Portable Fridges

A fridge keeps food cool enough so that it can be stored safely and it cools drink and food (chocolate, ice-cream) to make them more enjoyable in the Australian heat.

James Harrison in Australia was the first person in the world to use refrigeration commercially. That was in 1860 and he used it for beer and meat - some things just don’t change much over one and a half centuries! Some people feel that being able to enjoy a cold drink or cold fresh food on a hot day is one of the great achievements of our civilisation.

When we’re at home, we take it for granted that we can have a big fridge and freezer, but that’s because our city homes have plenty of cheap fulltime electricity available. But keeping our food and drinks cool when travelling is more of a challenge and this article will help you to find the best fridge for your needs, to make efficient use of the fridge and minimise the cost in buying the fridge.

The minimum size that most travellers use is around the 40 litre size. With careful packing, this can hold enough to feed two adults and two younger children for at least several days. Obviously you will not be able to keep all your drinks for the whole trip cold in this, but if the fridge is working well, you can refill it with food and drink early in the morning (see tips below on improving fridge efficiency).

The maximum size of fridge you can take will be limited by the space available in the vehicle, weight limits and the power available (bigger fridges will draw more power or use more gas).

There are many different types of fridges available, so firstly you will need to think about - the climate you’ll be travelling in (cool/tropical, summer/winter); how much food you’ll be taking; how much will be frozen; will you be staying in one place or moving every day; how much do you want to pay etc.

If you’ve never travelled with a fridge, then before spending a lot of money on a fridge, it would be a good idea to hire or borrow the type and size of fridge you are thinking of buying and use it on a shorter trip.

There are three fundamentally different ways that fridges cool food, and each of these types has advantages and disadvantages in different situations.

Compressor (2-way)

These fridges use a motor to pump the working fluid, just like your home fridge, but they run off 12, 24 or 240 volts.

• Electric power: 12v, 24v, 240v (some)


• Can run on gas: No


• Can warm food: No


• Can freeze food: Yes


• Cooling at 20 degrees: -18


• Cooling at 40 degrees: -10


• Maximum amps on 12v: 4 to 7


• Average amps on 12v: 1 to 3


• Amp hours per day: 24 to 72


• Noise level: Medium


• Heat output: Low


• Fast cooling: Yes


• Capacity range (litres): 13 to 110


• Price range: $600 to $2000

Absorption (3-way)

These fridges use a source of heat running on 12 or 240 volts and even gas.

• Electric power: 12v, 240v


• Can run on gas: Yes


• Can warm food: No


• Can freeze food: In cool weather


• Cooling at 20 degrees: -10


• Cooling at 40 degrees: +3 to +15


• Some higher priced models may also have a freezer compartment,
  which can hold -15 degrees on a 40 degree day


• Maximum amps on 12v: 10


• Average amps on 12v: 10


• Amp hours per day: 240


• Noise level: Low


• Heat output: High


• Fast cooling: No


• Capacity range (litres): 39 to 50


• Price range: $350 to $1200

ThermoElectric (Peltier)

These fridges use an electronic heat pump running on 12 or 240 volts.

• Electric power: 12v, 240v (some)


• Can run on gas: No


• Can warm food: Some


• Can freeze food: No


• Cooling at 20 degrees: +3


• Cooling at 40 degrees: +15 to +20


• Maximum amps on 12v: 4 to 6


• Average amps on 12v: 2 to 6


• Amp hours per day: 48 to 144


• Noise level: Medium


• Heat output: Medium


• Fast cooling: No


• Capacity range (litres): 7 to 32


• Price range: $50 to $400

Eutectic Fridges

These fridges don’t use a different way of cooling - eutectic pods are only a way of storing energy. E.g. if you have a eutectic fridge, then while you are driving, the eutectic pods are freezing, and when you stop and disconnect power from the fridge, the eutectic pods will start to thaw (just like an iceblock) and keep the food cold for the next day - but without needing any external power. Because eutectic pods need large amounts of energy to freeze them, they are usually only used with Compressor fridges.

There is a choice in fridge construction also. The exterior can be fibreglass, moulded plastic, aluminium or steel (could rust), with metal offering the most secure tie-down. The inside can be stainless steel, aluminium, fibreglass or moulded plastic. The insulation can be Styrofoam or polyurethane. With portable fridges you still have the option of buying an Australian-made product.

Extra Compartments

Experienced campers would understand the advantages in being able to have variable temperature zones, and separate compartments in their fridge, however most makes/models of camping fridge are simply a rectangular tub with top entry via the lid. The Twozone is an Australian invention, with a fridge compartment that can be fitted quickly and easily onto your existing fridge. This increases the height of the fridge, thereby providing more storage space, and creating another temperature zone where food items such as fruit and vegetables that only require moderate cooling can be placed. Accessibility is another advantage of having two compartments because lifting the top compartment, which also acts as the lid, allowing you to get to the food items at the bottom far more easily than pulling everything out.

There are a few simple techniques you can use to reduce the amount of electricity or gas that your fridge uses, or to let you get by with a smaller fridge.

Cable Size

It’s essential to use a cable size that will deliver adequate voltage to the fridge. You need to calculate voltage drop using the maximum current that the fridge draws - not the average - for Compressor fridges and Thermoelectric fridges this is up to 7 amps and for Absorption (3-way) this is 11 amps. But if the same cable is used to feed other loads or charge batteries, then you need to add this additional current when calculating the cable size needed to keep voltage drop below 0.4 volts. The amp rating stamped on the cable is not adequate to indicate acceptable voltage drop. If the cable feels warm when running at full load, then you are losing significant power due to the cable being too thin. Here is the smallest cable size to use (in Gauge and copper diameter) for 11 amps and 0.36 volts drop over several distances (assuming same size wire is used for the supply and earth leads)
2.5m = 12 gauge - 2.3mm 4m = 10 gauge - 3.2mm 6m = 8 gauge - 4.1mm 16m = 4 gauge - 6.4mm

Battery size

To calculate how much battery capacity (in Amp hours) you will need, you need to know the current consumption of the fridge (in Amp hours per day, which equals average amp hours times 24) under the conditions that you will be using it, plus the number of days before you can recharge the batteries. For long battery life, you need to plan to use only 50% of the battery capacity and to recharge them within a few days. You would only use up the full battery capacity if there was no sunshine at all for several days

Solar size

To avoid damage to your batteries you should always plan to recharge batteries fully in one day, although in winter this may mean the equivalent of 2 hours of peak charge current from a panel, depending on whether you are in Tasmania or Cape York. For details on solar calculations go to the Solar article.

"Amps aren't amp hours"

Don’t be dismayed if you get confused when you read about fridge consumption - many web pages and magazine articles have errors in the wording used for power consumption. Here’s a quick summary.

Instantaneous current flow - amps - for a compressor fridge this could be 6 amps when working hard, reducing to 2 amps when the contents have cooled down, then zero when the thermostat cuts out.

Average current - amps - for a fridge you need to average out over 24 hours, because the instantaneous current consumption varies so much with air temperature. For a Eutectic fridge, you may need to run the fridge for a day or two before taking measurements, because the Eutectic banks can store so much energy.

Total Current Consumption - Amp hours - as the name implies, you multiply amps by hours, so if your fridge uses 2 amps average, then over 24 hours it will use 48 amp hours. The daily amp hour usage of a fridge is easiest to use, because it is comparable with battery capacity in amp hours. Also you can add amp hour usage for different loads - e.g. if your Tent light uses 1 amp and you use it for 4 hours a night i.e. 4 amp hours per day, then your total draw from the battery per day will be 48 + 4 amp hours. You should not plan on drawing more than this from a 100 amp hour battery before recharging it

Better Connectors

Here are some alternatives to the Cigarette Lighter Plug that are suitable for all portable 12 volt fridges and will be more reliable.

Hella/ Merritt Plugs - these look like a smaller version of the Cigarette Lighter Plug, but are more reliable and rated for 8 amps continuous, so do not use them for 3-way fridges. Some Cigarette Lighter plugs have a removable collar which converts them to a Hella/Merritt plug.

Anderson Powerpole 30 - these Red and Black connectors with silver contacts are rated at 30 amps and are very small, will not fall out and are genderless (plug and socket are identical). They can take cable up to 10 Gauge or 5mmsq.

Anderson SB50 - these are the smallest of the grey Anderson connectors, they have silver contacts, rated to 50 amps, will not fall out and are genderless. They can take cable up to 6 Gauge or 13mmsq.

Connectors

Many fridges come with Cigarette Lighter plugs, and the only advantage of having these on your fridge is that you can plug it into any car - but only when the Ignition is switched on. Cigarette Lighter plugs can be a hazard because they were never designed for drawing power continuously - they can overheat or fall out, leading to warm, spoilt food ! If you must use a Cigarette Lighter plug, then make sure that you use plugs with a collar that latches onto the retaining springs (that are only found in some sockets).

Emergency Mode

This bypasses the Thermostat so at least you have full-power cooling if the thermostat electronics fail - a handy backup in the middle of nowhere!

Easy-to-use Controls

It’s much easier to get the right temperature if the controls show actual degrees in 1-degree steps. Some controls only allow 5-degree steps. Some have unmarked steps and you have to experiment and see what temperature you get from a setting.

Multiple Temperature Zones

To keep food for as long as possible, different temperatures are needed for different food types - Minus 18°C (frozen foods) - 0°C (meats - unfrozen) - 4°C (dairy and most foods) - 10°C (vegetables). However many small travel fridges only have one compartment, so you will have to compromise - e.g. no frozen foods, or keep them frozen but closer to zero degrees - which is OK if you are only keeping them for a week or two. Some a have small shelf to keep butter and cheese a bit warmer than other food.

Dual thermostat

Basic two-zone refrigerators only have one thermostat, so you can only directly control the temperature of one zone, so you have to hope the other zone will be close to what you want. With Dual Thermostats each zone will stay at the temperature you have set.

Turbo or Boost Mode

This sets the compressor to run harder to cool food down faster.

Built-in 240 volt supply

Some 12 volt fridges have the 240 volt supply built in, so saving you the cost of buying a separate supply ( or forgetting to take the 240 volt supply with you ! )

Suspension Base

Unless your vehicle has a very hard suspension and you travel on very rough tracks, you shouldn’t need a spring suspension base to protect the fridge mechanism or your food. This still won’t help with Absorption (3-way) fridges to keep them level so that they will work properly.

Slide

It’s a lot easier to get the food in and out, if you can slide the fridge out of the rear of the car, but make sure that the slide is designed to survive the impact load of the fully laden fridge on rough tracks.

Vibration-resistance

If you plan to mount the fridge on the drawbar of a trailer, then the fridge will have to be more vibration-resistant than if it’s mounted inside the vehicle.

Secure Tie down

A fully laden fridge could become a deadly projectile in an accident, unless it’s mounted so it won’t break loose from the enormous shock-loading in an accident. You could use handles or screw-holes already on the fridge, but consider that 4mm screws will not hold a 50kg fridge in place in an accident - think about how big seatbelt bolts are. You may need to have a securing strap that goes around the fridge and the Slide for it.

Lid Options

A support or gas-strut to hold the lid up is very handy, especially when the fridge is sitting on top of an extended cargo drawer. This provides travellers with excellent accessibility because it allows both hands to be free to reach in and search for food items that may be lying at the bottom.

Some optional high-rise lids allow you to fit in more food or tall bottles, even if it may be at a warmer temperature than lower in the fridge. Some lids also have magnetic seals, which helps to minimise heat loss.

Interference to HF Radio

The internal Inverters used to drive the motors in some compressor fridges can cause interference on HF Radio reception (VKS737 etc) that will make it impossible to receive any calls when the compressor is running. Test this before buying the fridge - it is very hard/expensive to eliminate it.

Quiet running

If you are sleeping near the fridge you won’t want any clunks whenever the fridge starts or stops, although you can get used the hum/hiss/gurgle of a properly running compressor or fan.

Cool running

In a heatwave you don’t want to have a heat generator beside you, so an Absorption (3-way) fridge is the least desirable here.

Low Voltage cut-out

To prevent damage to your battery, it’s important that the Fridge switch itself off and remove all current drain, when the battery voltage drops below a set limit e.g 12 volts, 10.5 volts. It helps if the cut-off voltage can be set by the user, based on the type of battery, the depth of discharge desired and the voltage drop in the wiring. Keep in mind that as battery voltage drops, a Compressor Fridge will draw more current to keep input power constant, so this means that voltage will drop fairly quickly towards the end, and therefore manual monitoring of battery voltage is not really practical.

Overload cut-out

Not really optional, but the more sophisticated ones will protect against fault conditions - over-temperature; over-current; over-voltage; or under-voltage (that can damage some appliances).

Automatic Fan Cooling

To get rid of waste heat in fridges over 200 watts, it really helps to have an internal fan and usually these are automatically switched on only when needed, to minimise power used.

Temperature Indicators

Imagine arriving at your campsite after a long day’s drive and finding your fridge contents at air temperature because a connector has fallen out; a control has been bumped; or an Absorption fridge was at the wrong angle! The best way to avoid this is to continuously monitor the temperature inside the fridge - this way you will get a warning as soon as something goes wrong and well before the food heats up. Some fridges have this built in, but it’s easy for you to add at low cost, preferably with an audible alarm to warn you immediately that the temperature rises. Some also have Maximum Temperature Memory so that you’ll know if your food has warmed at any time to a temperature that could cause dangerous bacterial growth.

Fault Indicator

So your food doesn’t spoil, you want to know immediately if there is a problem so a flashing light or a buzzer are best to catch your attention, rather than just a tiny light.

Service and Warranty

Check if there are Service dealers around Australia, if spare parts are readily available and how long the warranty is.

Internal Light

This is handy, particularly if the fridge is in a location where the car light doesn’t reach the interior.

Handles

A big, fully loaded fridge can be a chore for even two people to move, so you want strong handles that are easy on the hands. Folding handles save stowage space, but check that they don’t pinch your fingers when the fridge is full.

Insulating Cover

These are available for some models to reduce power consumption, but if they aren’t available for your chosen model, it’s not too hard to make one or have one made - just make sure that the fridge’s ventilation slots are not restricted.

Warm beer can be a nuisance, but food which has been stored at unacceptably high temperatures can cause food-poisoning - this could be just a nuisance - or it could be life-threatening when you are a long way from medical services.

A big fridge full of food will weigh more than 50kg and you don’t want that bouncing around on a 4WD track or come flying forward in an accident. You need to make sure that all mounting points on the fridge and the vehicle won’t break during an accident, where the impact loading will be many times the static weight of the fridge. If the fridge isn’t secured, then it could become a dangerous projectile that could even break some of the mounts on a cargo barrier.

If you have a gas fridge, then a Gas leak can be an explosion hazard or the fumes can poison you while you’re asleep. If clothing falls over the hot air outlet of a portable gas fridge, it could catch fire.

Battery capacity

All the power for the load has to come from the battery, so with high power Fridges or appliances you want to run for a long time, you need big, heavy expensive battery banks.

For Gas fridges, you need to transport enough gas for the whole trip. Gas fridges must be stored level otherwise they will stop working.

If you don’t go travelling often, then ice in an Esky/Icebox is an economical solution. Make your own ice bricks before you go by freezing briny water in plastic milk bottles. This type of ice-brick is more effective at keeping the Esky/Icebox cool for longer than if using block/crushed ice and also avoids food floating in water when the ice melts. Alternatively, you can use your portable fridge set to freeze to make the ice-bricks which can then be transferred to the Esky or Icebox.

If you just want to keep food cool, rather than chilled when camped, then you could do as the early settlers did and depend on the breeze cooling a wet towel draped over the Esky - like a ‘Coolgardie Safe’

You can keep meat for many weeks without having to keep it frozen, by having it packed in Cryovac bags to keep out all air - just be careful that the bags are not damaged by bones etc. (See Food and Water article)

Many bushwalkers and yachties eat very well without using any form of refrigeration - just by planning different menus and choosing foods and alternatives to fresh (eg. freeze dried, dehydrated, UHT - see Food and Water article). There are many new products and convenience foods on supermarket shelves so look into these options before planning a menu for a trip without refrigerated foods.