Fridge; short compressor time on 12v

[mattonthesea]

We have a standard danfoss refrigeration unit. The sort you find in all the chandlers. When we are on shore power, even at low settings, the unit will get down to freezing quite easily and the compressor works for several minutes at a time. However, on battery alone, it will only go down to cooling around 6 degrees Celsius with the compressor only working for about 20 to 30 seconds at a time. The unit sits below the water line and there is a computer fan to circulate air around it.

I have done all the checks that Viv recommends and it all seems to be working correctly.

With hindsight I would say that my insulation attempts are just that; attempts!

My thoughts are running to getting one of those portable fridges that are more efficient and lower on power. I am though, quite interested into why the present fridge only runs as it does. I have 3x 80aH batteries.

 

[sfellows]

Many refrigerator systems have voltage cut-out when the voltage gets too low. Sounds like it is your batteries that are the problem. Test the batteries to ensure that they are working properly. 3 x 80aH will only give you approx 120aH useable power. Refrigerators can easily use 40aH per day (4 amps @ 12v at 10 hours on per day).

 

[Graham376]

If it's OK on shore power, how can it be an insulation problem? Sounds like battery or voltage drop problems. Our Danfoss draws 7 amps and needs fairly heavy gauge cable but no problem freezing food.

 

[yotter]

I would also check the voltage that the compressor is seeing at its end of the wiring. It could be cutting out due to low voltage caused by a bad connection while the ship's battery voltage is normal. I would put a digital voltmeter accross the compressor supply and monitor it on and off load.

 

[Boater Sam]

Falling voltage causes it to shut down prematurely before the thermostat tells it that it is down to the set temperature. Most likely failing or incompletely charged batteries and or undersize wiring exacerbating the problem when on batteries. On mains the output of your battery charger is overcoming the problem.

Test the batteries properly, use the cable volt drop calculator to see if the wiring is heavy enough. Clean battery connections and any connections to the fridge from the fuse box.

 

[vas]

also get a small 5mm LED and connect it to the right terminals at the back. You'll be surprised to see that mostlikely when it cuts off on battery power it will "think" it's hit the low voltage limit, when you believe that batteries are OK and provide enough juice.

 

[coopec]

I am sure the Dometic Fridge uses a Danfoss compressor. This thread may help?

Dometic CRX110 refrigerator - 12 or 24 volt or both?

Waeco Adler/Barbour Refrigeration Problem - Fridge 101
https://c34.org › bbs

27 May 2012 — Small 12/24 volt boat refrigeration using Danfoss compressors manufactured after 1996 will have a BD 35 or BD 50 variable speed compressor with ...
15 posts · , he is an expert, and provides complete checkout procedures that you then can us

 

[vyv_cox]

I agree with most that the power supply seems to be the problem, either batteries or the cables and connectors. First action is to wiggle the fuse in its holder!

 

[Graham_Wright]

You can connect an led to emit a message. E.g. 7 flashes (on mine) indicates "compressor failed to reach the correct speed".
You can find the decodes on the net.

 

[mattonthesea]

Thanks. Two of the batteries are fairly new but one is old but this happens even when the battery monitor indicates 13v while compressing. Also the compressor behaved the same before and after I added the two new ones so I suspect it's the wiring. Furthermore, I rewired most of the boat but left the fridge wiring through laziness. It is routed through the engine compartment and does weird things through the battery box! And I suspect it's domestic lighting wire.

I'll read the links and do some testing next time I'm on the boat. Planning to rejig the galley so I'll take the unit out and test it with good wiring.

 

[Richard10002]

We have a standard danfoss refrigeration unit. The sort you find in all the chandlers. When we are on shore power, even at low settings, the unit will get down to freezing quite easily and the compressor works for several minutes at a time. However, on battery alone, it will only go down to cooling around 6 degrees Celsius with the compressor only working for about 20 to 30 seconds at a time.

Has it only started doing this recently, or has it been happening since you i) bought the boat, ii) fitted the fridge unit?

As others have said, if it works fine with shore power, then it is to do with power supply, and the fridge unit is fine.

 

[Graham_Wright]

It is sobering to measure voltage at the destination when under load. The difference between that and battery volts can be significant.

BIG WIRES!

 

[MattS]

Mine was doing exactly this - whenever running on battery only, and it did seem to be a low voltage thing. Would only run the compressor for 10 seconds before cutting out again.

I replaced the house battery, and it hasn't done it since - but then I also try not to run it when not on shore power anyway because it's so hungry!

 

[mattonthesea]

Thanks everyone.

I tested the voltage across the terminals at the fridge and they were down about 1.5 v compared with battery. That's on a very quick and dirty test. The fridge was wired in with domestic 1.5mm2 lighting flex; and about 4m of it. It's about the only wiring I haven't replaced, and probably a good thing as it's unlikely I would have known/done it in 2.5mm2. Tested again with tinned, 2.5mm2 and hardly any drop.

However, in doing so, we found that a fridge wall was being leaked into and the ply has delaminated. So one job done and another appears

Thanks again

 

[Boater Sam]

Thanks everyone.

I tested the voltage across the terminals at the fridge and they were down about 1.5 v compared with battery. That's on a very quick and dirty test. The fridge was wired in with domestic 1.5mm2 lighting flex; and about 4m of it. It's about the only wiring I haven't replaced, and probably a good thing as it's unlikely I would have known/done it in 2.5mm2. Tested again with tinned, 2.5mm2 and hardly any drop.

However, in doing so, we found that a fridge wall was being leaked into and the ply has delaminated. So one job done and another appears

Thanks again

Glad its working. As a rule of thumb you use cables 1mm2 per metre run to the fridge from the battery so 4m away I would have used 4mm2. But, hey its working!

 

[superheat6k]

As an aside have you considered keeping the fridge as full as possible with liquids - cans or plastic bottles, water, beer, etc. Liquid has a far greater heat density than air and the coldness does drift away when the fridge is opened. Get the liquids cold when on shore power, and this then acts as a cold sink when away and on the batteries.

If you need space for food then simply remove some of the cans / bottles.

 

[mattonthesea]

@superheat6k Yes We keep three 500ml bottles of water in the element space. Works a treat to keep cool overnight.

@Boater Sam I reduced the cable length to just over 2m. It means the fridge switch is not on the main panel but I can live with that. It also means that I can keep the convention of having the switches on the +ve side. When I got the boat they were nearly half and half +ve and -ve side switched. I blew the first RS232 - USB cable because the laptop tried to charge through it from the AIS receiver when I switched the charger off! I'd say re-wiring was one of the most rewarding of 'totally and utterly frustrating' jobs I've ever done, in that I now know what is happening behind the panels and why. I just didn't do the fridge or the water pump because they went the other way ?

 

[mattonthesea]

UP~DATE

finally finished and fitted new fridge last weekend. Then realised the I have spent almost as much as I would have done on one of those Dometic, portable ones!

It's a sandwich construction of 9mm ply, 10mm Aerogel, 6mm ply under the work surface. The lid is 9mm ply with the inner lid of 10mm aerogel encased in acrylic and still in the process of being shaped to fit airtightishly. The body is then surrounded by varying thicknesses (depending on proximity to other things including hull) of other types of insulation all sealed and glued with expandy foam.

Quite pleased with it. Before I fitted I put a cup of ice in it - having not prepared the box in any other way - at room temp; and another on the floor beside it. When the control ice had melted the fridge cup was still more than half ice (difficult to tell proportions because I hadn't remembered Archimedes at the time!) On the boat I ran the fridge for half an hour and the evaporator plate was still cold to touch three hours later. Result methinks!

Thanks again