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Fridge voltage drop


Jo Green

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Hi, hoping someone can advise. My 12v is wired in with 12mm cable. Recently turned it on for the first time since it was put it. When the engine is running or the sun is shining directly on the solar panel, it runs. Now the sun is going down the red light is flashing indicating a voltage drop. I've measure the voltage and it is 11.9 whereas the batteries read 12.9. The question is, instead of taking the cable out and replacing it with thicker cable, can I double up and run another 6mm piece of cable alongside? Thanks, Jo

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Hi, hoping someone can advise. My 12v is wired in with 12mm cable. Recently turned it on for the first time since it was put it. When the engine is running or the sun is shining directly on the solar panel, it runs. Now the sun is going down the red light is flashing indicating a voltage drop. I've measure the voltage and it is 11.9 whereas the batteries read 12.9. The question is, instead of taking the cable out and replacing it with thicker cable, can I double up and run another 6mm piece of cable alongside? Thanks, Jo

That is some volt drop. How far is the fridge from the battery bank? Is there one of those red isoltaing switches between the battery bank and the fridge? If so I would measure the voltage on either side of that; they often drop voltage with age and need renewing (with a proper one.)

Edited by Guest
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To answer the exact question, yes you can.

 

That is quite a big voltage drop, and would probably result in a very big drop when the motor switches on (current surge) which is probably what is making the warning light flash as 11.9v should be enough to run on; so yes it's worth looking for a possible different cause such as a poor isolating switch. The usual recommendation is that the cables (both of them, + & -) should be 1 sq mm for every metre of distance between the fridge and the battery so 12 sq mm (if that is what you have) should be OK for up to 12m of separation.

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I've not done the underlying maths but a 12mm2 cable is pretty hefty and I'd not expect that large a voltage drop. HOWEVER its quite possible that there's a poor connection at one or another of its ends, or anything in between (such as an isolation switch, distribution panel, etc etc). There's voltage drop calculators on the internet but I don't have a favourite one and Google to find a different one each time I search for one.

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There's voltage drop calculators on the internet but I don't have a favourite one and Google to find a different one each time I search for one.

 

I use this formula, simply because it works smile.png

 

The size of cable required can be calculated with the following simple formula:

cablesize = 18 x metres x amps/(V*1000)

 

To go the other way, and find the voltage drop for a specific cable size and current draw you end up with

V = (18 x metres x amps)/(Cable Size x 1000)

 

V = Voltage Drop

metres - Distance there and back

Cable size is in mm2

 

So, for a 4A draw, 5 metres from the batteries, with 4mm2 cable you end up with (18 x 10 x 4)/4000 = 0.18V.

 

If nothing else, this shows that cable size is unlikely to be OP's problem.

 

Tony

 

edited to make the equation simpler

Edited by WotEver
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Paul C, on 20 Apr 2016 - 5:42 PM, said:

 

The OP reported voltages of 12.9V at the battery; and 11.9V at the fridge.

 

 

Mike the Boilerman, on 20 Apr 2016 - 6:13 PM, said:

 

 

If the batteries are flat the voltage dropped will still be proportional, i.e. WAY too much!

 

 

Whoops my bad...

'pologies

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Of course, it's possible (in fact probable) that those two voltages were measured at different times. If the fridge had been off when reading the 12.9v battery figure, then on during reading the 11.9v at the fridges terminals, then it would go a long way to explaining the issue here. I took it on trust that the OP ensured the fridge was on during both measurements, thus ensuring the readings are meaningful and showing that voltage drop.

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  • 1 year later...

Just noticed this question. A year or so old and I hope you fixed it.

My fridge packed up the other day. I was a faulty on/off switch. This thread is a timely reminder to fix it, but in terms of cable size my fridge works well on 2.5mm cable.

12mm cable is more than enough - I would be inclined to look at all the joints for the whole circuit (switches/breakers/fuses/connectors) it only need one to become 'loose' or corroded, I'd pick the battery terminals for starters. Take them off, clean them, put it back and tighten.

 

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I am going to take issue with the above and Wotever's conclusion for a 4 amp draw.

The problem with electric compressor fridges is the exceptionally high starting current surge, not the running current. The compressor manufacturer for very good reason states you need 1 sq mm of cable CCSA for each metre between battery and fridge. Even a professional harness maker seems to have got it wrong in at least once case I know about and also the volume builder from Scousland.

If the run from battery to fridge is 18 metres or less (and it probably is on most narrowboats) then The above is correct but if its longer then its not.

 

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2 hours ago, Tony Brooks said:

I am going to take issue with the above and Wotever's conclusion for a 4 amp draw.

The problem with electric compressor fridges is the exceptionally high starting current surge, not the running current. The compressor manufacturer for very good reason states you need 1 sq mm of cable CCSA for each metre between battery and fridge. Even a professional harness maker seems to have got it wrong in at least once case I know about and also the volume builder from Scousland.

If the run from battery to fridge is 18 metres or less (and it probably is on most narrowboats) then The above is correct but if its longer then its not.

 

Agreed on all points, but not relevant, I don't believe, for the OP.

OP posted the question over a year ago and never returned to offer any feedback. She was reporting a >1V voltage drop down 12mm2 cable. My post was intended simply to demonstrate what others had posted, namely that the cable size wasn't the culprit; she must have had a poor connection somewhere. 

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Agreed, but just with 12V compressor fridges I wanted to make sure less technical readers understood they should not determinate cable size by calculating voltdrop from the running current.  I was far more concerned about what looked like an unqualified statement that 12 sq mm CCSA would be more than enough when the OP did not seem to provide a cable run length. (That is the statement was not qualified by run length).

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1 hour ago, Tony Brooks said:

I was far more concerned about what looked like an unqualified statement that 12 sq mm CCSA would be more than enough when the OP did not seem to provide a cable run length. (That is the statement was not qualified by run length).

Yes, agreed, it was a foolish comment. It is completely impossible to say that "cable size x" will be fine without simultaneously giving the distance. 

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Danfoss - the makers of most 12V fridge compressors (there is one other newer entrant that says the same)  states 1 sq mm CCSA for each metre between battery and fridge. You then use the same size for the return.  This equates to 0.5sq mm CCSA for each metre of the total run that you are talking about.

It is interesting that Waeco do not conform and also that it seems their chart requires cables nearly three times as large as for Danfoss (I jhave just checked the Text in my Waeco manual) but they seem to run ok on Danfoss speced cables. I really can not see even a Chinese rip off compressors and control boxes are that much different consumption and surge wise to the Danfoss ones, especially as I get the impression the Waeco  are more efficient on a day to day basis. I suspect a manual translation cockup.

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17 minutes ago, Tony Brooks said:

I get the impression the Waeco  are more efficient on a day to day basis. I suspect a manual translation cockup.

I have been reading my manual  'slowly' and trying to look for any hidden clues - maybe Waeco have already taken the return length into account (similar to some of the internet based volt-drop calculators), so the length they quote can be taken as the battery to fridge length.

Hence their 2x (+) size recommendation when compared to others.

My fridge is about 8 feet away from the batteries (Bus-Bar) and I used 8.5mm2 and now have no problems.

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It seems odd to me that manufacturers wouldn't make it crystal clear in the instructions whether the cable lengths are out and back. These fridges are only likely to be used in two situations; metal-bodied vehicles where the body acts as the return conductor, and boats/caravans where a return cable is needed. If the bodywork is used the return length can be ignored.

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All those I have seen do. Even the Waeco make sit clear its the total run length in their case but their chart indicates their cable size needs to be over twice as thick (CCSA) as other similar makes. If you stick with the instructions there will be no problem but as I said something seems a bit odd with the Waeko size.

So much of the internal structures of caravans are made of non-conductive materials I do not think you can ignore the return length. I suspect the are wired like boats with a domestic negative bus bar but the return length is probably shorter.

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15 minutes ago, Tony Brooks said:

So much of the internal structures of caravans are made of non-conductive materials I do not think you can ignore the return length. I suspect the are wired like boats with a domestic negative bus bar but the return length is probably shorter.

Erm. I did group caravans and boats together as needing the return cable.

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