Modern car electrical systems seek to reduce fuel consumption in all sorts of fancy ways, one of them being to minimise use of power sapping ancillaries. This has led to to fancy alternators that sense power requirements and prioritise where they send power to. This can mean that after starting up, priority is given to sending power to the car's battery to recharge it ready for the next start. It also means that when there are substantial power demands from within the car, the amount of power available for things like powering caravan battery charging and fridge chilling is substantially reduced.

As I understand it this can lead to periods when little power is getting to the fridge. However this isn't really a big deal provided that the fridge has gone down to temperature via gas or mains before travelling, as the insulation will ensure the interior remains cool, with occasional periods of the 12V element working when spare power is available.

Quote: Bob wrote in post #45

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...my vehicle will not even charge the leisure battery whilst towing (never mind chill the fridge) unless all of the above conditions are met.

Do you concur?

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Quote: Bob wrote in post #45

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As for my BMW, I think I'll need to get the tow-ball socket opened to see what's inside and go from there.

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Thanks. Will report back when I know more.

Update: - Factory fitted retractable tow bar supplied new with car is from Westfalia.

I have been inside the socket and find that pins 10 and 11 (the switched supply/earth) are not wired... as suspected.

Researched Westfalia's 2016 catalogue and see that the vehicle-specific wiring harness (P/N 303 352 300 113) can be supplied with "Charging cable optional” (with extension kit 300028300113). It seems that this "Charging cable optional" (which costs about £50 online) is the part that I am missing to enable a supply for the fridge and leisure battery charging.

However, when I spoke to my local tow bar fitting specialists about retrofitting the "Charging cable optional" they did not want to touch it due to likely issues with coding the car. They said that if I want to continue seeking a solution with the car then I should talk to BMW. Hmmmm...let me think about that...

I asked about bridging the existing permanent 12v supply (pin 9) across to pin 10 and they suggested that I would be best speaking with an expert on caravan electrics to see what could be done. So that's my next step.

If anyone can recommend an Eriba electrics wizard within a hours drive of Poole then please do let me know.

Going back to earlier posts in this thread, my experience is that the problem with poor 12v fridge performance on the move is often down to the thickness (or otherwise!) of the cabling from the car battery/alternator to the tower. In a previous life I had a large twin axle towed by a Volvo 940 and it struggled to keep the fridge cool whilst on the road - so I rewired the positive supply on the car in 1mm or 1.5mm multi strand copper core wire (I forget which) to the socket and everything worked much better. That was in the good old (bad old?) days of 12N and 12S sockets and before the introduction of "intelligent" power supply routing.
Also there was a statement I saw that said 12v is not as good as 240v or gas. I wouldn't agree - as long as the wiring is man enough you should get better performance on a camping absorption fridge from 12v, than 240v. My Dometic portable camping fridge plugs into my boot (Volvo V70) and it most certainly cool stuff down whilst travelling.

In other words - investigate the power handling capacity of the cabling before blaming the fridge - after all, you can't put out a fire with a domestic hosepipe because you just can't force enough water through it however high the pressure... That's what the fire brigade's big hoses are for!

The 1mm or 1.5mm refers to the diameter of the copper core. Don't use solid core, because it won't be flexible enough. use high quality multi strand copper cable. Pricey, though...

1.5mm diameter is a cross sectional area of 1.8 mm²

That's far too small for a fridge supply - the auxilliary conductors in a 13 way towbar cable are 2.5 mm² and Dometic recommend a minimum of 4.0 mm² for the caravan's internal wiring to the fridge.

I don't understand your assertion that running a fridge on 12V (with adequate wiring) is more effcient than 230V operation.

Both elements have similar power ratings, eg Dometic RM8501 is rated 125W @ 230V, and 120W @ 12V.

Sorry, i meant as good as, rather than better than... As to cable diameter, i am sure you are right - it's a long time since I did it and my memory's not what it was...!! 4mm sounds much more likely.

2nd update

Believe I have now solved problem of how to charge my leisure battery and operate the fridge whilst towing with my BMW (which does not have the switched 12V supply but does have the continuous 12V supply in the 13 pin connection).

I have modified the van's electrics by inserting a 2-pole 20A switch in the continuous 12 supply. When I close/connect the switch, the continuous 12v supply is bridged to the 12V switched-supply. This then closes the protection relay which connects the 12 continuous supply to the bus which charges the leisure battery and feeds the fridge.

When the switch is open/disconnected there is no change in behaviours and everything works as usual.

Clearly, if the switch is connected and I stop the towing vehicle's engine, there is a risk of the fridge flattening the towing vehicle's battery and the leisure battery. So I know that I'll need to disconnect the 2-pole switch whenever I turn off my car's engine for more than a few minutes.

Just to recap on why I've done this...I am planning a long European tour later this year (1 day driving / 1 day static) and I wanted to be sure of 1) a charged leisure battery when arriving at each stop (I am not confident that I'll always get a hook-up) and 2) our food is preserved in the high temperatures.

You lost me at "2nd update"
Dave

Perhaps you could change your two way switch to a relay, triggered by the switched supply. That way you wouldn't flatten your auto battery when the engine wasn't running.

Sorry, just re-read your post and you don't have a switched supply.

Voltage sensing relay perhaps? Set it to just below 12.7Vdc which would cut out a short time after the auto stopped running.

Nice suggestion which I believe would work. Thanks!

But I think I'll see how good I am at remembering to switch off when I stop before investing any more time. This has already cost me hours and hours of research.