Maybe I’m misunderstanding the comments in this post but the 12v car bit has me wondering. When my caravan has stored on driveway fridge is off. A couple of days before a trip I switch it to mains to cool it down as 240v is always plugin.
When towing I switch the power switch on fridge to off. Car electrics supply the power to fridge. My understanding is that 12v battery selection on fridge switch runs off battery, at greatly reduced efficiency. No ?
Caravan fridge "efficiency" is often misunderstood and the 12V operation has gained a reputation for poor performance when its not actually the fridges fault.
The "efficiency" and "effectiveness" of the fridge is affected by a number of factors, which conspire in caravans to make it seem running on 12V is less effective. These negative factors are more to do with the installation and the way we use the caravan, than the fridge it's self.
Most caravan fridges use the Absorption refrigeration process. This counter intuitively requires a heat source which drives the process. Each of the energy sources, 12V dc 230V ac and gas , do the same thing, they convert the energy into heat, and broadly speaking each will provide roughly the same amount of heat to drive the cycle, so their thermal efficiencies are also much the same. Most under counter caravan fridges consume about 130W of power
If the conditions a fridge is used in are all the same, then the fridge would perform equally well on any of the energy sources including 12Vdc. However in touring caravans, when you select to run it on 12Vdc the installation is wired, so it can only receive 12V power supplied by the tow vehicles alternator is producing power. All this switching is not controlled by the fridge, but by the towing harness in the tow vehicle and the habitation controls in the caravan.
130W of power taken from a 12V source will produce a current of 10.8 Amperes (A) This would be enough to fully discharge a good 100Ah battery in about 9hrs. This is the reason they are only wired to take power from the tow vehicles alternator.
Especially in more modern vehicles Alternators do not produce electrical power all the time. The cars engine management systems only switch the alternator on when it detects a need for it, and unfortunately a caravan fridge is not always recognised by the ecu as a legitimate need. Consequently the fridge will not always be powered during towing. Some cars need to have be reprogrammed so the alternator does produce power mor consistently.
Another factor that can degrade the fridges performance ion 12V s the size of the cable used to make the connection back to the alternator. All wires do have some resistance which restricts the current that can flow through them. Most caravans use wire with a conductor size of 1.5mm CSA this has a resistance of 0.012 Ohms per metre With a current of 10A that means it will cause a voltage drop of approx 120mV per meter length.
The total conductor length between the fridge and the tow vehicles alternator will be dependent on the model of car and caravan but realistically it will be at least 3m in the caravan and 5m in the car a total of 8 or probably more in most out fits, but if we assume an 8m length 8 x 120mV =960mV but that is for the positive feed only, we also have to consider the negative return which could add up to another 8m which will add a further 960mV to make the total voltage losses of about 1.92V. which is removed from the voltage reaching the fridge. This reduces the fridges cooling capacity when on 12V power by about 20%.
As 12V powering is restricted to being sourced from the tow vehicle's alternator, its reasonable to assume if the fridge is receiving power from the alternator it is also likely to be moving and subjected to the stresses and strains of towing. You will see from other threads on the forum how towing can produce some surprising vibrations and motions in a caravan, and of course there is also the air moving around the caravan.
The performance the absorption refrigeration cycle is also affected by physical shocks and movement, both of which are present when the caravan is being towed. And as the fridge also needs external vents to allow the condenser to exchange the heat removed from the fridge, any air flow in the external vents caused by towing will also affect the fridges performance on 12V power.
The combination of all these factors can render the 12V performance of a caravan fridge to be quite poor compared to gas or mains 230V operations which can only be used when the caravan is sited and stationary.Despite all these negatives it can be useful to use 12V power, it will have some cooling capability even in a caravan thats being towed.
It should be noted that some three way fridges have been used very successfully on 12V only in other situations where travelling at high speed is not a factor. Some are fitted to boats, and some have been used in conjunction with a solar panels and battery storage for storing medical supplies in remote locations.
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