Rewiring 12 volt 3 way fridge in campervan

Hi Ross M,
Thanks for the reply!
The fridge is currently powered up from the car battery, is only powered when ignition is on and if a switch on the dash is on. It is a long wheel base van so I'd need probably 6 metres of 6mm b&s neg and pos cable each. The battery for the car is a very cheap starting battery, but does the job of starting the car fine.
The aux battery sits a metre from the fridge at the back, and is a large commercial 960cca battery (not deep cycle) as it needs to be maintained from my alternator.

1. Should I power the fridge from the aux or car battery?

2. When trying to run the fridge while driving the other day, I noticed the car battery volt meter slowly dropping from a charge of approx 14 volts down to 12 volts, is that normal?

Thanks for your input!

If the engine is running and the fridge is on AND the car battery voltage plummets down near 12v, I would think the alternator some a blown diode in its rectifier or the alternator regulator is not making the alt make full charge output.

If the car battery voltage does/can reach 14 or so in normal operation then it is most likely the a diode in the rectifier plates.

When running, the alt should easily be able to hold 13.8-14v with a 10 amp load on it.

Ross M

Hi Teds
[1]You should supply it the CAR battery
The current set up of ignition control is OK
DO NOT feed fridge from the AUX battery ,you will affect its rate of charge AND final state of charge[SOC]
But you do need heavy cable, 6b&s mininum, [that has a COPPER core dia of 4.11 mm]
If all is correct, with engine running @ fast idle you should have in excess of 12.6v@ the fridge when the fridge is operating, The Fridge 12v element in that fridge IS the same wattage as the 240v element [AS are most Dometics]
It will work just as effectively as the 240v ,IF it recieves the CORRECT voltage
The Majority do not work as effectively on 12v due to inadequate wiring
{2]NO, that is not normal
The al;ternator should be able to maintain the voltage around 13 plus volts
Perhaps a faulty regulator ,alternator
OR bad battery connections or battery
First step, take them off & clean
A direct check of voltage @ the battery may reveal where the problem lies
Peter

Hi Teds
Edit [1]You should supply it from the CAR battery
The present set up of ignition control is OK, if via a relay, which I would expect it is.
How is the Aux bat seperated from the car battery???

Peter

Hi Ross and Peter,

I'll try and run 6b&s cable to the fridge next and then have the alternator looked at. It slowly drops down to just above 12volts when driving with the fridge on.

It looks like the cable becomes quite thin at the switch on the dash. If I have no or little voltage drop at the switch, then I will try and run the new cable from the switch in the dash, would that be ok? Just seems easier then trying to trace the cable behind the dash and to the car battery behind/under the drivers seat.

Peter: The Aux battery is separated via some sort of relay/isolator. It seems to do the job.

What is the best way to connect the large 6b&s cable to the tiny wiring/terminals at the fridge?

Thanks!

Hi Teds
REQuote
[1] looks like the cable becomes quite thin at the switch on the dash. If I have no or little voltage drop at the switch, then I will try and run the new cable from the switch in the dash, would that be ok? Just seems easier then trying to trace the cable behind the dash and to the car battery behind/under the drivers seat.

[2] The Aux battery is separated via some sort of relay/isolator. It seems to do the job.

[3]What is the best way to connect the large 6b&s cable to the tiny wiring/terminals at the fridge? End quoye]


[1]I would expect that tiny wire is supplying a relay coil for the Fridge
If so, THAT is Ok ,but you need to find the relay & run the 6B&S Positve from the battery, into one side of the relay contacts & the other side out to the fridge
[2]Good
[3]It is Ok to reduce to a smaller size to fit terminals
Either solder some smaller wire on ,keeping as short as possible or cut back some strands of the 6B&S cable

One point, if not already fused with a 10<15amp fuse near the battery you should do so in the pos of the 6B&S
You will need a good heavy duty fuse holder

Peter

Great, thanks for the info. Should be enough to get me started!

I'm pretty sure there is a fuse for the fridge in the fuse box... I'm pretty sure all the wiring and the fuse box has been modified earlier at some stage. Will check though.

Will let you know how I get on.
Cheers!

Hi,
Just a quick update, I tested the voltage at the fridge terminals, with the car at idle I got a reading of 12.4v and approx 12.8v with some revs.

Is that adequate power to the fridge?

If so, thinking the 12v element may need replacing?
Thanks,
Peter

Hi Teds
If that is with the fridge switched on & running, then That shpuld be Ok it must have fairly heavy wires
Check again, Then,try turning the FRIDGE "'OFF' AT THE FRDGE & see what the voltage rises to
If the voltage does not rise to the same as the battery crank voltage something is wrong
Lets Know what you get
You should come back with 3 voltages
[1] At Fridge,engine @ fast idle, fridge switched on & RUNNING
[2] AT fridge,Engine still running & fridge switched "off" at fridge
[3 At crank battery,]Engine still running ,fridge still 'off @ the fridge ,'Crank battery voltage


Peter
Do them as close together as possible[time wise]

Hi Teds

One other test while you have the fridge on & doing test [1]
If you can get to the 12V element check the voltage @ it's power in [the centre wire in the sheath & the sheath itself
This will help to show if there is any problem with the thermostat or the selector switch.
Peter

MAKE SURE ALL 240V IS DISCONNECTED

Hi Peter,
Thanks again for your help!

1. At Fridge, fast idle, fridge on = 9.7 volts
2. At fridge, Engine running, fridge off = 9.45 volts
3. At crank battery, engine running, fridge off = (idle)13.98 volts (fast idle)14.07 volts.

I was unable to test the thermostat or selector switch at this point.

Looks like the cabling is too small!

It looks like the fridge is piggy backed off of the cigarette lighter, I do not think there is an individual relay.

Would I wire up the fridge in the following way:

1. Using 6mm b&s connect the positive to the cranking battery to a 15 amp inline fuse.
2. From the fuse connect to a relay.... need instructions on how to wire up a relay!
3. from relay to positive terminal on fridge.
4. connect the negative from the battery terminal to the relay and then to the negative on the fridge......

Will that work?

I will not bother with the switch in the dash, will probably use it for something else, maybe to power some fans for circulation around the fridge or something?

Thanks again!

Just read up on wiring relays, I think I'll tap into the ignition circuit to power the relay... is that the way to go, so the fridge can only obtain 12v power when the car is running/ignition is on.

How does that sound?

I'm not sure if I'll need a switch after the relay to control power to the fridge either....

Hi Teds
No wonder it does not work

Re "
2. At fridge, Engine running, fridge off = 9.45 volts
3. At crank battery, engine running, fridge off = (idle)13.98 volts (fast idle)14.07 volts


2]something is seriously wrong if you only get 9.45v with the fridge 'off"
& engine running
it should be virtually the same as the crank battery
Not just wiring size, something else must be on the same circuit
Perhaps .That will be the aux battery which I suspect is cactus/finished
Can you check the aux battry voltage ?
Engine running & engine not running voltage", no load on aux battery in both cases
Do you have a charger? try recharging the AUX ,if voltage is low

[3] Is OK, alternator does not seem to be a problem


Wiring the relay :-
Take that line you already have from ignition switch to one side of the relay coil[only needs light cable ]
from the other end of the coil, take a small negitive to earth
Take a heavy positive wire from battery pos to fuse [heavy duty 20amp blade type] to one side of the relay contacts
Take another heavy wire from the other side of the relay contacts to the fridge pos via whatever plugs are needed
The relay needs to be heavy duty continuous rated such as a horn relay [jaycar part #Sy-4068
Peter
The relay should have the terminals marked

Peter

Hi Peter,

Thanks for the reply.

I noticed that the fridge is piggy backed off the cigarette lighter socket! That may have also added to the problem.

I have ordered some 6b&s cabling and will wire it how you have outlined.

Will let you know how it goes!

Thanks

I have wired up the fridge with 6b&s cable, this has fixed the voltage drop.

At the battery with a fast idle: 14.10 volts
At the fridge with a fast idle, fridge on: 13.7 volts.

A big improvement in regards to less voltage drop, however went for a quick drive with the fridge on to see if it will cool. After about five or so minutes, the voltmeter for the cranking battery slowly drops down to 12 volts. When I switch the fridge off it slowly climbs back up to 14 or so volts.

Is this normal? Ross had mentioned that the alternator may have faulty diodes? Will take it to an auto elec tomorrow and have them test it.

Cheers

Hi
Sorry for the late follow up
You may have your problem sorted
But I would suspect you have two problems

A stuffed Aux battery& possibly crank from being pulled down to >,10v volts Also your isolator is faulty or wired wrong, it should not allow the crank to be pulled down below aroungd 12.4<12.6 v
Peter

Hi Peter,
You weren't late at all, only just posted an update less than an hour ago. What makes you think the aux battery is stuffed?
The fridge is powered off the crank battery, I'm pretty sure the aux battery has been charging fine and has not been drained much.
Not sure why the cranking battery starts to drain when the fridge is switched on the car is running.

Hi Barry,

Having a browse now, thanks for the link!