Extension lead from car to caravan - 13 pin electrics

[DerekT1]

Hi, can anyone give me any advice on the following problem please?
I have recently bought a new Coachman caravan with 13 pin plug and lots of unnecessary electrical features inside. I like to spend time on sites without mains hook ups. On a recent holiday the leisure battery went flat in 5 days and I assumed that I could make an extension lead with a 13 pin plug and socket to connect the car to the caravan just like I did with my old caravan with the 12s plug. However although this worked the lights it would not allow the fridge to light even though this was connected and switched to gas. It seems that the Thetford fridge in the Coachman requires a 12 volt supply even when it runs on gas and it appears that this 12 volt supply is separate from the one that powers the lights and water pump, etc. The only way that I could solve this problem was to buy two crocodile clips, disconnect the caravan leads to the leisure battery and connect the wires from pins 9 and 13 on the 13 pin car socket directly to the caravan battery leads.
This doesn't seem very satisfactory so has anyone else solved the problem of connecting these new vans to the car's 12 volt electrics?
Thanks for any help I get

Derek T

 

[Damian]

When the van is connected to the car, the Habitation Relay cuts power to everything except the battery charging circuit, the 12v fridge element(when the engine is running) and some interior lights.
You are correct that the Thetford and Dometic new series fridges require a permanent 12v supply to power the electronics before they will work, and this supply is via the Leisure Battery(and the power supply unit when connected to an EHU)
The only way to overcome your problem is to have a Solar Panel to keep you rleisure battery topped up.(or an inverter technology generator)

 

[DerekT1]

Damian, thank you, I was assuming that this was the case but it seems daft when so many of us don't want to use mains hook ups and we have a decent battery in our cars that will function well as long as the car is used regularly to ensure that its battery does not go flat. I was thinking of making an extension lead that plugged into a socket connected to the caravans main battery leads and which had a switch to isolate the caravans battery from it. Any comments?

Derek T

 

[ProfJohnL]

Hello Derek,
I understand you concept, but one of the reasons this is not a popular approach, is to do with the type of battery you have in the car. The car battery is designed to provide high cranking currents for short periods of time, with the expectation the battery will be rapidly rechrged by the cars alternator. They dont like slow long discharges and standing for long periods at low levels of charge. Under such conditions the battery will deteriorate and it will shoten its life, and it will be more likely to let you down when trying to start the car. Car batteries are often as small as 60Ah capacities when new, where as caravan batteries are typically 85 or 110Ah capacities. All batteries tend to loose their ability hold a charge as they get older, buy caravan batteries are designd to better withstand partial discharge states.

Don't be tempted to fit a caravan battery in the car, apart from teh fact it probably wouldnt fit, The trade off for better discharged state characteristics is a reduction in the peak or cranking current the battery can deliver, so whilst caravan batteries may have a larger total charge capacity, they can't deliver as much peak current as a car battery, thus no good for starting cars.

You may think that a caravan does not use much 12V power, but infact if you work it out, a typical caravan lamp will use 1.2A of current, so three of them used for 3Hours in the evening will consume, the fan blower on the heater will also use about 1.2A, TV areail amplifier, fridge, space and water heater control electronics, Ventulator fans, etc all these small consumeres are used for long periods of time so the total Ah used adds up, ad into the equation things like TV's andradios, and it is quite easy to consume 10 to 20Ah of battery charge each day. supprisdingly intermittently operted items like water pumps may have significant peak currents, but because they are used for very short periods the total power they drain from tah battery is quite small.

A 10 to 20AH discharge on a car battery is a significant chunk, and it may impare the batteries ability to crank the engine.

 

[Gafferbill]

Derek………..UK Caravan 12 volt electrics have always been rubbish and the newer caravans with their 13 pin connection and habitation relays are worse still in my opinion…… as you have found out.
The caravans that have proper wiring connections to the towing vehicle are to be found in Australia where the power supply to the caravan is via an Anderson plug/socket. Only a plug of this type can handle the electrical demand the caravan can make on the towing vehicle’s battery out there. They have fitted sensors so that the vehicle battery does not fall below a certain voltage, so retaining enough charge to start the engine.
I retain an older UK caravan that is wired to make use of the car battery when on site; it has a switch in the caravan to choose either the caravan battery or the car battery.
Of course you need a 12S extension lead in order to do this.
You say this is what you did in your old van.

I have completely uprated the wiring systems on my towing vehicle and caravan to minimise voltage drop for battery charging, road lights and 12-volt fridge operation. I have used much heavier gauge wiring and solenoids.
I use 2 x 12S connections to carry all the wiring and 12 volt load to the caravan!
I also have a 110amp leisure battery permanently carried on the towing vehicle in an underslung carrier attached to the chassis of my 4x4.
This is electrically connected to the caravan 110amp battery in parallel and is charged whenever the engine is running but is isolated from the engine starting electrical circuit.
When on a non EHU site I use two 12S extension leads connected to two 12S sockets to connect car to caravan.
With this set up I can use either the car battery (160amp) or the two leisure batteries 2x 110amp to power the caravan.
In use on site you get a voltage difference between the two leisure batteries as the one on the car gets charged and the one in the caravan does not. This is the reason for the second 12S connection because as soon as you connect the two batteries together current flows from the higher charged to the lower until they are equalised.
This second 12S connector is wired with pins 7&3 as earth and 12456 as positives to carry the electrical load and is fused at 30amp.
I have also altered the refrigerator 12-volt wiring so that it will operate without the engine running. It will now work efficiently for up to two hours in this mode drawing current from the two leisure batteries only.
With gas and a 150watt inverter for the TV and Skybox who needs EHU!!
You just have to use the car every day for trips which I usually do.

Derek……your wiring modification sounds all right to me but you would need a fuse to protect the wiring and you would need to consider warranty implications on your new Coachman, which for me would scupper the idea.
Most people with newer caravans seem to go the solar panel route.

 

[DomDom]

The habitiation relay prevents the use of a 13 pin extension, that much is correct. The answer is one or both of a solar panel and Anderson leads.
Solar panels have been done to death so I'll assume you know about their merits.
Anderson plugs, sockets and cables are simply high current, easy connect/disconnect jump leads. They can be used either to connect the car battery and use that in the caravan or to charge the caravan battery with a running engine. My car has a 140 amp alternator which will 90% charge the leisure battery in 20 minutes or so. Since I never run it flat we only ever do this once a day. Parts to make them can be found on the bay and should cost no more than 20 quid.
HTH
Dom.

 

[Sproket]

Parts to make them can be found on the bay and should cost no more than 20 quid.
HTH
Dom.

Just as DomDom said :
ANDERSON-SB-175-GREY-BATTERY-CONNECTOR-JUMP-START-KIT