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dbowring
dbowring

Posted: Tue Jun 08, 2010 9:57am

Post Subject: Low voltage on 12v fridge

Hi I am having a problem with a relatively new Shoreline 12v fridge on our narrowboat. The domestic circuit is powered by 2 110amph batteries (again, fairly new). The batteries are monitored using a Stirling Power management panel. The fridge works fine whilst we are cruising - nice & cold & plenty of ice for the gin & tonic! After a day's cruising (usually about 4 or 5 hours these days) the management panel will show the batteries at about 13v or more. This fairly quickly falls back to 12.6v which I understand is the fully charged reading. However, at this point the fridge's low voltage warning light will start to flash. After a few hours the temperature of the fridge will have dropped & the ice in the ice compartment started to melt. The fridge has a dedeicated feed from the distribution point & we are fairly economical in our use of power with all lights being low voltage LED lights. In the morning the management panel reads around 12.4v and shows we have used about 27amp hours of electricity. There is definately no ice in the fridge! Any help will be much appreciated.

Tony-B
Tony-B

Posted: Tue Jun 08, 2010 10:36am

Post Subject: Low voltage on 12v fridge

First of all, just to avoid perpetuating something that is not quiet correct, the fully charged volatge after the battery has been stood with no load on it for several hours is 12.7 to 12.8 volts. 12.4 volts show that the batteries are still over 50% charged (we say that discharged is 12.3 ish but that is because to optimise life typical boat batteries should not be discharged to below 50%). Now the $64,000 question - who wired the fridge or provided the wiring? I think its another case of people thinking they know better than the fridge and component manufacturers. The flashing light (in groups of 1 flash) mean the fridge electronics have detected a low volatge AT THE FRIDGE. Despite what the manual may say it has nothing to do with battery voltage save that a low battery would give a low voltage at the fridge. If the flashing is in groups of more than one please come back and tell me how many flashes in a group. Measure the distance of cable run from the batteries to the fridge and this gives the cross sectional area of the conductor in the cable you need. My fridge is 13m from the batteries so I used 13 sq mm cable for the POSITIVE and NEGATIVE cables. This looks like, and is, car starter cable. I bet yours is much thinner and is thus causing voltdrop every time the fridge tries to start. The cable between the battery and master switch should be large enough to carry all your domestic loads PLUS the size calculated above. The fridge wiring should run from master switch, via a fuse to the fridge and back to the battery. It is not ideal to run it via a switch/fuse panel unless the supply cable to the panel is suitably upgraded. All the above is in the fridge instructions. However 27 amp hours use overnight with LED bulbs and a fridge that has turned itself off seems a bit high to me so are you using something you have not mentioned? It should not affect the fridge though. By the way any amps in - amps out counter type battery monitor will gradually build up the size of the lies it tells unless you regularly reset it. Tony Brooks

dbowring
dbowring

Posted: Tue Jun 08, 2010 10:50am

Post Subject: Low voltage on 12v fridge

Thanks Tony. I think the problem lies with the size of cable. The wiring is as it was when we bought the boat second hand - the cable to the fridge is nowhere near the size you suggest (the fridge is about 10m from the battery so I guess a 10mm cable will be required). Our extra use of power is probably accounted for by a couple of hours watching a digital flat screen TV!

Tony-B
Tony-B

Posted: Tue Jun 08, 2010 12:01pm

Post Subject: Low voltage on 12v fridge

All the 12V fridges with a Danfoss compressor have a way of altering the voltage at which the fridge refuses to start and flashes the light. It involves a resistor connected between terminals C and P on the unit behind the fridge. If the present resistor is replaced by a wire link the unit will not protest until the voltage at the back drops to 9.6 volts on start up and then cut back in again at 10.9 volts. The standard settings are 10.4 and 11.7 volts respectively. There are other settings available by varying the value of the resistor. Often playing about with the resistor value can appear to cure problems such as you are suffering but at the expense of potentially allowing the fridge to flatten the batteries. Full details are in the Danfoss Direct Current Compressors R134a & R600a data sheets which are on the Danfoss website for download. Alternatively Shoreline are normally very happy to advise. However the proper cure is the correct cables. Tony Brooks

dbowring
dbowring

Posted: Wed Jun 09, 2010 4:16pm

Post Subject: Low voltage on 12v fridge

Hi Tony. I am in the process of taking your advice & re-wiring the supply to the fridge. I favour your recommendation of coming from the master isolation switch to the fridge via a fuse & back to the battery. However I am having difficulty in sourcing a suitable fuse holder to take a 15amp fuse & be fitted in a 10mm2 cable! Shoreline have no suggestions. Can you suggest a supplier or solution? Thanks

Tony-B
Tony-B

Posted: Wed Jun 09, 2010 4:59pm

Post Subject: Low voltage on 12v fridge

The whole problem is voltdrop but if an imaginary cable run is zero distance the voltdrop will also be zero. This means that if you get a typical terminal and then cut away a number of conductor strands to fit the distance with the reduced CSA will be so small as to make no difference. If you are using coloured crimp terminals use the yellow ones and thin the conductors in two steps. the smallest to fit into the normal conductor crimp and then with less removed to fit the insulator crimp. If you get quality terminals with a metal sleeve around the insulator crimp so much the better. Then I would use a length of adhesive heat shrink sleeving to further strengthen and insulate the connection. This is all bad practise but its the best you can do. wWw.vehicleproducts.co.uk list several single blade type fuel holders which would do the job but unless you want to pay £5 postage on a £1 item you will need to get the holder locally. FWIW. I used a twin pole, mains 40 amp cooker type switch with the cable split and fed to both contacts for my fridge to minimise voltdrop across the switch. Tony Brooks


Posted: Wed Jun 09, 2010 6:41pm

Post Subject: Low voltage on 12v fridge

Thanks Tony. That all seems like good advice - I'll let you know how I get on.

dbowring
dbowring

Posted: Thu Jun 10, 2010 10:24am

Post Subject: Low voltage on 12v fridge

Hi Tony. Just one more question (I hope). Does it make any difference where the switch is positioned? In the manual it says "close to the appliance". My switch is currently located in the distribution cupboard at the rear of the boat & it would be most convenient to leave it there. Will that affect the volt drop? I will be upgrading the switch as you suggest - did you split and connect both the positive & negative cables or just the positive? I think all switches of this amp rating are double pole allowing for connection of the + & - for supply and load.

Tony-B
Tony-B

Posted: Thu Jun 10, 2010 10:45am

Post Subject: Low voltage on 12v fridge

I think the idea is that a big switch near the fridge is likely to be more visible than one elsewhere so is less likely to be left turned on accidentally. It also avoids people wiring the fridge from one of the silly little circuit breaker/switches on the main board which probably has an inadequate sized feed cable. I split the positive only, stripping back the lot into two bundles of conductors then heat shrunk some insulation over the two legs. My idea was that I wanted a big red switch that clearly showed ON on the switch toggle itself when turned on and as you should not really use AC switches on DC and because I know voltdrop was an issue I used both poles for the positive. This doubles the contact surface area and hopefully extends the life of the switch (9 years so far). Tony Brooks

dbowring
dbowring

Posted: Thu Jun 10, 2010 11:13am

Post Subject: Low voltage on 12v fridge

Thanks Tony

dbowring
dbowring

Posted: Tue Jun 15, 2010 10:39am

Post Subject: Low voltage on 12v fridge

Hi Tony. It will take a weekend cruise to prove it out properly but having rewired according to your suggestion the compressor is buzzing away happily with a battery reading of 12.5v. Many thanks for your help

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