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AlwynMike

Van battery charging from car bodge

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OK Chaps.

Its raining outside. The dog has been walked. The wife is at church. I'm sitting musing with my Yorkshire tea.

 

Please, I don't want the pedants posting with "you can't do that, its not standard" etc. I know this. Can we treat it as a theoretical exercise?

 

We all know that van battery charge power comes from the permanent 12 volts from the car.

We know that the van battery won't get charged until the fridge supply is energised.

 

Some of us have cars that have not been fitted with a fridge supply (Thank you Herr BMW).

I've never used the fridge on 12 volts. Ever. In 30 odd years of caravanning. Doubt if I'll start now!

Van battery charging would be useful however, and it is my cunning plan to fit a fridge supply to my car when I can be bothered to sort out the socket parts required.

Still with me?

 

What if. ......

I got a voltage sensitive relay and wired it into the caravan permanent 12 volt feed (inside the van). Adjusted the VSR to a relatively high set point. Took the VSR output to the fridge feed - and disconnected the fridge 12 volt power.

My thinking is that it SHOULD work to put a bit of charge back into the van battery whilst on the road.

 

Hooking up to a properly wired car wouldn't cause a problem as the additional VSR is designed to see 12 volts (or 14 ish!) anyway.

A flatish van battery may drag a significant current for a short time - I've no idea on what sort of hysteresis these VSR's have, so it may chatter - I'm assuming they use a discrete mechanical relay rather than solid state switching.

Back feeding power from the van battery to car battery when starting the car will be just the same as if the VSR was fitted at the car end, so not an issue.

 

Am I missing something??

 

Remember. If a job's worth doing well, its worth bodging ☺

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The main thing is to ensure that the caravan battery is in some way disconnected when the engine is cranking. You do not want the caravan battery trying to push starting current through the charging wire.

 

The second issue is to prevent the caravan battery from draining the car battery when the engine is either not running or idling.

Meet those two requirements and all should work.

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Why not just wire the car with the two extra wires ? For a caravan 

 

 

Dave

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1 hour ago, Stevan said:

The main thing is to ensure that the caravan battery is in some way disconnected when the engine is cranking. You do not want the caravan battery trying to push starting current through the charging wire.

Agreed

 

The second issue is to prevent the caravan battery from draining the car battery when the engine is either not running or idling.

Meet those two requirements and all should work.

The VSR will effectively do both duties. With the car off, idling or cranking, the car battery voltage will be lower than set point, so no power to fridge/habitation relay to connect batteries together.

 

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1 hour ago, CommanderDave said:

Why not just wire the car with the two extra wires ? For a caravan 

 

 

Dave

Whilst on paper, this is the "easy" option, in practice, it isn't! The plan is to do the job properly at some stage. I've had the car 18 months and have always had enough ooomph in the van battery to use the movers to get it in a suitable parking spot. But there will come a time. ...

 

The car has the manufacturers retractable towbar, and whilst I know the socket is a bespoke Westfalia one, I know I will have difficulty in sourcing the pin connectors. BMW don't sell the socket separately. It has all of the pins present, but it has push on soldered connectors at the back. No doubt there is no suitable transit into the car interior, and very flexible cable will have to be used due to the retractable bar. The bumper is easily undone, but its a 2 person job because 6 feet of floppy plastic isn't easy to hold on your own - without scratching something.

On the upside, the battery is in the boot, so no great cable length!

 

So doing it in the caravan would be a lot quicker and, in this weather, warmer and drier.

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Two thought.  

First. Why would the Towbar fitter wire a permanent 12v. But not the switched 12v. That’s a proper bodge.  

Second. Can’t figure why you would want/ need to disconnect the fridge supply. If you don’t want fridge working on 12v then don’t turn the fridge onto 12v

 

beyond that, I don’t see anything seriously wrong with your proposed “upgrade “

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3 minutes ago, Lost in the wilderness said:

Two thought.  

First. Why would the Towbar fitter wire a permanent 12v. But not the switched 12v. That’s a proper bodge.  

Second. Can’t figure why you would want/ need to disconnect the fridge supply. If you don’t want fridge working on 12v then don’t turn the fridge onto 12v

 

beyond that, I don’t see anything seriously wrong with your proposed “upgrade “

If you had a European van without leisure battery, the permanent live feed would enable occasional off grid use by drawing on the car battery.

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Our car tow bar has a smart relay supplied with permanent 12 volts but as the engine runs the voltage rises which switches the relay so no need for a separate ignition controlled supply. Being an older car it does not have a clever alternator that might vary its voltage which might not switch a smart relay.

 

On your tow bar socket, has it not got all the 13 wires to it to allow normal wiring up at the other end so you could fit a smart relay from the battery ?

 

 

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3 hours ago, AlwynMike said:

OK Chaps.

Its raining outside. The dog has been walked. The wife is at church. I'm sitting musing with my Yorkshire tea.

 

Please, I don't want the pedants posting with "you can't do that, its not standard" etc. I know this. Can we treat it as a theoretical exercise?

 

We all know that van battery charge power comes from the permanent 12 volts from the car.

We know that the van battery won't get charged until the fridge supply is energised.

 

Some of us have cars that have not been fitted with a fridge supply (Thank you Herr BMW).

I've never used the fridge on 12 volts. Ever. In 30 odd years of caravanning. Doubt if I'll start now!

Van battery charging would be useful however, and it is my cunning plan to fit a fridge supply to my car when I can be bothered to sort out the socket parts required.

Still with me?

 

What if. ......

I got a voltage sensitive relay and wired it into the caravan permanent 12 volt feed (inside the van). Adjusted the VSR to a relatively high set point. Took the VSR output to the fridge feed - and disconnected the fridge 12 volt power.

My thinking is that it SHOULD work to put a bit of charge back into the van battery whilst on the road.

 

Hooking up to a properly wired car wouldn't cause a problem as the additional VSR is designed to see 12 volts (or 14 ish!) anyway.

A flatish van battery may drag a significant current for a short time - I've no idea on what sort of hysteresis these VSR's have, so it may chatter - I'm assuming they use a discrete mechanical relay rather than solid state switching.

Back feeding power from the van battery to car battery when starting the car will be just the same as if the VSR was fitted at the car end, so not an issue.

 

Am I missing something??

 

Remember. If a job's worth doing well, its worth bodging ☺

Surely this is what your proposing.  

 

https://www. towsure. com/self-switching-smart-split-charge-relay-for-towbar-electrics?gclid=EAIaIQobChMI4ufdorqx4AIVrZPtCh0E1gWgEAQYAiABEgK9yPD_BwE

 

Hide it in the boot, supply with a heavy cable direct from the battery v+ via a 30 amp fuse.  I have done this in the past by threading through a length of hose and running under the car as bulkheads are very difficult to penetrate.

 

Some BMW’s already position the battery in the boot which makes life easier.

 

From there connect to your twin 7 pin or 13 pin sockets.

 

John

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2 hours ago, Lost in the wilderness said:

Two thought.  

First. Why would the Towbar fitter wire a permanent 12v. But not the switched 12v. That’s a proper bodge.  

Second. Can’t figure why you would want/ need to disconnect the fridge supply. If you don’t want fridge working on 12v then don’t turn the fridge onto 12v

 

beyond that, I don’t see anything seriously wrong with your proposed “upgrade “

 

The towbar fitter was a German robot who knows nothing of fridge wiring (Factory fitted)

 

Putting fridge current, charging current and ATC through one cable is, IMO, far too much.

2 hours ago, Paul1957 said:

Our car tow bar has a smart relay supplied with permanent 12 volts but as the engine runs the voltage rises which switches the relay so no need for a separate ignition controlled supply. Being an older car it does not have a clever alternator that might vary its voltage which might not switch a smart relay.

 

On your tow bar socket, has it not got all the 13 wires to it to allow normal wiring up at the other end so you could fit a smart relay from the battery ?

 

 

 

The German penny (Euro) pinchers deemed it not necessary to fully populate the cable. This is better than previous models where they don't even populate the socket fully.

The voltage sensitive relay I mentioned is what is also refered to as a Smart relay

1 hour ago, JCloughie said:

Surely this is what your proposing.  

 

https://www. towsure. com/self-switching-smart-split-charge-relay-for-towbar-electrics?gclid=EAIaIQobChMI4ufdorqx4AIVrZPtCh0E1gWgEAQYAiABEgK9yPD_BwE

 

Hide it in the boot, supply with a heavy cable direct from the battery v+ via a 30 amp fuse.  I have done this in the past by threading through a length of hose and running under the car as bulkheads are very difficult to penetrate.

 

Some BMW’s already position the battery in the boot which makes life easier.

 

From there connect to your twin 7 pin or 13 pin sockets.

 

John

 

As I noted earlier. This is planned. But it's not a simple operation to do - even if I can source the relevant parts.

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AlwynMike I do think you have come up with a very simple solution to where a vehicle manufacturer does not install an ignition switched supply on a factory fit tow bar.  

 

You will need to find connections in your caravan for the incoming supply from pin 9 (permanent live) and pin 10 ( switched supply) and simply bridge between them with a self switching relay as part of the bridge. The self switching relay will need an earth connection.

 

My only reservation would be to ensure that you never tow with the fridge switched on, as power for charging the caravan battery and the supply to the fridge will all come through pin 9. The Ryder diagram for the self switching relay in my tow car does say the supply fuse should be 30 amp and I would be wary if the supply cable in your BMW would take this much current if you were to change the fuse in your car to this size. If you have an Al-Ko ATC fitted to your caravan that will add to the supply current through pin 9 if it needs to become operational.

 

Looking back I see you are going to disconnect the fridge 12v anyway.

 

John.

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11 minutes ago, John19 said:

AlwynMike I do think you have come up with a very simple solution to where a vehicle manufacturer does not install an ignition switched supply on a factory fit tow bar.  

 

You will need to find connections in your caravan for the incoming supply from pin 9 (permanent live) and pin 10 ( switched supply) and simply bridge between them with a self switching relay as part of the bridge. The self switching relay will need an earth connection.

 

My only reservation would be to ensure that you never tow with the fridge switched on, as power for charging the caravan battery and the supply to the fridge will all come through pin 9. The Ryder diagram for the self switching relay in my tow car does say the supply fuse should be 30 amp and I would be wary if the supply cable in your BMW would take this much current if you were to change the fuse in your car to this size. If you have an Al-Ko ATC fitted to your caravan that will add to the supply current through pin 9 if it needs to become operational.

 

Looking back I see you are going to disconnect the fridge 12v anyway.

 

John.

 

Thanks John19.

Without looking, I would assume the fridge disconnection would be simply to remove the fuse!

I do have ATC, (which I do manage to activate a couple of times per trip at least!) and the fuse is 20 amp. I have since mentioned the current carrying capacity of the single cable permanent feed would be suspect.

 

JCloughie's link to a VSR was an eye opener - I'm sure the last time I looked they were £30-odd.

 

Mike

 

Edited by AlwynMike
ATC

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Mine is this one and hence the 30 amp fuse:-

 

Ryder Tf1170-3 Super Smart Combi Caravan Split Charge Towing Relay 30 Amp

 

Available from ebay. Never had any problem with equipment from Ryder.

 

John.

 

 

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I think there's a fundamental misunderstanding here of how caravan switching works. When the battery is being charged the supply to charge the battery comes from the fridge connection (pin 10) not from pin 9. The latter is only used when the vehicle battery is being used to supply 12V to the trailer - as someone said because most continental-built caravans don't have their own battery - in a static location.

 

Mr Beamer decided a couple of years ago that there is insufficient requirement in the caravan market to justify the cost of the extra wire to the socket: note that this cable comes from the fusebox as it has to be switched and - more importantly - the ECU has to be told about it so that the supply is not switched on and off by the actions of the smart alternator. Ergo, anyone buying a Beamer should request the towbar be dealer supplied and fitted and make the presence of the fridge supply a condition of contract. You could try that with BMW when ordering a factory fit but I suspect they would bounce the order or simply instruct the supplying dealer to fit it.

 

I realise it is a bit late to do anything about it now but any good dealer - or better still a competent towbar fitting company -  would be able to remove the socket and replace it with a UK equivalent complete with pins 10 and 11.

 

As a point of interest, Westfalia bars as fitted on our Passat don't come with the fridge wiring as standard but at least VAG have the sense to make an optional extra cable assembly available in case it is required and the supplied socket is 13-pin ready. The cable, about 12 ft or so long costs - wait for it - just over £100, and you've got fitting on top of that!

 

I wish the OP good luck but I suspect he's onto a non-starter as the voltage drop on the cable between the vehicle battery and the caravan will vary with current and the VSR will chatter despite the hysteresis built-in.

 

Oh, and finally, to the OP you don't want to get into trouble with the Discrimination Police so remember next time its chaps and chapesses. .......  :D

 

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Further to my previous post and link.   And to add my understanding to Woodentops post above.   While it would never be wise to do anything which may invalidate a guarantee.   Taking a fused supply directly from the vehicle battery to the VSR should in no way interfere with the vehicles sensitive electronics as they are bypassed.   It also has the added advantage of being able to utilise a heavy cable to minimise voltage drop.

 

I imagine there are many who would not agree with that .

 

My current car is the first I have ever had in which I have not installed the towing electrics myself splashing out £800 for a Volvo detachable bar and 13 pin electrics.   Previously I did it in the I way described above and never a problem.

 

John

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1 hour ago, Woodentop said:

I think there's a fundamental misunderstanding here of how caravan switching works. When the battery is being charged the supply to charge the battery comes from the fridge connection (pin 10) not from pin 9. The latter is only used when the vehicle battery is being used to supply 12V to the trailer - as someone said because most continental-built caravans don't have their own battery - in a static location.

 

 

Sorry, but you are the one with the misunderstanding or perhaps you haven't explained yourself very well.

 

When a caravan is connected to a towing vehicle (without the engine running) a 12v supply is available through pin 9 which can power the 12v equipment in a caravan, with the exception of the fridge. Pin 10 should be dead.

 

Start the engine and pin 10 should come alive to power the fridge. This also switches a relay (which may referred to as a habitation relay). The use of this relay is to remotely divert the flow of current through pin 9, from power to the caravan, to provide a charging supply to a caravan battery.  

 

If this link works you will see this relay alongside the fuse boxes at the cable entry point of this caravan:-

 

https://www. caravantalk. co. uk/community/uploads/monthly_01_2015/post-68862-0-79695300-1422473037. jpg

 

AlwynMike is correct in his description in the first few sentences of his original post.

 

John.

 

 

Edited by John19

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1 hour ago, Woodentop said:

I think there's a fundamental misunderstanding here of how caravan switching works.  

 

Yup

1 hour ago, Woodentop said:

When the battery is being charged the supply to charge the battery comes from the fridge connection (pin 10) not from pin 9. The latter is only used when the vehicle battery is being used to supply 12V to the trailer - as someone said because most continental-built caravans don't have their own battery - in a static location.

 

Nope

 

A relay is energised from pin 10 connecting pin 9 through to the leisure battery.

 

 

image.png.91ef5b043dc5fe7e51ee785c4c6612e1.png

image.png

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2 hours ago, Woodentop said:

I think there's a fundamental misunderstanding here of how caravan switching works. When the battery is being charged the supply to charge the battery comes from the fridge connection (pin 10) not from pin 9. The latter is only used when the vehicle battery is being used to supply 12V to the trailer - as someone said because most continental-built caravans don't have their own battery - in a static location.

Yes, this could be. It was my understanding that the fridge power energised the habitation relay to enable van battery charging via pin 9. I stand corrected.

My initial response thought was that the van battery would be connected almost directly to the fridge if the charge current was routed via pin 10 - so there must be a relay or diode in circuit to prevent this.

 

Cross posted. No further comment.

 

Quote

 

I realise it is a bit late to do anything about it now but any good dealer - or better still a competent towbar fitting company -  would be able to remove the socket and replace it with a UK equivalent complete with pins 10 and 11.

A good BMW dealer is an oxymoron

 

As a point of interest, Westfalia bars as fitted on our Passat don't come with the fridge wiring as standard but at least VAG have the sense to make an optional extra cable assembly available in case it is required and the supplied socket is 13-pin ready. The cable, about 12 ft or so long costs - wait for it - just over £100, and you've got fitting on top of that!

There was an "additional loom" listed for my model. Which has now disappeared from the parts list.

There is now a "replacement loom" including the socket but who knows how many wires are in it. Cost may be similar to VW!

 

I wish the OP good luck but I suspect he's onto a non-starter as the voltage drop on the cable between the vehicle battery and the caravan will vary with current and the VSR will chatter despite the hysteresis built-in.

I had considered chatter. Rainy afternoon investigations reveal various hysteresis figures from 0. 4 to 0. 6 volts. Ryder / Ring ones seem to be 0. 5v with the Maypole 30A unit 0. 6v but with an adjustable trigger voltage, so I may go for one of these for car fitting anyway.

I don't know what effect the car's fancy charging system will do with the battery voltage anyway.

 

Oh, and finally, to the OP you don't want to get into trouble with the Discrimination Police so remember next time its chaps and chapesses. .......  :D

I am famously Politically Incorrect so the Discrimination Police are no stranger to me.

 

 

Another concern now - possibly a practical non-concern - is that Ryder (and I concur with John19 that they are a known reliable make) state that thier relays may take 30 seconds to de-energise. How many of us have stop/start cars? Or you may stall the car and do an immediate restart. Ryder relays may well still be energised - as you restart your car - with the van battery still in circuit.  

I'll leave that one with you. I've coded my car's stop start to default off!

Edited by AlwynMike
Cross (not angry) post

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33 minutes ago, AlwynMike said:

Another concern now - possibly a practical non-concern - is that Ryder (and I concur with John19 that they are a known reliable make) state that thier relays may take 30 seconds to de-energise. How many of us have stop/start cars? Or you may stall the car and do an immediate restart. Ryder relays may well still be energised - as you restart your car - with the van battery still in circuit.  

I'll leave that one with you. I've coded my car's stop start to default off!

It really isn't as big a problem as people make it out to be. The length and cross sectional area of the wiring gets in the way. If you had a fully charged caravan battery, and a low voltage on the car during starting eg it dropped to say 11. 5v (very low) there would only be 2. 3v difference between the two. I would guess the wiring from the caravan battery all the way through the socket and relays would show at least 0. 5 Ohms resistance, using ohms law V/R=I  the maximum current would be 4. 6 amps, no wiring catching fire, no fuses blowing etc.

Edited by AJGalaxy2012

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8 hours ago, Stevan said:

If you had a European van without leisure battery, the permanent live feed would enable occasional off grid use by drawing on the car battery.

mmm not sure this is a good idea, risk of flattening car battery .

macafee2

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11 hours ago, AJGalaxy2012 said:

It really isn't as big a problem as people make it out to be. The length and cross sectional area of the wiring gets in the way. If you had a fully charged caravan battery, and a low voltage on the car during starting eg it dropped to say 11. 5v (very low) there would only be 2. 3v difference between the two. I would guess the wiring from the caravan battery all the way through the socket and relays would show at least 0. 5 Ohms resistance, using ohms law V/R=I  the maximum current would be 4. 6 amps, no wiring catching fire, no fuses blowing etc.

 

A problem in theory, but not in practice or we wouod have had lots of posts about melted insulation etc.

9 hours ago, macafee2 said:

mmm not sure this is a good idea, risk of flattening car battery .

macafee2

 

Welcome back to the 1970's and 80's !

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12 hours ago, AlwynMike said:

 

 

Another concern now - possibly a practical non-concern - is that Ryder (and I concur with John19 that they are a known reliable make) state that their relays may take 30 seconds to de-energise. How many of us have stop/start cars? Or you may stall the car and do an immediate restart. Ryder relays may well still be energised - as you restart your car - with the van battery still in circuit.  

I'll leave that one with you. I've coded my car's stop start to default off!

 

The time to de-energise a self switching relay depends partly on the load the car battery is carrying. Charge a battery at 13 to 14 volts and it will hold that voltage for a period of time after the charge is removed.   Add a load to the battery and its voltage will reduce and hence a self switching relay will cut out. For those of us powering a fridge it is enough of a load to quickly reduce battery voltage if the charging stops. I did a test with an 55w foglight  to see its effect on pin 10. Without the light (load), pin 10 did remain energised for about 25 to 30 seconds after the engine was stopped. With the light connected as a load, pin 10 de-energised almost immediately the engine was stopped ( light went out).

 

Btw the handbook for my A6 (with smart alternator) does say stop/start should be switched off when towing.

 

John.  

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I don't have latest British standard only BS7671:2008 so there may be a change? However it lists two supplies pins 9 + 13 for Continuous power supply and 10 + 11 for Power supply controlled by ignition switch. It also states "The circuit to charge an auxiliary battery should be separate from a circuit to operate a refrigerator." So to comply the caravan has a relay so the supply on pins 9 and 13 can be used for battery charging only when the supply on pins 10 + 11 is live.

 

On my caravan I had a fault and to cure I needed to work out wiring, it seems the printed circuit relay and fuse box had been replaced and the new one had extra fuses, to the printing on box.

 

I hunted for wiring diagrams and found there were many versions of the fuse and relay box with more or less relays and some had diodes. This means not all caravans are the same.

 

Pre alternator control by engine management we used voltage dependent relays to switch on the engine running supply, that is no longer an option, as the supply could turn off while towing, and as already said it needs some method to stop the caravan battery feeding the car. For two reasons, one it could supply excessive current when cranking car, and two it could inhibit the engine management from charging car battery.

 

I have been told not to use even a Smart charger on our Jaguar XE battery as it can tell the engine management not to charge battery once engine is started. There are relays which turn off power to some areas when the battery voltage drops, I would assume the official caravan towing wiring takes power from a point controlled by the cars automatic shut off system which will likely change from car to car.

 

If not using the cars official system, you still need to ensure no back feed, easy method seems to be use of blocking diodes, although the new schottky diodes do not have the 0. 6 volt drop of the old silicon diodes, there must be a volt drop, plus the volt drop of the wiring, so connecting the caravan battery to the car without a DC to DC inverter is in real terms pointless. Yes on starting car engine you may see a 15 amp charge, but after 10 minutes likely that will have dropped to less than one amp. So a three hour run will likely only put around 5 Ah of charge into the battery, which is hardly worth the effort.

 

A DC to DC inverter will prevent any back feed, however you don't what to discharge the car's battery to a point where the car will not start. The inverters have a cut out voltage limit, however because of the cable volt drop with some set ups that may be exceeded.

 

Sterling, ring, and Durite all make versions and I would consider around a 10 amp would be about right, I have tried charging my caravan battery with a Lidi Smart charger, and after using the motor mover was surprised to see charge rate drop from 3. 8A to 3A within the hour, in fact often within 15 minutes. So in real terms within 5 minutes a 30A DC to DC inverter is likely giving same charge as a 10A version. It is what the battery can take rather than what the car can give.

 

So the three hour run will still likely only put in 15 Ah of charge into caravan battery which could clearly make the difference of getting caravan parked with motor mover or not, so in some cases may be worth it? 

 

However the post starts with word "Bodge" so consider this Bodge, my battery pack for jump starting, compressor, light, 230 volt outlet has a cig to cig lead to charge it from car, it has a 20 Ah sealed lead acid battery in it, and using the 200 VA output it can easy work the caravans built in charger, so a 13A to 16A lead and I can plug caravan into the pack until the battery becomes flat charging the caravans own battery, then return it to the car plug into cig socket in boot, and it will recharge while I am driving on all trips, not just those when caravan attached, so I can charge caravan battery from any car does not need to be tow car, and it will cost me nothing as I already have it all.

 

So why mess around trying to charge from 13 pin socket? Yes I agree there is a British standard and for a car not to comply with that standard it could be said the car is not fit, and you should return it to supplier demanding it complies with the British standard, however I have never tested either the Kia Sorento or the Jaguar XE to see if they charge the caravan battery or work the fridge, with the Kia the car battery is big enough, but the Jag battery is rather small so would be worried that it may inhibit the Jag from charging.

 

I read the Jag auto monitors the battery and corrects the charge as required, but some cars when replacing the battery you have to tell the engine management it has been changed, so it can re-map the battery, connecting a second battery can upset this process, as can using battery chargers when the car is not used for a time. So unless I know the charging system, I would not want to connect a second battery.

 

I am not sure if even the fitters in the dealerships know exactly how the charging system works today, it has become so complex.  

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21 minutes ago, ericmark said:

 

I am not sure if even the fitters in the dealerships know exactly how the charging system works today, it has become so complex.  

From my experience, most "fitters in the dealerships"  do not "know exactly how" anything works!

They seem to just blindly follow the idiot's guides provided by the makers. Getting to Talk to the one guy in the workshop who actually understands stuff is a challenge in it's own right.

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36 minutes ago, John19 said:

 

The time to de-energise a self switching relay depends partly on the load the car battery is carrying. Charge a battery at 13 to 14 volts and it will hold that voltage for a period of time after the charge is removed.   Add a load to the battery and its voltage will reduce and hence a self switching relay will cut out. For those of us powering a fridge it is enough of a load to quickly reduce battery voltage if the charging stops. I did a test with an 55w foglight  to see its effect on pin 10. Without the light (load), pin 10 did remain energised for about 25 to 30 seconds after the engine was stopped. With the light connected as a load, pin 10 de-energised almost immediately the engine was stopped ( light went out).

 

Btw the handbook for my A6 (with smart alternator) does say stop/start should be switched off when towing.

 

John.  

 

Many, perhaps all, VW Group cars automatically turn the stop-start off when towing

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