2 Fridges on my wagon

[Chris Holmes]

Hi All,

For many a year now we have taken a camping fridge (approx 80W rated consumption) and it has been connected to the split load charging contacts on the 12S socket on the back of the cars.

In theory, this keeps the fridge running on the move, but prevents it from discharging the car battery when the ignition is off.

This year we are expanding our tents and the plan is to take 2 such fridges and I?m hoping they will both fit in the DIY fridge box on the front of the trailer.

I reckon that means a combined load of around 160W

I don?t know if these fridges (which I THINK are a basically (in ?12V? mode) a Peltier device across the input) are likely to draw more than 80W each at startup, nor do I know how low the volts have to go before what I believe is a voltage sensing relay cuts the power.

I do know that ?The 12S plug is fed by a 20A and and a 30A relay (I don?t yet know if these are in series, or if one feeds the split load pins and one feeds the ?Caravan Interior Lights and Fridge? pins.

If I were towing a modern(ish) caravan, it would have a voltage sensing relay fitted internally to disconnect its fridge when the engine isn?t running.

So, what I think I have on the car, is a pair of permanently live pins, and a pair of pins switched by a voltage sensing relay.

My options are?

Leave the existing 2 core 1.5mm lead to the fridge box and run the second fridge in //.

Replace that cable with a 2 core 2.5 and run the fridges in //.

Or use 4 core cable and run one fridge of the pins I?m using now and run the other from the ?Interior lights / fridge? pins via a voltage sensing relay fitted in the fridge box.

I think it all comes down to how much those fridges might draw at switch on / continuously and how close that is to the ratings for various relays / fuses / cables / connectors.

I?m inclined to go for the belt and braces option.

What does the team think?

Chris

[Harry Bloomfield, Esq.]

Chris Holmes wrote on 25/06/2021 :

There is only usually one relay, the voltage sensing relay in your car )boot). The supply cable needs to be chosen for volts drop, rather than current rating, to avoid the voltage sensing relay cycling. My own choice is at least 4mm. In my case, as it is the main feed for road lighting, battery charging and the fridge, I used 2x 4mm for the main feed. The car's manufacturers feed for the purpose, is sensibaly around

6mm, from main fuse panel, to rear of glovebox.

It runs my 120w fridge quite well, but as installed, it lacked any indication that it was receiving the 12v with the engine running. I therefore added an LED which comes on when the voltage sensing relay closes, with the engine running.

The 12v is only capable of holding the fridge temperature down, it is not intended to initially get it down to freezing, so I run it on mains for 24hours before we leave home and only put already cold/frozen items in it.

[Harry Bloomfield, Esq.]

Chris Holmes laid this down on his screen :

I don't think Peltier involves much switch on surge.

Caravans have a second relay, in the actual caravan, called an 'habitation relay' - that is nothing to do with running a fridge.

[Chris Holmes]

Thanks for that, according to t'interweb, the wiring of 12S sockets has changed over the years.....

Gives one wiring diagram and shows one pair of pins (3&4) for Interior lights and battery charging (I think the battery in the caravan would be charged by this via a voltage sensing relay in the caravan so as not to drain the car battery when the engine isn't running).

It shows another pair (6&7) for Fridge which are only live when when the car's engine is running / the ignition is on (done via a voltage sensing relay in simpler cars these days).

I am told the car has a 20A AND a 30A relay. I'm not sure why this is (It's possible that it is a typo and the fitter meant to write something else). If it does have two I assume one will be the voltage sensing relay for the fridge pins (6&7) and that Pins 3&4 are permanently live but for some reason are fed via a relay. Or perhaps the volts for the two live pins mentioned above come via the 30A for some reason and then 3/4 are fed direct and 6/7 go via the voltage sensing relay?

I think to reduce the risk of pin burn out, etc, I WILL fit a voltage sensing relay in the fridge box to feed one of the fridges from 3&4 and keep the other fed directly from 6&7 as at present. As they should only draw 80W each, I reckon a short length of 1.5mm cable should be fine both for the amperage and to avoid voltage drop.

I think I will also invest in some sort of LED indicators to show the volts are on.

[Harry Bloomfield, Esq.]

Chris Holmes formulated the question :

No, there has never been a voltage sensing relay installed in the caravans. Before there were voltage sensing relays, a relay was used powered by the ignition warning light.

The whole idea was to isolate the caravan battery, from the car battery, so cranking current was not drawn from the caravan as well as ensure the caravan battery received a charge once the engine was running. That idea was extended to include running the fridge only once the engine was running (whether a battery was fitted in the caravan or not).

The habitation relay simply cuts power to the caravans internal lighting, heating blowers and etc. - the unnecessary loads whilst the caravan is under tow.

[williamwright]

Not that the caravan ever got any worthwhile charge, because of the voltage drop on the cable.

Bill

[Harry Bloomfield, Esq.]

williamwright pretended :

Exactly, which is why you rate the cable for volts drop along the length of it, rather than the current rating.

My caravan's battery charges up just fine. A good check to make, is the voltage across it, when the car's engine is running, versus the voltage across the car's own battery.

I also find the fridge works much better with a properly sized supply cable. Most installers just rate the cable for the current.

[charles]

Indeed so, Sombody designed a central battery system for emergency lighting in our village hall. He used the current rating for 240v to specify the cable. I managed to stop the project going ahead.

[Harry Bloomfield, Esq.]

Well done!

[charles]

by then, self contained battery emergency lights were just coming in and I used those, so we didn't have to rely on a gas emergency system. You can tell it was some years ago.

[williamwright]

Ideally yes. In practice the caravan battery never gets fully charged.

If you use the battery much on site it will initially charge quite well, but it will never reach completion. And the slow charge rate that results from the 0.25ohm (or whatever) cable resistance means that once the battery is half charged it takes 110s of miles to charge much more.

I have a trailer that carries disability equipment including a battery for emergency charge of a big scooter. In order to charge that battery on the move it has a 10A intelligent charger adjacent to it, powered by

240V from an inverter in the motorhome.

Bill

[williamwright]

Did you call him a dimwit, because it would have been apposite.

Bill

[charles]

i didn't, because he was the Chairman and I was just a newbie on the committee. He was a "power" engineer.

[NY]

Do you mean current or power? I can see that wire suited for 1 kW mains voltage will be rated for a much lower current than wire suited for 1 kW at

12V. But wire of a given cross section and therefore current-carrying capability will be equally suited for mains and 12V, won't it?

[charles]

Cable rating is based on a voltage drop over a given length at normal mains voltage, While 5 volt drop is quite allowable when the supply is 240v, it's not quite so good with a supply of 12v or even 24v.

[williamwright]

No because to get the watts at the end the current goes up as the voltage goes down.

To turn a water turbine you can either use a small volume of high pressure water or a large volume of low pressure water.

Bill

[Chris Holmes]

So I decided to go for the belt and braces option and run one fridge off each pair (via 2x 1 metre 2.5mm 2 core flexes) ..........

I only have the one fridge to test with at the moment.

I checked the "Fridge pins" (switched by a voltage sensing relay on the car). With the engine running and no fridge attached, they gave out 13.5V. With the fridge plugged in only 12.35, which I assume is just about enough.........

I then connected up (instead) to the other pins (normally used for lights / battery charging) (via a second voltage sensing relay which I intended to mount in the fridge box). This again gave 12.3ish under load, but the relay kept cycling every second or so.

So, plan Z is to wire both fridges to the car's fridge output wires, perhaps via 2 pairs of pins in a non standard configuration. But I'm worried the volt drop will be excessive when I've got 2x 80W rather than 1x.

I think I will have to "get out and get under" and see how the socket on the car is wired (I suspect that even if the car has butch cable(s) running front to back, the last foot or two may be done with a standard 7 core cable made of bugger all.

[williamwright]

A foot or two of thinner cable won't have much effect. Cable resistance is per unit length.

Bill