12v fridge question

[sthurley]

Hello can anyone guide me? I have installed a 240v to 12v 40a converter so that I can run the 12v boat circuits from shore power and isolate the batteries. Everything except the fridge works great. Whilst the supply is more than capable of delivering the fridge requirements, it simply does not run, however when I switch to the batteries it works away merrily? Can it sense the available power from the batteries before it draws? Where as with the converter there is no 'power bank' as such so can the fridge be tricked? Stumped!!

 

[David2452]

A fridge compressor requires an initial high current draw for a very short time which may be the issue, people sometimes experience similar problems by using over light cable, the cross section required is far higher than the advertised amperage draw suggests, in fairness manufacturers cover the issue in the fitting instructions.

 

[sthurley]

I used 2.5mm cable from the converter to a change over switch which then feeds the main circuits/switchboard, same cable thickness as originally installed. The cable run is Max half metre. The start up draw is approx 4A. The output voltage was set at 12.55v however I have increased this to 13.9, basically the same as the reading when the engine is on. The invertor is well over sized and can supply 40a at 12v!

 

[superheat6k]

Have you tried running the inverter in parallel with the batteries. The inverter will maintain the batteries and the batteries will run the fridge.

I would suspect the inductive load of the compressor motor is why the inverter, which I expect is using substantial capacitors the electrical opposite of an inductor, cannot start the compressor.

Might be worth trying disconnecting the batteries with the compressor running and see if the inverter can maintain the compressor running.

40a might be the peak load rather than the continuous, and a fridge motor could require a considerably higher start surge, even if you only measure 4 amps, the spike could be far higher.

 

[LadyInBed]

The inverter isn't outputting some sort of chopped signal is it?You might try a very large capacitor across the inverter o/p, to act as a resivour like a battery.

 

[nigelmercier]

Is this going off at a tangent? I see no inverter mentioned.

 

[abraxus]

I used 2.5mm cable from the converter to a change over switch which then feeds the main circuits/switchboard, same cable thickness as originally installed. The cable run is Max half metre. The start up draw is approx 4A. The output voltage was set at 12.55v however I have increased this to 13.9, basically the same as the reading when the engine is on. The invertor is well over sized and can supply 40a at 12v!

I'd be surprised if the start up draw was 4 amps. The running consumption of most 12v fridges is around 4 amps, and start up draw can be as much as 10 times that.

Also, have you checked the voltage at the fridge when running off the converter? Some fridges have a cut out voltage to save excessive battery drain, and so if the voltage is too low then it might simply be a case of replacing the 2.5mm cable with something bigger, which is no big job for just a half a metre run.

 

[William_H]

My little Waeco CF25 compressor fridge has a sensing circuit in it which decides if it is being fed with 12v battery or 24v battery. It also has sensing circuits to cut the fridge out if the battery voltage is too low. This means that there is a window of acceptable input voltage at nominally 12v. Aprox 11.5 min voltage and a max of about 14v before it decides it is actually a flat 24v battery. Now I have not checked these voltages accurately though. I did try to build a simple 240v power supply for it. Using crude transformer rectifier the voltage on no load necessary to maintain about 12v under load was so high as to make it think it was a flat 24v battery. As a matter of interest I adjusted the tap on the transformer to give about 13v no load and used a reed switch to sense current draw to switch to a higher voltage tap under load. (I wanted to avoid a linear regulator with heat losses and did not want to tackle a switch mode regulator).
Any way for the OP I suggest he check voltages at the fridge when on the mains supply. If your fridge is like mine they may be too far from 12.5 volts desirable. The simple answer being to connect batteries in parallel to stabilise the voltage when on mains power. good luck olewill

 

[geem]

Is your converter providing DC or AC? It won't run on AC.

 

[Mrnotming]

now we are splitting hairs!

 

[Dougal]

Be interesting to hear the outcome.

 

[LadyInBed]

Is this going off at a tangent? I see no inverter mentioned.

Excuse my French! Should have said CONVERTER. :sleeping:
PS I'm surprised you didn't pick me up on the spelling of reservoir . . . V

 

[nigelmercier]

Excuse my French! Should have said CONVERTER. :sleeping:
PS I'm surprised you didn't pick me up on the spelling of reservoir . . . V

I wasn't picking you up, just steering the thread back to being relevant. Or trying to ☺