Fridge power

Agree that it is best to pre-cool the fridge before setting off on a journey as the 12 volt system from the car just seems to maintain the tempreture rather than drop it enough by itself.

Be aware that the 12V operation of your fridge is not very effective. Switching it to 12V is better than nothing, though, and will keep it cool for longer.

Interesting to hear that on 12v the fridge doesn't get propperly cold. I'm new to caravaning and have just wired in the twin electrics on my Defender so I can keep the fridge cold whilst traveling - setting an awning up without a cold beer? I think not!

I was intending to have the caravan plugged into the mains at home anyway to make sure the fridge is cold, battery charged etc. and then have the fridge running off the lesure battery in the Defender whilst on the move, I curently do not have a voltage sensitive relay cutting the power to the towbar electrics as they are fed from the second battery on the tow vehicle - if that was discharged the vehicle would start and operate fine.

Does anyone know how quickly the fridge would drain a 120ah battery? Whilst driving it'll be charged by the 120A alternator which should keep up with it no problem; not sure what draw the fridge takes just to maintain it's temperature?

Everyone will tell you that a fridge is not very effective on 12v.  Everyone includes me.

However if you look at a standard Dometic fridge, they typically  have a 230v element rated at 125W and a the 12v element rated at 120W which I would contend is no significant difference in power.   What's more the 12v element is not thernastaically controlled it carries on heating to cool the fridge as long it has a 12v supply.   By contrast the mains element once the fridge gets down to temperature switches off. So in theory over a period of timea 12v fridge should be as cold or even colder than a 230v fridge.  

Of course reality is nothing like this and there are two principal reasons. Firstly the wiring is to thin and the cable run is too long to get anything like 12v to the fridge so the element is not producing anything like 120W.  The other reason is that the fridge circuit on the tow bar wiring if it is actually wired in at all. Normally runs on a smart relay. These switch off if the voltage they see reduces below a minimum level and dont switch on again until a highr voltage is seen.  This is to protect the tow vechicle battery from going flat because of the fridge taking more than the alternator could put back in.   This is sensible, however I have found that switch off voltage tends to be too high and the switch on voltage is too low   On a previous car I had a Ryder towing relay  it was possible to adjust the trigger voltages on these to make them less sensitive and more practical  

Final the 12v element on the fridge should be independent of the caravan system so should only work when it is connected  vechicle to prevent the battery being flattened within a couple of hours   

Interesting to hear that on 12v the fridge doesn't get propperly cold. I'm new to caravaning and have just wired in the twin electrics on my Defender so I can keep the fridge cold whilst traveling - setting an awning up without a cold beer? I think not!

I was intending to have the caravan plugged into the mains at home anyway to make sure the fridge is cold, battery charged etc. and then have the fridge running off the lesure battery in the Defender whilst on the move, I curently do not have a voltage sensitive relay cutting the power to the towbar electrics as they are fed from the second battery on the tow vehicle - if that was discharged the vehicle would start and operate fine.

Does anyone know how quickly the fridge would drain a 120ah battery? Whilst driving it'll be charged by the 120A alternator which should keep up with it no problem; not sure what draw the fridge takes just to maintain it's temperature?

To keep the sums easy, assume that your fridge will take 10A and that you can only discharge the battery to 50%. So you have 60Ah available and that would last 6 hours. However, it’s not good practice to let a battery get that flat, especially if not recharged straight away. You’ll probably be fine if you chill the fridge well before setting out.

Interesting to hear that on 12v the fridge doesn't get propperly cold. I'm new to caravaning and have just wired in the twin electrics on my Defender so I can keep the fridge cold whilst traveling - setting an awning up without a cold beer? I think not!

I was intending to have the caravan plugged into the mains at home anyway to make sure the fridge is cold, battery charged etc. and then have the fridge running off the lesure battery in the Defender whilst on the move, I curently do not have a voltage sensitive relay cutting the power to the towbar electrics as they are fed from the second battery on the tow vehicle - if that was discharged the vehicle would start and operate fine.

Does anyone know how quickly the fridge would drain a 120ah battery? Whilst driving it'll be charged by the 120A alternator which should keep up with it no problem; not sure what draw the fridge takes just to maintain it's temperature?

To keep the sums easy, assume that your fridge will take 10A and that you can only discharge the battery to 50%. So you have 60Ah available and that would last 6 hours. However, it’s not good practice to let a battery get that flat, especially if not recharged straight away. You’ll probably be fine if you chill the fridge well before setting out.

AFAIK on our Swift, you cannot run the fridge on the leisure battery (you do need this for the fridge electronics which controls it when on mains or gas but this is a tiny amount) The two cooling elements are only  supplied from mains or when the tow vehicle engine is running?

Best to make sure your trailer connection is correctly wire to supply fridge and power for ATC etc. (if fitted) Check that fridge is running when engine is started before you leave and everything should be fine.

I see that you say you have a non-standard arrangement with additional battery supplying caravan electrics so this may well work as stated BUT if you do have ATC fitted, as the battery discharges it may reach a point where it does't work correctly and IMHO is a possible safety issue? Better to keep it standard (your choice of course)?

Thanks for the info! Just to add a bit more detail the Defender has a split charge system and a separate lesure battery it's self which is charged as I drive so I'm sure it'll be OK.

The feed from the lesure battery to the tow bar electrics is a decent size so there won't be issues there, not sure about the caravan it's self so I'll have to assume Avondale did that bit right!

If it is a 10A draw it should all be fine, I have voltage read outs for both batteries in the Landy so can keep an eye on things if I do stop. Sorry for the thread hijack BTW...

I might do a bit of a test over a couple of hours with the engine off just to see what the actual timescales are to drop it, it'd be nice to check it only takes power from the tow vehicle too.

I'd not heard of ATC before so I've already started leaning - I don't have it BTW, just the normal ALKO hitch. 

 ..... I'd not heard of ATC before so I've already started leaning - I don't have it BTW, just the normal ALKO hitch. 

ATC acts on the wheels more or less directly and an ATC eqiped caravan uses the same normal ALKO hitch 

I had never realised one had to switch on the main panel for the fridge to work when towing? Is this correct? How do I know if my car has a relay as mentioned?  I'm sure my fridge doesn't have an option to select 12volts, please enlighten me.