Powering 3way Dometic via 12v from Tow Vehicle

Thanks RMD, I am not sure I am going to run the Setec as well, I might just isolate and cap those cables and just run it through to the Fridge as it is my main priority.

I am running the 12v extractor fans and will do the circulating fan mod shortly as well.

Thanks again for taking the time to comment.

If the 12v is set up correctly they will work fine on 12v.

Things like wire size, connectors etc will cause voltage drops in the supply. I only use 50A Anderson plugs for the connection from car to van and the largest cable you can find that will fit the Anderson plug connectors.

I have driven for 12 to 14 hours in a day using 12v and still had a very cold fridge at the end of the day.

My Dometic runs beautifully on 12V from the car using 6B&S via a 50A Anderson plug; I carry ice cream in the freezer part with no problem!!!!! I do have Karls Kool RV extractor fans on the top vent, plus a circulating fan inside the fridge.
To isolate the fridge from the car battery while stationary, there are several ways. One is a relay switched via the ignition as mentioned above. A similar thing is achieved by a VSR. The last alternative is to fit a Fridgeswitch in the van on the 12V wiring to the fridge. This isolates the fridge unless the van is moving.

Tonysmc
I am one who disagrees. Only after making sure the electrical supply is A1+ can you assume the travel action retarded cooling action of fridge. ie, 13v or so at the fridge inputs and knowing through testing the current draw is as designed can you blame travel. Mine draws 15 amps. If wiring crap then cooling crap!
You can't assume. Mine was poor before it received attention. If you make sure it will work then it does!
After altering, I suppose it is called upgrading, wiring and supply issues, my fridge went from around 6C or 7C at beginning of trip down to 2C after a 500km run. Certainly not a placebo effect at all. It did same on return journey. It all depends on the energy you ACTUALLY have present AT the fridge terminals. No good measuring anywhere else because it is the fridge you want to have working. Often people fit really good gear but the actual wiring to the fridge acts as a resistor and negates the good work, effort and money for the supply. I haven't seen any OE van wiring suitable to run a 3 way fridge effectively. Mostly too small a cable or Looooong unnecessary runs of small cable.

Thanks for the comments all. I am running the 2 extractor fans on the top fridge vents, while not the KeverCool solution, it was a DIY with identical outcome. It is switched and thermostat controlled. Makes a massive difference to temps and the Bench top heat as well :) I will look at doing the interior circulation fan mod as well at some time.

Looks like I will go the approach of 40 amp midi fuse --> 40 amp 12v switched relay (switched by 12v accessory feed with fuse piggyback) - 6B&S cable run to 50A Weatherproof Anderson socket.

Does anyone know if using the relay with cause issues with voltage drop due to the type of connectors they use?

Marcus
Some relays just have spade connections which may be ok but most have small contacts inside. I haven't load tested any to read the voltage drop across them. Depending on budget, you could use two normal relays in parallel so the running load is shared. There are other larger duty constant duty relays ie used for dual battery systems etc. they can be switched as required. I think some VSR units have the ability to be manually switched ON as well as acting automatically.

Regarding fans. Two fans at top is ok and do extract heat as you mentioned, but some of the air they expel is simply drawn up past the fridge and not necessarily heated air from the condenser fins. If one fan is aimed AT and airflow is across the condenser fins it is blasting cool air against the items you want heat to be taken from and the top fan simply takes it all away. Both ideas work but I found the two placed strategically make a significant difference.
The insulating of sides and top is also a major benefit. ie, no airgaps between cabinet and fridge so heated air is excluded from those panels to stop heat ingress. You just got rid of the heat, no good having it short cut to the inside again. Caravan makers seem to not care about it!

That is the logical way to go but do yourself a favour, use a relay or continious duty solenoid with at least 6mm or 8mm stud type terminals for the secondary circuit. 1/4" spade terminals wont cut it.
And dont forget to make sure the negative cable size as well if not already mentioned.

Marcus
A PS. If you add a selector switch to just the fans 12v supply they can be used while on gas, 2van battery 40v or 12v travel. Just switches from can input to to make then operate. Fairly easy to do and suits, on any site use, as well as travelling.

Thanks RMD and qldcamper.

I think the Narva HD Relay Normally Open 12V 4 Pin 200A should do the job, screw terminals that I can run crimped ring terminals on.

Spot on, although Narva is one of my least faverite brands for no other reason than when they first hit our market their product range was just very cheap knock offs of reputable brands like Hella but Apparently their quality has improved over the years....so say their reps.
Cant go far wrong with a simple relay can you?

Marcus,
I agree with RMD regarding getting the airflow over the fins. Our current Dometic in the old Jayco Freedom has the fins almost hard up against the outer skin of the van and the top fans work well in drawing air over the fins. Our old Jayco Penguin had about 65mm between the fins and the outer skin; in this instance, I fitted a horizontal deflector plate (corflute and duct tape) to close this gap and funnel all the air across the fins - worked really well.

Same here.
At 12.8 v most VSR's disconnect and keep battery healthy, above 13,v the VSR starts and runs the fridge. I use an LED indicator to check it starts and stops as it should. Just so I can do something if anything malfunctions.

Thanks for the comment William!

I am not sure that a VSR will work for me with the smart alternator. Are you able to tell me what VSR you are running?

I fitted a Projecta 140 amp.
I have had other brands before.
The beauty about a VSR it is automatic.
I can't comment on the smart alternator sorry.
William

William B and others.

VSR's, or Voltage Sensitive Relays will NOT work with 'Smart' alternator, but will work with most LOW voltage alternators that are often used on the Toyota range of vehicles.

Smart alternators will often run as low as 12 volts, and sometimes even less, when the vehicle engine is under load (and when towing is not that most of the time), and then go above 15v when throttle is closed. In my Isuzu it only takes a minute amount of throttle and you can watch the alternator voltage dropping. I did fit a VSR with a light on the output side and took my vehicle for a drive in some hilly country, the light was out for far longer than it was illuminated.

As three way fridges rely on a heater ellement for their operation, then to achieve good results, that heater must recieve a CONSTANT good supply and kept hot, anything less and they are nothing more than an ornament.

Regards
Athol

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Hi Athol,

Voltage-control (aka 'smart') alternators do present problems when supplying auxiliary batteries and loads such as 3-way fridges.
VSR's (voltage-sensing relays and solenoids) will fail to operate when needed but isolator relays controlled from an 'ignition' source will provide a connection when the engine is running. However there is still a problem with obtaining sufficient voltage to charge the aux battery or supply 3-way fridges.

This problem, and a number or other problems, can be overcome by employing DC-DC chargers that incorporate an ignition-derived control function. These are designed to operate typically as low as 9v input and are active whenever the vehicle ignition is on. No modification or re-programming of the vehicle electrics is required. They have a significant advantage in overcoming volt-drop in cabling from vehicle to van or trailer and can incorporate solar input. The alternator will continue to behave as intended and the vehicle battery acts as a buffer to maintain supply at all times.

To enjoy their benefits the DC-DC charger must be located in the trailer close to the aux battery, Accordingly an ignition source must be available to the trailer. It may be thought that this will require an additional dedicated wire from the engine bay to the charger in the trailer but this can be avoided by connecting the heavy-duty feed cable via an ignition-controlled solenoid in the engine bay. This means that the feed is only available when the engine is running and the trigger wire of the charger is now connected to that wire and hence senses the 'ignition' function.

The 3-way fridge can be connected directly to the cable from the vehicle but would be happier if connected to the aux battery as it will be supplied with full voltage at all times. It should be connected via a relay whose coil is supplied from the incoming cable from the vehicle such that the fridge ceases to draw current from the aux battery when the engine is off..... unless of course if you consider, at your peril, that the aux battery is capable of supporting the 15A fridge load!

I have made a couple of installations with this arrangement successfully.

Allan.
I agree with most of what you have said, however by running the fridge from the van battery when using a BCDC charging system you are effectivly REDUCING the available charge to the battery, as the fridge is taking its requiements regardless. (ie a 25 amp BCDC is only delivering around 7 amps charge when the fridge is asking for 18amps).

It is far better to have a dedicated supply of 6B&S (depending on charger used that may even be 4B&S) cables for the charging system, and a SWITCHED supply of 6B&S for the fridge, and each of these come directly from the starting battery via 2 core cables (return to battery via the shunt that is common with Smart alternators, not via the vehicle body/chassis).

I have tried an installation as you are promoting whereby the fridge draws power from the van battery and found that the van battery did not recieve sufficient charge to fully recover from the previous night's use during a 5 hour drive day

Whatever sytem is used it is always a compromise, that is dependent on where you wish to allow for that compromise. There is no perfect machine yet made by man (we may be getting better, but still a long way from perfect)

Regards
Athol

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Athol,
As you say.... there is always a compromise.

Firstly, for the benefit of other readers, I am not proposing "running the fridge FROM the van battery" as Athol put it and I don't think he meant that. However some readers will grasp on that and bang-on about 'running the van battery flat'! The fridge is only running from the charger as Athol went on to deliberate on.

But Athol, I am very aware of the issue of robbing charger output in order to feed loads. However in this instance of an installation with a 3-way fridge I had assumed that there is not much other load on the aux battery so recharging should not be an issue, particularly if using a 40n or 50 Amp charger. If there is heavy consumption on that battery then that is a different consideration.
Certainly, a second supply line dedicated to the fridge has some advantage but at the cost and inconvenience of providing a second well-sized cable and plugs. Furthermore, if the vehicle has a voltage-controlled (Smart) alternator, then this second line may well be fed by a very reduced alternator and something well below 12v may be arriving at the van. A second dedicated line is only satisfactory if its feed voltage is adequate, and it rarely is in my experience!

If there are concerns about sharing the charger with the aux battery and fridge then simply use a single 6B&S cable and connect it to two DC-DC 20A chargers, one for the fridge and one for the battery charging. Both of these consumers will then receive their specified inputs. As I have lectured elsewhere, the loss in Wattage to the fridge is the square of the voltage loss. So if you are down just 1 volt from the nominal 12v then the fridge will be operating with only 80% of its design power..... and these absorption fridges need every bit of help that they can get!

Allan
I totally agree that they need all that they can get, the only issue with using 2 BCDC chargers is that you will then require 2 batteries, as most, if not all, BCDC chargers must 'see' a battery voltage (usually above about 10.5 to 11) before they start to charge.

Regards
Athol

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Pinko, I'm not surprised that you had problems.......
Power losses are not simply proportional to the volt-drop. With resistive loads, such as the heater in the fridge, the shortfall Wattage is the square of the volt-drop.

With 8B&S cable and a 15A fridge load you would be losing 1.28v and only supplying your fridge with 10.72v from the nominal 12v car battery. This would be creating only 80% of the design wattage to the fridge.
I assume that the Intervolt charger you installed is charging a battery in your van to which the fridge is connected. Its terminal voltage will now be at about 14v so the fridge will be operating at 130% of the 12v design rating. It certainly should be performing well!

The downside is that the life of the fridge heater may be shortened.

You need to be careful doing this, many alternators use a remote sensing wire to sense the voltage at the battery terminal or fuse box buss, on such systems under heavy loads the voltage at the alternator can be considerable higher than the battery voltage. I read of one instance where the owner said the voltage measured on the alternator stud was 2 to 3 volts higher than the 14.4V volts at the battery which he thought was good as it gave him a better charge rate in the van.

LeighW
Is the sensor you speak of the current sensor often placed at chassis neg connection? If the Voltage at an Alternator terminal was 2 or 3 volts higher than the battery terminal then the +ve cable between alt and battery would have to be relatively thin to create such a degree of potential difference and be a resistor in reality!

No, the alternator has a voltage sensing wire that goes to either the battery or the fuse box so that the alternator can compensate for any drop there may be across the main power cable from the alternator. The model concerned was a Land cruiser that had the batteries located on the opposite side of the engine bay to the side of the engine the alternator was mounted on and there was along cable run between the two.

Say a 100A trying to charge a couple of under bonnet batteries, with the voltage measured from the chassis to the battery and a 2V drop we end up with a resistance of around .02 ohms. For a couple of metres of barely adequate cable including a fusible link plus some terminals and some earth straps and panel metal that's not all that bad.

I believe you will find on a lot of model cars with a reasonable load on the alternator quite large voltage drops are not unusual hence the remote voltage sensing. It would seem it is cheaper for the manufactures to design alternators with external voltage sensing and run a separate sense wire rather than use a decent bit of copper wire between the alternator and the battery.

Example of people that know what they are talking about and those that think they know prepaired to challenge those that actually know.
Those that thought they knew will be googling the shit out of it so they can come across as an expert next time it comes up.
I gave up long ago.
Leigh your quite right, before ECMs got involved almost all automotive alternators changed from machine sensing to battery sensing voltage regulators, since the VN commodore era when Bosch started fitting RE60 style regs to all Australian produced alternators and before that in many jap vehicles.
Now many alternators dont have voltage regulators but a field control module completely controlled by the ECM, some still run a voltage regulator only marginally controlled by the ECM.

Leigh W
I understand many alternators have a sense wire so actual battery voltage is "seen", but if the feed is taken from the alt terminal, then the load on the at to battery cable is small and the voltage "seen" at the battery terminal is and must be similar to the alt terminal because current between the two is low. Can't see how the alt will be significantly above the battery.
The field control module seems to have appeared so as to save fuel to a small degree.

Yes if all the accessories are connect directly to the alternator then you won't get the drop across the cable to the car drawn by those devices but keep in mind these days the cars electrics use a lot of power which is why 100A to 300A alternators or more are installed. When you start turning on the cars accessories and or the cars batteries are low then the current starts to skyrocket and you can get a significant drop across the cable between the alternator and the battery or fuse box which is why they use remote voltage sensing.

I'm just saying if you intend to connect to the alternator output stud I would be checking the voltage with all the cars accessories turned off and batteries charged then start turning on accessories etc and see how stable the voltage is on the stud.

Think your starting to over think it a bit now, after all we are only talking about getting 15 amps or so to a fridge.
By the way, what cars have been fitted with a 300 amp plus altetnator? I have only seen that sort of capacity on american trucks that still cling on to 12 volt systems and still only 200 amp. Everything else that I know of that need that sort of power are 24 volt and they are fitted with 2 alternators. Just going on what I have actually seen and there is a lot I havent seen so still learning, not saying it cant exist cause I havent seen it.
I think Niehoff has just recently released a 24 volt alternator close to 300 amps for heavy mining equipment with multiple AC systems and some pretty serious lighhting.

300amp alternator? Which modern cars have this?
What electrical gear in a vehicle demands that? Pixies in the garden!

Bad choice of words but there are plenty of 300A alternators available if you need one such as.

High output alternators

Models that use have 150A-200A to many to list

Some models that use over 200A or more alternators:

BMW
Chrysler
Jeep
Mercedes
Toyota

Well there is the answer Leigh, you have nailed it.
All the OP has to do is fit a 300 amp alternator with 4 runs of 0 B&S cable through a pair of 175 amp Anderson plugs to six 40 amp DC/DC chargers to six 100 AH lithium batteries, and 6 VSRs as not ro risk flattening the van batteries, that should keep the fridge cold but if he wants to run lights as well might have to change up to a 24 volt system.
Oh, better look at a performance chip as well as it will take over 10 horse power to drive that system.

I really don't know why you have got so narky, you suggested someone do something, I suggested it might not be a good idea depending on the vehicle involved to blindly do what someone is suggesting without doing some testing first.

Some quotes by others:

"At the alternator it was showing 14 volts plus, but at the batteries could barely muster 12 volts"

" I am getting 14.8 volts at the back of the alternator but when I measure at the battery I'm getting around 13.5 volts"

"Only just 13 v at battery 14+ at the alternator"

"I just got 13,4v on the starter battery when the car is running and the lights and fans are on. I got 14.34v at the alternator"

"BUT I have 2 volts between my alternator and the battery positive post"

Leigh W
Yes, there are losses because of resistance. in the alt to battery OE feed line. What were the quotes for? What uses or needs a 200amp alternator. And also you can get large output alts for specific uses. You mentioned vehicles which have them, do they? WHY? Early Toyota's had a fusible link in the Alt to Batt circuit. When if got tired it burned out and left the alt uncontrolled and fried anything attached including the alt. I know, it happened to me. Hence a larger direct to alt cable was fitted.

The quotes show that there are people are seeing voltage drops of between 1V and 2V between the alternator and the battery, so it does happen.

The models with 200A have them because they need them to run the accessories. The new 300 series Landcruiser has a 200A alternator from the specs I have seen:

300 series specs:

227kW/700Nm 3.3-litre V6 twin-turbo diesel
10-speed automatic transmission with manual shift mode
Low-range transfer case
Lockable centre differential
Three-mode drive mode select
110-litre fuel tank (80L main, 30L sub-tank)
Front double-wishbone independent and rear four-link rigid axle suspension with coil springs
Ventilated front and rear disc brakes
Hydraulic variable power steering
200Amp alternator ***************************************************

Leigh W
What accessories require a 200amp alternator on a 300 series. They might have one but would run ok on 100amp and never demand more than 1/2 of it. All a large alternator does is hammer a discharged battery harder and shorten their life. Just because something is found on the net doesn't mean it has great importance.

Car manufacturers don't waste money putting alternators in cars that have huge reserve capacity. Example Prado 120 has a 80A alternator, it has about 50A reserve with no accessories turned on.

Head lights high beam 10A
Running lights say 5A
Airconditioning say 15A
Aircon rear 5A
Stereo / satnav around 10A
Wipers say 5A say on low

That's 45A gone and we haven't consider charging the battery.

Then you still have the rear screen heater 10A, rear wipers, 12V sockets, Fog lights and no doubt other things I have forgotten, turn on every thing at once and the battery will be discharging and that's with the alternator producing 80A continuously which it can't as it only has a continuous rating of around 75% so after a few minutes it will start getting hot and start to de-rate.

Then you have the intermittent loads, Windsrceen washers, Reversing lights , Stop Lights etc, power mirrors ABS/Stability control/down hill assist/electric power brakes and other things I have forgotten, yes temp loads but the alternator still has to hold 13.8V or they run as slow as a wet week and that's just the GXL version without the upmarket accessories.

Now consider the 300 series,

Lights and theres a lot of them
Heated and ventilated seats
Satnav
Power seats
Power folding seats
Power rear door
Moonroof
Auto dimming, auto tilting and heated Mirrors
Heated steering wheel
Power adjustable steering wheel
Illuminated vanity mirrors
4 zone climate control
rear auto cooler
Power mirrors
Windscreen wipers
rear wipers
Electronic parking brake
Electric power booster
Power windows
Rear screen heater
Front wipers
Rear wipers

All the safety gear
Pre collision system
Active Cruise control
Lane departure
Road sign assist
Lane trace assist
Blind spot monitoring
Rear cross traffic
Reversing camera
Electronic suspension
Diff locks
Adaptive suspension
Down hill assist
Media screens
Multi functional displays
12V accessory ports
Heads up display
220V inverter
Alarm
Tracking
and probably other items and all the electronics modules and systems to support the above.

The 200A alternator can most likely only put out around 150A continuously, that's a lot of electrical gear its got to support. Toyota wouldn't put the "200A" unit in if it wasn't needed. Then you have to add in the after market accessories.