Power to rear of 6.4 pvc (1 Viewer)

[Durham's dad]

Hello all
I've got a Dometic cf36 12 volt chest fridge/freezer which is really good and useful. It tucks under my single bed in the locker at the rear of the van. For most of the time it's worked fine, though thinking back we've had a mains hook up at many of the sites we've visited when it's been in use. However, we've recently been off grid and it's not been firing up. I know it'll cut out when the battery gets to a certain level. It's set to do that at about 10.3 volts but we've got 200 watts of solar and 220 amp hours of battery and I've not seen a voltage below 12.5 in all the time we've been away.
On returning home I've checked the voltage at the freezer plug and it has been about the same, perhaps .1 of a volt less than at the front of the van where the batteries are, reading 12.7 today.
I managed to contact Dometic via e mail and they suggested a power issue (stilted conversation really, not the same as speaking directly but anyway) so I took the freezer out and wired it, using it's lead alone, directly to my car battery and, Yes it works!
So my question is, why won't it work at the back of the van when there appears to be more than adequate voltage? Does amperage drop regardless of voltage over a distance?
It's a teaser!!
Thanks in anticipation...

 

[gus-lopez]

Could be the drop over that distance when it attempts to start?

 

[cyberyacht]

Beef up the wiring to the supply at the back of the van. Thin cabling will result in voltage drop over a longer run.

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[Guigsy]

Beef up the wiring to the supply at the back of the van. Thin cabling will result in voltage drop over a longer run.

This is correct. I'm not entirely sure, but I think you also need to measure the voltage under load. The wire is effectively a resistor, so the voltage drop is proportional to the current being pulled. So when the fridge is off, the voltage might look pretty healthy. But when the fridge kicks in, the resistance in the wiring becomes significant enough to drop a couple of volts.

 

[berni109]

get a length of 20amp cable equivalent to length you would need to replace the currently installed wire and temporarily connect to see if it will run your device.

 

[Guigsy]

get a length of 20amp cable equivalent to length you would need to replace the currently installed wire and temporarily connect to see if it will run your device.

Or, for testing purposes, move the fridge to nearer the battery so the cable length is short to see if it works?

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[autorouter]

So my question is, why won't it work at the back of the van when there appears to be more than adequate voltage? Does amperage drop regardless of voltage over a distance?

Actually it's the other way round. The amperage (flow of electric current) is absolutely constant all the way round the circuit, on the positive feed and negative return. If there's any resistance in the wires, the voltage drops along the wire so that the voltage available at the actual freezer connection is reduced. The more amps, the more voltage drop. At zero amps (fridge thermostat switching off for example) there's zero voltage drop.

There is always some resistance in the wire, so the idea is to choose a wire thickness so that the voltage drop is small - below 3%, or even below 1%. But the voltage drop depends on the amps, so you have to measure the volts when the fridge is drawing its full amps value.

 

[Lenny HB]

Just looked at the spec it draws 5 amps but start up current will be a fair bit higher, if your cable run is 5m you will need at least 10mm sq cable.
If you need to buy cable be aware thin walled cables are the size is often quoted including the insulation.

Calculator here, when I checked I used 8 amps to allow for start up current.

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