Fridges:

When we go camping or caravanning one of the first items is generally a fridge – it tends to be one of the first upgrades from an esky-and-ice, and it’s well worth the investment – and I speak from painful experience!

There are a number of different choices, and in this area we will encounter a good deal that will ultimately boil down to personal choice. What I’d like to do here is to help us base that personal choice on some facts, so that the choice fits best with the way that you camp and travel.

Types of Fridges:

There are 3 basic types of fridge, each with an underlying technology:

•  Compressor fridges e.g. Engel, Waeco, etc.

•  Gas fridges

•  Thermo-coolers

So let’s be different and start with the last one first.

Thermo-coolers:

The thermo-cooler has no compressor, and uses a so-called Peltier-module which is a solid-state electronic device. When a voltage is applied to a Peltier-module, one side becomes hot, and the other side cold. So if we put the cold side in the fridge compartment, and keep the hot side on the outside, hey-presto we have a fridge!

Typically these thermo-coolers can get the inside temperature down to about 20 degrees below the ambient temperature of the air around it. Another feature is that if we reverse the current, the hot and cold sides swap around too, so we get heating on the inside instead – this is typically done with a little switch indicating heating or cooling.

Now, it’s that “20-degrees below ambient” that pretty much defines its usage – that and the fact that it’s relatively power-hungry. If we are sitting inside the cab of an air-conditioned truck at say 25°C then the thermo-cooler will get things down to about 5°C which keeps our drinks nice and cool as we travel along.

I say “as we travel along” because these coolers draw in the region of 5 Amps, which is fine if the motor is running and the alternator charging, but not so good if we’re hoping to keep it going 24-7 off a battery. A current draw of 5 Amps over 24 hours means an energy consumption of 120 Amp-hours per day – enough to completely flatten a 100Ah deep-cycle battery in less than a day, with ease! So this definitely limits its use to while we’re driving – and hey, in that situation it works just fine, no problems.

Now to that thing again about 20 degrees below ambient. If we’re not inside an air-conditioned vehicle, and the ambient is a beautiful 40 degrees in the Kimberley, then our drinks will be at a less-than-lovely 20°C. I have personally measured over 50 degrees in the back of our 4WD, not even at the height of summer, and with that kind of ambient temperature the thermo-cooler is well and truly out of the game!

But that said, thermo-coolers are the least expensive option of all – they eliminate the costly compressor so they’re just a fraction of the price of a small compressor fridge – and that can often be a big plus. And if you’re only going to use it while you’re driving, this is a great option, and provides a level of convenience that makes our travels a whole lot more enjoyable.

Gas Fridges:

I avoid calling them a 3-way fridge, because essentially its whole design is premised on it running off gas – the 12Volt and 240Volt options are essentially add-ons to get us through times when we don’t want to run it off gas.

There are many good websites, books, etc. that describe the inner workings of gas fridges (absorption fridges), and how best to install them, so I won’t go into that here. Let’s just summarise it all by saying that a gas fridge works by making its working bits hotter than the surrounding air, and in so doing it cools the interior compartment of the fridge.

Longer Stays:

The gas fridge is ideal for longer stays where we are wanting to camp up at a particular place for a week or more. It then works very well, and some of the more modern ones even include a thermostat which switches off the gas when the fridge is cool enough, and reignites the flame once it needs the cooling again. They can be very frugal on gas as well, and there is also no need to worry about the draw on the 12 Volt system when gas is being used.

Hot Climates:

Being a heat-driven device, its efficiency is however affected by the temperature of the surrounding air, so they tend to suffer a bit in the tropics. Proper installation, ventilation, and choosing the right Climate Class of the fridge can certainly help though. There are four Climate Classes, from Sub-Normal, to Normal, Sub-Tropical and then Tropical. However as most of us travel across all of these climatic regions, selecting just one of these classes for our fridge means that there is really only one region where the fridge will work optimally.

Operation off 240 Volts:

When there is 240 Volts available, such as at a caravan park, then instead of using up the gas, a 240 Volt element supplies the heat instead of the gas flame. For instance, a popular 150-litre gas fridge uses a 210 Watt element to achieve this – not very efficient, but typically we pay a flat fee for power in caravan parks, so this inefficiency is pretty much immaterial.

Operation off 12Volts:

Gas has to be switched off when travelling, and 240 Volts is also not available, so we must lean on the 12 Volt system to keep things cool while travelling. This is probably where gas fridges get their worst reputation, but most often it’s not entirely the fridge’s fault.

The same 150-litre fridge mentioned above uses a 175 Watt heater for the 12 Volt system, so it’s drawing around 15 Amps. This is a pretty hefty current, and needs a lot of care if the fridge is to have a hope of operating correctly. Routing this current through a 7-pin or 12-pin trailer plug is usually the first obstacle. Despite the optimistic ratings on these plugs, they unfortunately don’t handle this sort of current well, and certainly not for any length of time. A 50Amp Anderson plug is a much better option, and a worthwhile investment – it will be safer, and minimise the volt-drop and heat-stress involved in trying to get this amount of 12 Volt power to your fridge.

The second important obstacle to remove, as the 15Amps of current makes it way to the fridge, is volt-drop in the cable that supplies this power. There is a volt-drop calculator elsewhere in The 12Volt Blog, and making sure we get less than the Australian Standard of 5% volt-drop along the line, will simply mean that you get more of the power into the fridge, and waste less power in the cable. Typically a 9mm automotive twin cable will do the trick for runs up to 5 metres; longer than that consider going to a 14mm twin. Most fridge installation instructions warn against using a single wire and chassis-earth – I would agree 100% with that[1], and by using the Anderson plug above, this automatically avoids this problem as well, and ensures the fridge’s earth connection does not come via the trailer plug.

Because of the high current draw it is also important to have the 12 Volt supply switched through the ignition, using a relay. This will prevent the fridge from flattening the battery while you park up for a quick cuppa! This is generally contained in the installation instructions for the fridge anyway.

And finally, if the same cable that feeds the fridge is also being used to charge the ‘van battery, please make sure that there is something in there to make sure the fridge doesn’t draw off the ‘van battery. Either a DC-DC charger or some other arrangement will do it – the fridge on 12Volt is a hungry beast and it’ll grab power from wherever it can, so just ensure the wiring arrangement prevents it from flattening the ‘van battery.

So, the gas fridge – it can be tamed to fit most situations, but as you can probably tell, it’s not my favourite. I’m quite happy to declare that as a personal preference, and leave everyone to reach their own decisions independent of that. And I’m equally happy when I hear from those who have enjoyed many years of faithful service from gas fridges.

Compressor Fridges:

Ok, these fridges are my preference for a number of reasons which will become clear as we go through this section. And of course there will be disadvantages, so let’s get those out there now.

Disadvantages:

First, they use 12 Volt power – yes I know, strange that, but it’s true – and what’s important is that they use it 24-7, so the point is that we’ve got to then have something to put all that power back into the batteries. For instance a 40 litre chest fridge needs roughly 150 Watts of solar to support it, year round. (for more accurate calcs take a look at How much Power 12Volt stuff uses).

Second, they make a noise – ok noise is perhaps a bit strong – let’s say they purr. I once spent time close to a compressor fridge at night while camping, and that convinced me to put it far from the tent for the rest of the trip – in the back of the 4WD in fact. That arrangement now works well on a repeat basis, so for me that’s problem sol-ved. In contrast, I’ve also spent time in a motorhome with a gas fridge, and it is almost completely silent – can’t beat that!

Types of Compressors:

Compressor fridges cycle on and off to keep a constant temperature inside the compartment, and this will vary with how many times it’s opened, the outside temperature, how much room-temperature stuff we put in it, and so on. There are two types of compressors used in camping fridges, the SawaFuji compressor which is used in Engel fridges, and the so-called Danfoss or Secop compressor used in Waeco fridges and many other brands.

There is heaps of technical info out there on these two compressors, so I won’t repeat that here. Basically the Engel is a one-moving-part unit with the compressor’s piston being moved back and forth directly by electromagnets. There is no separate motor driving the compressor – the piston is the compressor and the motor all in one.

In contrast the Danfoss, or Secop compressor as it now is, uses a standard combination of an electric motor driving a compressor, contained in a sealed unit. This standard arrangement has the advantage of being able to incorporate counter-balances within the motor-compressor which can help to reduce vibration and noise.

Advantages:

The biggest advantage of a 12 Volt fridge is the simplicity of wiring it into the system. Just give it 12 Volts (or even 24V) and it’s happy! And if there is 240V available they just switch over automatically to 240V and save your batteries[2]. So we don’t have to remember to switch from one power source to the other. And when we’re towing it will simply draw off the 12 Volt system which will probably be getting charged while you’re travelling along anyway.

Hot Climates:

Compressor fridges also operate very much better than gas fridges in the tropics – of course they will work a bit harder, and draw more current, but they will keep the drinks cold – and that’s important!

So, all-in-all it’s the simplicity, efficiency and climatic-flexibility that leans my vote towards these compressor fridges. That said, to each their own, and I’m quite happy to spend time around the campfire hearing why others prefer their type of fridge.

240Volt compressor fridges:

What about a little 240 Volt compressor fridge running from 12Volt through an inverter – they’re much cheaper, and inverters don’t cost much – so how about it? Ok, let me try and keep this short.

There’s a reason 240V bar-fridges are cheap – they’re made to a price, but they’re not efficient[3], whereas 12Volt fridges are made with efficiency as the main goal, so their price is higher. There’s also a thing called power-factor when we deal with 240V AC fridges, and the short version is that the inverter has to cope with this, and therefore uses even more power[4]. All of this means that to run a 240V fridge, our 12Volt system will need solar and batteries that are 2-3 times bigger than what they would be for a 12Volt fridge.

Next there’s the start-up current of the fridge – about 5-6 times its normal running current, so up goes the size of the inverter. And the fridge’s thermostat and other electronics is pretty sensitive stuff, so it’s pure/true sinewave only – and up goes the price again.

So, all-in-all, there’s very seldom merit in going this way, and it’s unlikely to save money either. There are exceptions, but generally the answer is no.

So, 3 basic types of fridges, and even when we’ve decided which type is best for us, there are still heaps of brands out there to choose from – have fun!

[1]: the problem with a chassis-earth is not the conductor – the chassis has heaps of metal to conduct the power – it’s connecting onto the chassis itself that is usually the issue. The smallest bit of corrosion, relentless vibration, heating, cooling, rain, etc. puts the chassis-connection seriously at risk. Just run a 2-core cable to the battery and be done!

[2] Some are made as 12-24V only, to reduce cost & bulk, often used on yachts

[3] Have a look at the Energy Rating of bar-fridges next time you have the chance – much worse than the bigger ones.

[4] Power-factor correction can save up to 40% power, so maybe worth it, but your electrician needs to know his/her oats.