(Updated August 2015)
– this article was originally called Making a Caravan Battery Charge Faster and Deeper
Making caravan fridges work as claimed is simple, cheap and easy. Most can also be made to draw less energy. Most fridges are badly installed and/or need better ventilation. Boat, camper trailer and motor home fridges too (and home fridges) do not work as claimed or expected. These too draw usually draw excess energy. This article for boats, camper trailers, caravans and motor homes shows just why and how.
Fridges do not generate ‘cold’. They pump heat from where it is not wanted to somewhere it does not matter. For making fridges work as claimed it is essential that heat cannot find its way back in or around that fridge. Making fridges work as claimed is shown later in the article.
Big fridges use more energy than small ones, but not in direct proportion to their size. If all else is equal doubling fridge internal volume increases energy draw about one and a half times, not twice. It feasible have one large fridge rather than two smaller ones.
Door or top opening fridges
Some cold air is lost when a fridge’s front door is opened. Top-opening fridges lose less, and accordingly use marginally less energy. Door-opening fridges door seals must be perfect. If not, energy usage soars. If they are over three years old, replace them.
Making caravan fridges work as claimed from solar is feasible but they need sufficient solar capacity to cope with days of little sun: about 150-200 watts of solar per 100 litres, and 100-150 amp hours battery capacity per 100 litres. They need less battery capacity if backed up via a generator charging the battery bank via a grid fed multi-stage charger. See Making a Caravan Battery Charge Faster and Deeper. Alternator charging assists, but may necessitate driving several hours each day.
In temperate areas, most 40-110 litre chest and door opening electric fridges draw 0.7- 1.0 amp hours a day per litre of their size. Larger ones draw slightly less per litre. For any fridge, energy draw typically increases by about 5% for every 1.0 degree C above about 25 degrees C. See also re consumption in tropical areas – below.
Most fridges control temperature by cycling on and off. Energy draw is related to ratio of on/off times so one that draws more energy but is on less often, or for shorter times, may use less energy per day. Some new fridges run constantly, adjusting speed to maintain temperature. Only the total daily energy draw has meaning.
Do not set fridge cooling below 4.0 degrees C, nor freezer below -18 degrees C. Here again, every 1.0 degree C warmer increases energy usage by 5%.
Two or three-way fridges
These run from gas whilst camping, from the alternator whilst driving (and most from 110/230 volts when available). On 12 volts, small-medium fridges draw 12-15 amps, 170 litre fridges draw 15-20 amps, 300-litre fridges draw 25-30 amps plus. All will run from a car alternator but their draw is far too high for solar.
Fridges and climate class
Three-way fridges work only over tightly defined ambient temperatures. There are four (CEN Standard) Climate Classes. Those designated ‘SN’, and ‘N’ (Sub Normal, and Normal) work up to 32 degrees C, ‘ST’, (Sub Tropical) up to 36 degrees C. ‘T’-rated (Tropical) perform up to 43 degrees C. A minor drawback (for travellers) is that T-rated fridges do not work that well below 14-18 degrees C.
Only ‘Climate Class T’ work satisfactorily in climates such as north and north-west Australia (or tropical areas generally). Such fridges are marketed in Australia by Dometic and Chescold.
For making caravan fridges work as claimed – competent installation (as outlined below) is essential. Very few are: almost all can be improved by following that shown below. Some dramatically.
Take this very seriously. When owners compare fridges – they are mostly talking about their respective competency of installation.
No caravan fridge will cool a carton of non-cold beer in an hour or two! Buy it cold and put it straight in the fridge. Nor will any caravan fridge adequately freeze a lot of newly caught fish. Power will be drawn continuously, doubling or tripling consumption, yet will still not freeze quickly. Doing this requires a chest freezer made for such use, and generator powered.
Correctly installed and sensibly used RV fridges will work as intended, but marketing mostly advises what a product may do, not what it cannot do. A fridge’s temperature range is thus usually revealed the technical data, but not necessarily in promotional literature.
Gas and three-way fridges must suit the climate in which they are used. There may also be unrealistic expectations. Fishers (particularly) may grossly underestimate the energy needed to freeze their catch.
Tropical areas have less usable sun
A common and often disappointing mistake is assuming there is more solar input in tropical areas year-round. This is generally true of midwinter (the maximum is Spring and Autumn) but so solar input is mid summer is less than many expect. It is then also hot all day and often all night, so fridges draw up to 50% more energy. Meanwhile solar modules lose energy through heat loss. All this is thoroughly covered in Solar that Really Works, Solar Success and Caravan & Motorhome Electrics.
A quick guide to making caravan fridges work as claimed on solar is this: unless your existing solar system brings batteries to float voltage by noon on most days in temperate areas it has no chance at all of coping in tropical conditions.
Installation – making caravan refrigerators work as claimed
Few RV fridges are adequately installed, including many done ‘professionally’. Making caravan fridges to work as claimed is generally possible – and sometimes even better. And often at little cost. It is usually not hard to do but in extreme cases, almost total re-installation may be required. Even if already cooling well, it’s also usually possible to cut energy use.
Whilst seemingly 100% obvious, a fridge must not be in direct sunlight. One character, who had his outside in full tropical sun, complained: “my b..y mongrel Electrolux won’t keep my %#@^& beer cold.” He would listen to nobody who attempted to explain why.
Essentially – the heat from a fridge must be allowed (or assisted) to exit to outside the caravan (etc), and such that it cannot re-enter.
To do so fridges must have a cool air entry at base level, and a hot air exit, ideally at roof level. Most need baffles to direct cold air such that it can only flow through or over its cooling fins. Rising warm air may need channelling so none is trapped. The sketches below show the vital requirements.
The cool air vent can be at the side or through the floor (but not above or behind an engine’s exhaust outlet). Cool air must enter below the lowest cooling fin and exit well above the highest fin. The warm air is ideally vented to and through the roof, or if not feasible, via a side vent well above that highest cooling fin.
Baffles to direct incoming cool air can be of foam covered in aluminium foil, aluminium, plywood or even heavy cardboard. They must be within a centimetre or two of the cooling fins. Some three-way fridges can tolerate 6 degrees tilt, others only 3 degrees. Electric fridges are less sensitive, but are best levelled.
In some installations it may help to use a small ideally solar powered extractor fan – as shown below. These work well as their cooling is most needed when it’s sunny.
Solar-powered extractor fans are readily available (Google for suppliers).
Alternatively, those used in the larger desktop computers cost only a few dollars and can readily be run directly from a 5-10 watt solar module. All fans are best used to extract warm air rather than pumping in cool air.
Upper: Left – the baffles are too short and need to be just below the cooling fins. Rising hot air is trapped in the dead air spaces – as shown. If not fixable (as at bottom centre or right), an extractor fan driven directly from a 5 watt solar module would enhance air flow. Right – the upper air vent is far too low – hot air is trapped in the fins above it. Baffles are also needed. Below: How to install fridges correctly. Baffles really do help yet are rarely used. Note how rising hot air is channelled to the outside. This drawing is copyright: 2014 Caravan & Motorhome Books.
Electrical problems with 12 volt fridges
Most 12 volt fridge problems are due to grossly inadequate cabling. Many have only a quarter of that required. Check by seeing if the fridge cools better on 230 volts (where relevant). Cable issues can be worsened by faulty fuse holders, and particularly cigarette lighter plugs and their associated wiring. Scrap any such plug and wire the fridge directly to the battery by the shortest possible route.
A certain way of knowing if cable is too small is to measure the voltage directly across the battery, then directly across the fridge. (Do this with the fridge door open to keep it cycled on.) Many caravan fridges have close to 1.0 volt drop. Accept no more than 0.15 – 0.2 volt drop.
Adequate cable makes an extraordinary difference to making caravan fridges work as claimed. For electric fridge to battery distance of less than four metres, use 6 square mm cable (AWG/B&S 8). Over four metres use 8 square mm cable (AWG/B&S 7). If over four metres, move the battery closer. Do NOT use the auto cable sold by auto parts and hardware stores without first reading about it below – or in more detail in Caravan & Motorhome Electrics.
Do also see DC-DC Charging – this ensure the caravan battery and fridge receive their full required voltage. Installing one of these can totally transform a caravan or camper trailer fridge.
Auto cable problems
Appliance makers specify cable by its cross section in square mm. Auto cable makers (in effect) specify it by the size hole you can just push it through. They rate it by its diameter including insulation.
Auto cable sold as 4 mm is typically 1.8 square mm, but may be only 1.25 square mm. Six mm auto cable is typically 4.6 square mm. Countless fridges are connected by 4.0 square mm auto cable. This may be only a quarter the size of the minimum specified. No fridges wired that way work as they should and usually can.
Direct comparison is not possible for auto cable as its size varies from maker to maker. On exception is that 6 mm auto cable (typically 4.56 square mm – or 10 AWG) can be substituted for 4.0 square mm cable, 8 mm auto cable is between 7-8 square mm (8 AWG).
Cable current rating trap
Cable ‘ratings’ (e.g. ‘50-amp’ etc) indicate only the current that cable can carry before it melts. It tells nothing at all about voltage drop: that is also a function of cable length. It is useless asking vendors about this. Few even know it’s even an issue. For caravans, locate the battery close as possible to the fridge. If alternator charged install a dc-dc alternator charger close to that fridge. See Article – Dc-dc Charging on this site.
Never use cable lighter than advised here. If you do the fridge will never work correctly. An exact way of establishing the best cable size is shown in my related books, Solar that Really Works, Solar Success and Caravan & Motorhome Electrics.
Fridge in cars and 4WDs
Making caravan fridges work as claimed in cars and 4WDs is more of a problem. Keep them out of direct sunlight, and leave an air space around the grill’s vent area. It is however fine to pack stuff close to or touching them – except for the eutectic types mentioned above (these must have a 50 mm air gap each side as the heat is dissipated from their sides).
All types of fridges can nevertheless be improved (some dramatically) by running a heavy cable (ideally 10 square mm (6 AWG) from the battery to that fridge.
A few boat and RV fridges, such as this Australian designed and made Autofridge, dissipate heat from their side walls. These fridges need an air gap of 50 mm at each side. Pic: Autofridge Australia.
Fridge service hints
Fridges with external cooling fins benefit from extra heat insulation but some fridges, such as the Intel and Autofridge, dissipate heat from their side walls. These ideally need a cool air feed at the base of their sides and must have an air gap (optimally of 50 mm) on either side.Door seals leak after a few years. You can buy replacements from stores such as Clarke Rubber.A three-way fridge that works on 230 volts but not gas may have an incorrect jet – or need adjusting. It may also be an imported fridge intended for LPG different from that in Australia. Here, you need expert advice.
RV fridge energy consumption
A fridge’s energy consumption is directly related to ambient temperature. All use about 5% more for every 10 degrees C above 25 degrees C. Exactly the same applies to the preset fridge and freezer temperatures. The fridge optimum is +4 degrees C, freezer optimum is -18 degrees C (some RV owners settle for -14 degrees C to save energy).Let food cool before placing in the fridge. Always defrost frozen food in the fridge section so that it cools that section. Keep bought frozen goods cold in a heat-insulated bag and put them in the fridge as quickly as possible. Let warm beer first cool overnight.Keeping the fridge full reduces cold air falling out when opened. Leave gaps for air to move, but fill otherwise empty spaces with plastic vessels full of water.
Particularly for boats, camper trailers, caravans and motor homes, you need the right fridge, competently cabled and physically installed. This is particularly so if travelling up north at it stays hot there also all night. Making fridges work as claimed (or even better) – and energy draw actually reduced by is totally possible by making the above changes.Except for the very cheapest fridges, dismiss claims of inherent deficiencies. Providing a fridge is appropriate for its proposed use, cooling problems are almost always due to installation failings.For domestic fridges, and fridges in cabins, virtually all of the above is relevant. It is however not likely that cabling will be an issue with a 110 or 230 volt fridge.A great deal more on three-way and electric-only fridges is in Caravan & Motorhome Electrics. It even shows how to build your own fridge that leaves commercial units for dead in cooling and economy. There’s also a lot of information about running them from solar in that book, and also in the 3rd Edition of Solar that Really Works! If the fridge is large and in a large home or on a property consider also Solar Success.