Wiring in fridge to 12V leisure battery - what I've done

[spandit]

I've hired for my holiday an Engel fridge. This runs off 220V, 24V or 12V and automatically switches between them (so you can leave it plugged in to the vehicle and the mains at the same time - the mains overrides the vehicle).

However, I don't want to leave it connected to the vehicle when parked overnight (or on the ferry) so bought a 12V leisure battery. However, manually moving the plug was a pain, especially as the Pinz has DIN/Hella sockets and the battery box has a regular cigarette lighter socket. I've thus effected the following wiring:



For those who are confused by wiring diagrams, what this does is switch between the vehicle and leisure batteries, depending on whether the vehicle master switch (and thus my 12V voltage dropper) is on or not. You may notice that if the master switch is on (i.e. the relay is activated) then you have 24V going through a 12V relay! Fear not - as discussed earlier, once the contact is made, between pins 87 and 30 is effectively a solid wire. As we know, 24V wires are thinner than 12V so there's actually more current flowing through this wire when connected to the 12V battery.

The Engel fridge only draws about 1A when actually running, so not worried about blowing the relay.

For those who are really excited about this sort of thing, the relay is from my old BMW E34 and terminal 85 is actually labelled 85a (as terminal 85 is a much smaller connection).

[David Dunn]

If you are worried about running the batteries dead, just unplug the fridge, they cool with "electric ice". If you don't keep opening it, or leave it open , it'll stay cold for a long time. I filled my ARB with 30 bottles of water and a 12 pack of sodas and they were cold in an hour.
Look up one of the threads on equalizers or solar units on this forum. You'll need to recharge the house battery eventually, and with those isolated from the vehicle's system does the same thing.

[spandit]

I've updated the diagram to put a feed in to charge the leisure battery on the move. Haven't actually got the diode yet but my forthcoming trip doesn't need me to run the fridge for very long. Just the ferry crossings and one night. On the return journey we might have it set to freeze which will take a bit more power but I'll have recharged the leisure battery from the mains in between.

Will look into those solar equalisers etc. for future use

[David Dunn]

You're reinventing the wheel... no need for a changeover. if it is on 12v, leave it on 12v...if the house battery is being charged, you don't need to change sources. It's all the same wattage.

You're making your life too complicated

[spandit]

David,

If I had the diode installed, which I don't (you probably didn't see the earlier version of the picture without it) then you would be right.

However, the fridge is more efficient on 24V than 12V (apparently) and switching it this way will maximise the battery life once the engine is switched off for the aforementioned periods.

Yeah, maybe it's a bit complex...

[David Dunn]

....However, the fridge is more efficient on 24V than 12V (apparently) and switching it this way will maximise the battery life once the engine is switched off for the aforementioned periods.

Yeah, maybe it's a bit complex...

Efficiency is a relative term... by adding more electric switching devices, you've used more power than you save and added more points of failure..... the ARB/Engel ( and who's ever) fridges were already design for efficency and flexibility.

Items that use high power is where you try to raise voltage to lower amperage.

But if you are dead set on it, go at it.

[karoja]

Why not use 24 volt on the house side? Thats what I did using the toolbox and a battery integrator.

[krick3tt]

I run my ARB (used to be Engle) direct from the battery. I used a lead for + and - from the starter to a fuse block to a lighter outlet on the back wall. I just leave the power on at the switch key while camping.
I have installed switches in the line for the 24 to 12 converters to shut them down while parked.
The 12v aux battery charges off a solar panel on the front of the pinz, no connection to the vehicle battery at all.
I cool the ARB with 120v house power before leaving on a trip and run 24v while traveling. Sitting at a camp site for long periods (days) uses the 12v aux.

http://s400.photobucket.com/albums/pp88 ... MG3361.jpg

Cold beer is great.

[spandit]

Thanks for the replies - that's why I posted it!

Krick - I'd be worried about running the vehicle batteries down overnight which is why I came up with the automatic switching idea. As David has pointed out, if I just connect the 12V feed from the dropper direct to the aux battery then it will achieve the same thing


Karl - 24V implies 2 batteries and don't want that much weight/bulk/expense - the aux battery box I have is useful for charging phones etc. as the cigarette socket is built right in

David, my only concern is how much current the battery will draw if plugged in to the dropper. I'm hoping that if the fridge is drawing, let's say, 3A, then the battery will only draw this to recharge - don't want to melt cables or the dropper. My comments about efficiency were misjudged when it would only be whilst the engine was running and thus amply powered from the alternator. Looks like a quick rewire in the morning before setting off!

[krick3tt]

Spandit,

I have run the fridge for two nights on the 24v from the battery with no adverse effects. I was in a no sun situation in Washington. Still had plenty of juice for a start up in the morning.

I have read that the military used the 710's for radio trucks and let the engine idle for power so it can't be all that bad to let it power a fridge that only uses maybe 2 amps. External temps have a lot to do with keeping stuff cold. When I camped in the desert finding the sun was not an issue and in the woods with the temps way down it is even better for the cooler.

I also have new batteries and keeping the vehicle tuned might be a contributing factor in starting easily.

Cheers

[spandit]

Just back from my holiday/expedition and the fridge was brilliant. Didn't use the relay in the end and as David (I think) suggested, ran the fridge off the 12V battery, with the battery plugged into the dropper to keep it topped up on the move (had to manually disconnect it as otherwise the battery was powering all the 12V stuff, radio etc.)

[Jimm391730]

FYI, I measured my Engel for current draw:

12V = 2.5A
24V = 1.5A

So it appears that it is slightly less efficient at 24, but I run it that way regardless. However it only runs for roughly 1/2 the time (10-15 minutes on, 10-15 minutes off) depending on temperatures of the fridge and outside so you could consider the currents cut in half.

With the stock batteries rated at around 75-100 AHrs and the fridge drawing about 18 AHrs a day (1/2 of 1.5A times 24 hours) in theory you can get four days of running the fridge from the truck batteries, but won't be able to start it after that. I've had no problem with 3 days/2 nights on stock batteries -- starts just fine.

Now my 712W has 110W of solar panels to charge 24V "house" batteries that power only the fridge and lights in the box, the panels are more than sufficient to keep the batteries up (at least in sunny SoCal).

Jim M.

[spandit]

Probably not so efficient once you take the power losses of the voltage dropper into account, but thanks for the information. I'm a bit more paranoid than you and didn't want to run any risk of draining the batteries whilst on holiday as finding another 24V vehicle to jump start the truck would be difficult

[Erik712m]

You can jumpstart with 12v.

[spandit]

You can jumpstart with 12v.

To one battery or to both?