battery banks, the newer ones became the older ones and the old ones got replaced by new ones--and so on. They WERE mixed, via the multiple switch, but each could be used separately. Both were charged simultaneously from the same twin-regulated solar arrays, but I could have charged them separately had I so desired. Both banks were usually fully charged before noon.

A properly maintained battery bank should last five years or longer. I have heard of farm lighting plants using heavy deep cell batteries properly maintained that lasted twenty years, but that is in a cool shed, not a vessel moving in the ocean.

This assumes that your battery bank is composed of 12 volt deep cycle batteries, and not tractor starting batteries. These heavy starting batteries are cheaper to buy and work OK as long as you have two or three of them--but they will need replacement about every two years--and time flies.

[OzePete]

Regards the OP. As others have suggested the problem is most likely voltage drop.
Ramblinrod properly describes the process of checking this but this diagram may add to that and make it easier to understand the process of identifying a poor or erratic power supply...

(Click on diagram to expand)



Cheers OzePete

[fnq grumpy]

All of the above about supply and voltage are reasonable. Danfoss like voltage but can work OK down to 10V but better on good voltage. I have battled fridge issues for years in hot part of northern Australia. New second hand boat with expensive Isotherm fridges, marginal in Brisbane (south), running continually and getting nothing like set temperatures as we came north. Prepared to replace but got fridge mech off next door boat to look. Three issues he pointed too. Operating voltage ,not supply voltage, 7-11v vs 12.7V supply when fridge off. Heavier wiring and fused relay from near by bus instead of long runs through nice switchboard. All seeing 12V+. Small computer fan in cabinet run off compressor fan and quicker more even cold distribution and ice build up on evaporators gone away.

BUT the biggie is compressor speed. Danfoss designed to run at 2000 rpm to 3500 rpm with proportionate increase in amps and cooling and very small efficiency loss at higher speeds and pressures. Simple resistor in RIGHT wire in thermostat circuit, speed increased. Gone from not getting down to temperature at 4 A draw 100% duty cycle to holding low temperature on 60% setting drawing 6A but at 40-50% duty cycle, sped up to 3000 rpm. Most manufacturers supply at 2000 rpm to be able to quote low power draw. All laid out in comprehensive Danfoss technical note.

I'll throw a couple of amps at current draw to have fridges that work properly on 50% duty cycle thanks. But check manufacturer has not already done this trick. This is basis of all expensive 'black box' load controllers, voltage sensing units.

All laid out in Danfoss article I have been blissfully unaware of.
files.danfoss.com/TechnicalInfo/Dila/06/bd_compressors_04-2007_pk100c802.pdf


Power supply is one issue but cooling capacity has to exceed cold loss from cabinet as some expert was quoted in a thread recently.

[a64pilot]

I’ve not seen loose or inadequate wiring cause an issue once running.
However I have seen several times it cause a failed start and the three flash code.
It seems that if the wiring and voltage are good enough to get it started, then it’s plenty good enough to run it once started.

Just from an experience of one, I have become a real fan of the AEO module, if you chose to replace the module, get the AEO and ensure there is NO resistor in line with the thermostat.
If there is I believe the AEO will run at a fixed speed, it only actually learns and changes speed as necessary if there is no resistor, I think.

[ramblinrod]

Originally Posted by fnq grumpy

All of the above about supply and voltage are reasonable. Danfoss like voltage but can work OK down to 10V but better on good voltage. I have battled fridge issues for years in hot part of northern Australia. New second hand boat with expensive Isotherm fridges, marginal in Brisbane (south), running continually and getting nothing like set temperatures as we came north. Prepared to replace but got fridge mech off next door boat to look. Three issues he pointed too. Operating voltage ,not supply voltage, 7-11v vs 12.7V supply when fridge off. Heavier wiring and fused relay from near by bus instead of long runs through nice switchboard. All seeing 12V+. Small computer fan in cabinet run off compressor fan and quicker more even cold distribution and ice build up on evaporators gone away.

BUT the biggie is compressor speed. Danfoss designed to run at 2000 rpm to 3500 rpm with proportionate increase in amps and cooling and very small efficiency loss at higher speeds and pressures. Simple resistor in RIGHT wire in thermostat circuit, speed increased. Gone from not getting down to temperature at 4 A draw 100% duty cycle to holding low temperature on 60% setting drawing 6A but at 40-50% duty cycle, sped up to 3000 rpm. Most manufacturers supply at 2000 rpm to be able to quote low power draw. All laid out in comprehensive Danfoss technical note.

I'll throw a couple of amps at current draw to have fridges that work properly on 50% duty cycle thanks. But check manufacturer has not already done this trick. This is basis of all expensive 'black box' load controllers, voltage sensing units.

All laid out in Danfoss article I have been blissfully unaware of.
files.danfoss.com/TechnicalInfo/Dila/06/bd_compressors_04-2007_pk100c802.pdf

Power supply is one issue but cooling capacity has to exceed cold loss from cabinet as some expert was quoted in a thread recently.

A word of caution about compressor speed.

The longer the on portion of the cycle, the more efficient the compressor will run (ie, the less Ahrs / day consumed).

The slowest speed is most efficient, but a higher speed may be required to achieve desired refrigeration temperature, depending on box size, insulation, door seals, ambient temperature, refrigerator temperature, how frequently opened for how long, and what is put into and taken out of it.

[BobHorn]

Have a Nova Kool freezer, compressor seemed to run continuously. Based on advice from Nova Kool I added a 500 ohm resistor in the thermostat line and now it cycles like it should.

[a64pilot]

Originally Posted by BobHorn

Have a Nova Kool freezer, compressor seemed to run continuously. Based on advice from Nova Kool I added a 500 ohm resistor in the thermostat line and now it cycles like it should.

Originally Posted by ramblinrod

A word of caution about compressor speed.

The longer the on portion of the cycle, the more efficient the compressor will run (ie, the less Ahrs / day consumed).

The slowest speed is most efficient, but a higher speed may be required to achieve desired refrigeration temperature, depending on box size, insulation, door seals, ambient temperature, refrigerator temperature, how frequently opened for how long, and what is put into and taken out of it.

Both of these are very valid points.
That is what the AEO module is in my opinion a game changer, because it will vary compressor speed as necessary to maintain optimum cycle times, if you don’t need the high speed due to your in cooler temps or not adding food every day etc., it will turn down the compressor speed and save power.

However if your heading South say from Canada to the Bahamas, as you get into warmer water and weather, it will automatically speed up the compressor as necessary to keep it cooling well.

You don’t have to do anything, it’s fully automatic.

[BobHorn]

What is an AEO module? There is some kind of speed control device mounted in the cabinet next to the compressor but it is not connected. Can't recall the brand.

[Ortolan]

Wow - thank you all! I am leaning towards replacing the control module when returning to the States, rather than adding another component (voltage regulator).


In case further details alter any opinions:


This is a Nova #RFS7501 dual fridge/freezer (5.5 cf fridge & 2.0 cf freezer), so asking a lot of a single compressor & not a lot of insulation. Yet when all is perfect it can keep the fridge temp below 39° even with 90° cabin temp. One way to keep all perfect is to defrost the fridge very often (every 1 1/2 weeks when humid) - mild frost on the fridge cold plate & especially even 1/4" of ice around the incoming fridge tube doesn't just cause diminished operation - it can cause an "episode" (see below).



When this unit has an "episode", the compressor runs continuously, yet the fridge cold plate completely defrosts -only the incoming tubing remains frozen. The freezer cold plate is never a problem. Is there a valve/device between the freezer cold plate & the fridge cold plate which could account for the extreme frosting differences between the freezer & fridge cold plates?


When it has an "episode", it often fixes itself if you cut power for 10 minutes. I don't know if this is helping the control module or equalizing the pressure. I have recently added an electronic timer which cuts the thermostat connection for 5 min. every hour daytime & 10 min. every hour nighttime.



I have added a LED to show the compressor running & a LED to show faults. I do get a 3-flash fault when I temporarily hook the fridge to an engine battery when below 12.2 volts (when equalizing the main bank). Hooked up to the main battery bank, it has never 3-flashed even at 12.1 volts. Also, about once a month, I get the fault LED glowing dimly - a hard fist hit on the door makes the glowing go away & the compressor starts. This by itself is a sign of a bad control module?


I have triple checked all connections, installed new 10g wiring (25') & checked the voltage at the control module - there is about a .3 volt drop when compressor is on.



I have replaced the factory 500 ohm resistor with a 1000 ohm resistor to increase the compressor speed. I have replaced the thermostat. I have replaced the fridge door gasket.



I have added a small fan (powered separately & always on) venting the compartment. This helps cool the compartment, especially since the unit isn't installed in a sided compartment or a sleeve - the factory fan which appears to want to exhaust hot air past the compressor & outward doesn't - the compartment is open so the same hot air was just blowing around.


Every change I've made has helped somewhat, but these "episodes" continue, especially during warm nights/lower voltage. I should probably have already replaced the control module, but weeks sometimes go by with no "episodes", so I keep thinking my latest mod has solved it. Am I missing anything?


Russ

[OzePete]

Russ, with this added info, and if it originally worked ok, I believe your fridge is slightly short of gas.

To explain: Refrigerant first services the freezer then on in series to service the fridge evaporator. Usually the thermostat sensor reads the temperature at the last of the fridge evaporator to be cooled so if a bit short of gas the sensor area of the evaporator misses out on refrigerating thus being warm and keeping the fridge running. A good indicator of this is if the freezer is colder than normal.

You may need to have the gas leak found/ repaired, the system evacuated and re-gassed.

Cheers OzePete

[john61ct]

Whatever other fixes/improvements are needed

> not a lot of insulation

that's one I would try to prioritize.

Not easy I know, but really takes the pressure off all the rest of the design factors, and nothing else will come close to the improvement in overall efficiency.

[ramblinrod]

Originally Posted by Ortolan

Wow - thank you all! I am leaning towards replacing the control module when returning to the States, rather than adding another component (voltage regulator).


In case further details alter any opinions:


This is a Nova #RFS7501 dual fridge/freezer (5.5 cf fridge & 2.0 cf freezer), so asking a lot of a single compressor & not a lot of insulation. Yet when all is perfect it can keep the fridge temp below 39° even with 90° cabin temp. One way to keep all perfect is to defrost the fridge very often (every 1 1/2 weeks when humid) - mild frost on the fridge cold plate & especially even 1/4" of ice around the incoming fridge tube doesn't just cause diminished operation - it can cause an "episode" (see below).



When this unit has an "episode", the compressor runs continuously, yet the fridge cold plate completely defrosts -only the incoming tubing remains frozen. The freezer cold plate is never a problem. Is there a valve/device between the freezer cold plate & the fridge cold plate which could account for the extreme frosting differences between the freezer & fridge cold plates?


When it has an "episode", it often fixes itself if you cut power for 10 minutes. I don't know if this is helping the control module or equalizing the pressure. I have recently added an electronic timer which cuts the thermostat connection for 5 min. every hour daytime & 10 min. every hour nighttime.



I have added a LED to show the compressor running & a LED to show faults. I do get a 3-flash fault when I temporarily hook the fridge to an engine battery when below 12.2 volts (when equalizing the main bank). Hooked up to the main battery bank, it has never 3-flashed even at 12.1 volts. Also, about once a month, I get the fault LED glowing dimly - a hard fist hit on the door makes the glowing go away & the compressor starts. This by itself is a sign of a bad control module?


I have triple checked all connections, installed new 10g wiring (25') & checked the voltage at the control module - there is about a .3 volt drop when compressor is on.



I have replaced the factory 500 ohm resistor with a 1000 ohm resistor to increase the compressor speed. I have replaced the thermostat. I have replaced the fridge door gasket.



I have added a small fan (powered separately & always on) venting the compartment. This helps cool the compartment, especially since the unit isn't installed in a sided compartment or a sleeve - the factory fan which appears to want to exhaust hot air past the compressor & outward doesn't - the compartment is open so the same hot air was just blowing around.


Every change I've made has helped somewhat, but these "episodes" continue, especially during warm nights/lower voltage. I should probably have already replaced the control module, but weeks sometimes go by with no "episodes", so I keep thinking my latest mod has solved it. Am I missing anything?


Russ

Here are some things that may be causing your problems:

1. Insulation: There should be at least 2" of well sealed Styrofoam SM, (or better) on all sides, bottom, and top of box.

2. Lid Seals: There should be no air leaks.

3. Opening: Should be kept to minimum frequency and duration.

Note that issues with items 1, 2, or 3, may cause excessive frosting.

4) Frost. A film of frost on the evapourator plate, that will develop fully withing about 24 hours of startup in a sealed and insulated ice box, is normal. Any frost build up, will really hurt performance.

In your case, the rapid high build up of frost at the evapourator inlet tube, may be indicative of:

1. Too high of a refrigerant charge.
2. Moisture in the system.
3. A restriction in the piping.

At this point, you may be well served to call in a "COMPETENT" HVAC/R tech.

(I perform installations and some servicing, but for real headscratchers, I call in a retired "Westinghouse" factory refrigeration field service tech. He has forgotten more about refrigeration than I will ever learn.)

[SoonerSailor]

I needed more control over my Danfoss BD35F system, so cobbled together a little box to do so. A bit cheaper than the AEO module for those so inclined. I posted a link to the schematic over in a new thread in the Marine Electronics section.