Camper Trailers Tech Tips

12volt compressor fridge draw



how long will my 12 volt fridge last
when camping?

suitable size solar panel & battery setup for long stays when bush camping

How long will a 12 volt compressor fridge run on a 12 volt battery when camping is a very common question, however it does not have a simple answer. It is a bit more involved than buying a fridge & plugging it into a battery. This article is not meant to be too technical, but a little theory is needed to explain how things work & how to select the right battery size to suit your needs without blowing your budget.

a little theory

Apart from Lithium Iron Phosphate batteries (LiFePO4) & the new Lead Crystal batteries that are now on the market, an AGM (Absorbed Glass Matt) deep cycle 12 volt battery is the best battery type suited for camping, as it can be discharged more deeply & accepts a charge more readily than a cranking battery which is used to start your vehicle. The capacity of an AGM deep cycle battery is stated in amp-hours (Ah).

On each 12 volt accessory there will be a label with electrical terms & figures. From this you can calculate the power an accessory will consume from an AGM battery? Listed on the label will be amperage, (A or amps) at 12 volt. There may also be 24 volt or 240 volt if the accessory has dual voltage input. If the label only shows watts & volts you can easily work out amps by dividing the watts by volts ie 100 watts divided by 12 volts = 8 amps.

Amp hour is the amount of current that flows in one hour. To work out the amp hours that will be consumed by an accessory from an AGM battery, you simply multiply the amp draw by the number of hours the accessory is running. As an example the accessory has 8 amps draw x 4 hours running time = 32 amp hours used.

You run a LED light under the awning of your camper trailer & it draws 2 amps.
* over 1 HOUR the light will draw 2 AMP HOURS from your AGM battery,
* if the light ran for 4 HOURS it would consume 8 AMP HOURS, that is 2 amps x 4 hours = 8 amp hours.

Let's say you run a 12 volt electric blanket for 2 hours before you jump into bed on a cold winters night so the sheets are nice & warm. The blanket draws 4 amps, therefore it will use 8 amp hours. That is 4 amps x 2 hours = 8 amp hours.

You set up a 12 volt water pump for showering which has a draw of 8 amps. When bush showering to save your limited water supply, you use the shower to wet down, turn it off & then back on to wash the soap off. All up the pump will be running for around 2 minutes per shower. In these circumstances the pump would use 0.25 amp hours per shower. If there were four people showering, the pump would use 1 amp hour.

fridge in awning shade on a hot day 40 litre Engel fridge in rear of our vehicle

12 volt compressor fridge

A compressor fridge/freezing may have a draw of 2.5 amps at 12 volt on the label as an example, however it does not run continually, but cycles or turns on & off to maintain the internal temperature you have set. There are a number of factors that will govern how often the fridge will run to keep the preset internal temperature. These include - 
* size of the fridge
* internal preset temperature
* ambient temperature
* how full the fridge is
* the number of times the fridge is opened
* the number of items at ambient temperature that are placed in the fridge
* the fridges position such as inside a hot vehicle.

So how do you work out what amp hours the fridge really draws over a twenty-four hour period. There are a number of meters around which you can plug into the power lead between the fridge & battery. The meter not only shows instant readouts, but also counts the amp hours used, which is a very handy tool to leave connected over twenty-four hours to give you the daily amp hours a fridge has used.

I have a Watts Up Meter which I have built onto a jiffy box so it can plug into different 12 volt accessories with cigarette lighter fittings. It can also be used between the solar panel & battery which uses Anderson connectors, so I can see what the panel produces over a day.

 If the fridge drew 2.5 amps & did run continuously, it would consume 60 amp hours over a twenty-four hour period. Here are the actual amp hour figures of our 40 litre Engel run as a fridge in the back of our vehicle when camping over a twenty-four period in three different ambient temperature ranges. As you can see the hotter the day, the more the fridge runs or cycles to keep the same preset internal temperature.

40 litre Engel actual power draw when camping
fridge dial fridge internal temp ambient min temp ambient max temp amp draw amp hours over 24 hours average amp draw run time
1.3 2.5 celcius 10 celcius 20 celcius 2.5 10 0.4 one-sixth
1.3 2.5 celcius 20 celcius 30 celcius 2.5 20 0.83 one-third
1.3 2.5 celcius 30 celcius 40 celcius 2.5 40 1.6 two-thirds

This is a test I did in the garage at home running the 40 litre Engel as a fridge & then on another day as a freezer after letting it settle at the freezer temperature

getting the best out of your fridge

By sticking to a few guidelines your fridge should work at the optimum performance when out camping.

* A cover protects the fridge & gives some insulation, helping to keep your fridge cool & reducing the power consumption.
* Cool the fridge & its contents before leaving home.
* When packing your vehicle make sure the fridge motor vents are given space to allow ventilation for best performance.
* Use the correct cable size from the 12volt battery for the amp draw of the fridge & cable length.  
* Keep your fridge cool by taking it out of the vehicle on a hot day & placing it in the shade under the camper trailer awning.
* Limit the number of times the fridge is opened. This can be hard with kids.
* Ice bricks or Techi Ice sheets placed inside the fridge will add to thermal insulation & reduce the power consumption.
* Keeping the fridge full rather than part empty will reduce the power consumption.
* Vacuum seal your meat & run as a fridge rather than a freezer will reduce the power consumption.
* Clean the inside after a trip with hot soapy water. We then wipe the inside with diluted vanilla essence.
* It is recommended that you run the fridge at regular intervals if its being stored for a period of time.
* Leaving the lid slightly open will reduce mould if not using the fridge. 

a little battery theory

An AGM or absorbed glass matt deep cycle battery is the best 12 volt battery type suited for camping as it can be discharged more deeply & accepts a charge more readily than a starter battery. As you can see on the graph below, continually discharging only 10% of the battery capacity may give you 5000 cycles, compared to continually discharging it to 90% which may give you only 400 cycles. It is better to have a large deep cycle battery discharged by a small percentage than a small battery discharging deeply.

depth of discharge v's number times recharge percentage of voltage at rest

Checking the state of charge of a deep cycle battery while it is charging using a voltmeter will only show the surface charge. When discharging a deep cycle battery the voltage will be below its true state of charge. You need to allow the battery to 'rest' for at least a couple of hours, preferably eight, for the surface charge to dissipate. It is like filling a glass with Coke & looking as if it is full, when in fact the top is froth. You need to let the fizz settle to see how much is really in the glass. The state of charge does not measure the storage capacity, performance level or the health of a battery. The best time to check your battery with a voltmeter is first thing in the morning before you open the fridge & before you place the solar panel in the sun.  

A battery monitor measures the amp hours in & out of a full battery via a shunt & is the most accurate way to gain the true state of charge. There are a number of different brands & types of battery monitors on the market. I installed an Everdrive Elite battery monitor on our camper trailer's AGM batteries

battery size

A basic 12 volt battery system can be as simple as a portable AGM battery in a plastic battery box to cover your 12 volt compressor fridge's power needs over a weekend & a multi-stage 240 volt battery charger around 10% to 15% of the battery capacity to maintain the battery at home between trips. From there you can add a portable solar panel to top up the battery during the day when camping & later perhaps a DC to DC charger to fully charge the battery from the vehicles alternator when driving between campsites on a touring holiday.

When setting up a 12 volt system to suit your needs always look at the bigger picture as a whole or what your ultimate battery setup will be. This will save you money in the long run as you can buy the correct size chargers & cable right from the start.

Lets say if your compressor fridge is run as a fridge & uses 20 amp hours over a 24 hour period. Over a two day weekend the fridge would use 40 amp hours. Therefore a 100 amp hour AGM battery would be suitable, keeping in mind not to discharge your deep cycle battery below 50% capacity.

If you are camping over four days, the fridge would consume 80 amp hours, so you need 160 amp hours of battery in total. One 150 amp hour battery is heavy & takes up a lot of space, so two 100 amp hour batteries would be a suitable choice installed in the camper trailer also spreading the weight. Even if you do have a solar panel, you cannot rely on the sun being out & you are more than likely to be running other accessories.

solar panel size

What size solar panel do you need to fully charge your battery? This depends on the season, your location & the available Peak Solar Sun Hours. Again it is not just a simple process of saying A+B=C.

Peak Solar Sun Hours is the number of hours when solar irradiance averages 1000 watts per square meter, however with a portable solar panel you can pick up more solar by pointing the panel to face the sun during the day. It is a good idea to choose your solar panel size so the battery is fully charged by after lunch.

You also need to take into account the output loss caused by temperature of the solar panel & cable voltage drop which can result in the panel only producing 70% of its rated output. Printed on the rear of a solar panel there will be two output figures, one at 25 degrees celcius & the other at say 47 degrees celcius. These are the actual temperatures of the panel itself & not air or ambient temperature. Typically there is 5% output loss for every 10 degrees celcius the panel is above 25 degrees celcius.

putting it all together

Richard's Solar Spreadsheet is an easy way to work out the battery & solar panel size you require by calculating the power usage of various 12 volt accessories

On the info page you will find a table for Peak Solar Sun Hours showing selected locations & months. In Sydney for example around the spring & autumn equinoxes when there is twelve hours daylight & twelve hours night, there are only 5.6 Peak Solar Sun Hours, if you have a fixed solar panel facing north.

In your calculations you can throw into the spreadsheet an overcast day with little or no solar input & see how the battery recovers via the solar panel. This can be important if you are setting up a remote camp for a period of time & relying only on solar to maintain your battery that is running everything.

solar regulators

A solar panel produces around 18 volts or more so this needs to be reduced as not to overcharge & damage the battery. This is done by connecting a solar regulator between the panel & the battery. A solar regulator can be mounted on the back of the panel or next to the battery that is being charged. There are two different types of solar regulators, PWM & MPPT.

PWM or Pulse Width Modulation slowly lowers the amount of power applied to the battery as the battery gets closer to fully charged. This allows the battery to be more fully charged with less stress, extending the battery life.

MPPT or Maximum Power Point Tracking regulators are able to convert excess voltage into amperage by calculating the output from solar panels to provide the maximum current possible at the required voltage at any given point. During low light level situations the MPPT will compensate for the low light level & find the new point at which the solar cell delivers its maximum power output.

Redarc BCDC1225 charger & Enerdrive battery monitor Ctek 240 volt 25 amp battery charger for home charging


After running the fridge from a suitably sized deep cycle battery over the weekend & keeping the battery capacity above 50%, you will need to charge the battery at home to maintain it in a state of good health. A rule of thumb for sizing a 240 volt multistage charger is around 10% to 15% of the battery capacity, that is a 10 amp to 15 amp charger is suitable to charge a 100 amp hour deep cycle battery. If you are looking at adding another battery sometime in the future, you will save money by buying a charger to suit your intended battery capacity from the start. 

There could also be times when you are on a touring holiday & run into some bad weather needing to drop into a caravan park to connect the 240 volt charger to your camper trailer batteries. A smaller charger will do the job, but the recommended 10% to 15% capacity will charge the battery overnight so you can continue your holiday with a fully charged battery.

What does multistage mean? The charger will go through three or more stages to fully charge the 12volt AGM battery. The most common multistage charger has three stages to give the battery a more complete charge. 

Stage one is also know as boost mode & charges the battery at a constant amperage until the battery voltage reaches around three quarters capacity.

Stage two is also known as absorption. This mode maintains the elevated voltage from the bulk phase, but adjusts the amperage accordingly. As the battery charge level approaches capacity, the current approaches zero. Absorption voltage output depends on the battery type, for example an AGM/Gel is 14.5 volts, standard lead acid 14.9 volts & a calcium battery 15.3 volts.

Stage three is also known as float. After the battery is fully charged the voltage is reduced to a lower level to reduce gassing and prolong battery life. This is sometimes referred to as a maintenance or trickle charge. Float voltage for an AGM/Gel, standard lead acid & calcium battery are all the same at 13.3 volts, the main purpose of which is to keep an already charged battery from discharging.

The same 10% to 15% rule of thumb applies when selecting a DC to DC charger which are also a multistage battery charger & charges the 12volt AGM battery in the camper trailer from the vehicles alternator when driving. Some DC to DC chargers also incorporate an inbuilt MTTP solar regulator. This means non-regulated output from the solar panel is connected straight to the charger which is installed next to the battery it is charging. Most commercial folding portable solar panels will already have a solar regulator on the back, however this can be kept on the panel & easily bypassed. The regulator on the back of the panel may come in handy one day to charge your vehicles battery or perhaps someone else's battery. 

cable sizes cable terminals crimped & heat shrink sleeving

cable size

Cable size is an important aspect of any 12 volt system. Undersize wiring is the most common problem as to why a fridge or any other accessory for that matter, does not work as it should. Undersize wiring causes heat & therefore voltage drop, which in turn is like running the fridge on a flat battery in extreme cases. Collyn Rivers recommends a 3% voltage drop is the acceptable maximum for runs from the battery to a 12 volt accessory.  

12 volt wire size can be very confusing as there are several wiring codes being used in the industry such as AWG, ISO, auto cable, B&S & square mm. To make it more confusing, auto wire measures the outer diameter of the insulation itself, having no reference to the actual copper conductor size.

The Solar Wind online calculator is an easy way to work out what size cable to use by placing the amp draw of the accessory & cable length in the windows Solar Wind has given permission to reproduce the cable size calculator found on their website.

The square mm result can be converted to the nearest B&S or auto cable size by using the table below. has permission from Collyn Rivers to reproduce his cable conversion table which gives a handy reference converting between the various cable sizes. Cable suited for 12volt work around the camper trailer under 6 square mm is usually referred to in auto cable size, while over 6 square mm is B&S. Twin core sheathed cable is used where the cable could be exposed to chaffing, giving it more protection. If the cable is exposed to the harsh environment on the outside of the vehicle or camper trailers it can be encased in plastic corrugated split tubing.

cable conversion table - ISO
















Auto Cable
















































Note: this table is copyright ‘Solar That Really Works' third edition, 2012
Caravan and Motorhome Books by Collyn Rivers

our 12 volt battery wiring diagram

our setup

On our camping trips we tend to roam, exploring as we travel from one bush campsite to the next, mostly staying only a day or three at each location The longest period we camp in the one spot is six days over Easter & ten days during the Australian CamperTrailers Group national meets, while the longest duration wandering has been over four months. We have no trouble keeping the batteries fully charged thanks to the Redarc BCDC1225 DC to DC charger. There is no need to visit a powered site at a caravan park.   

The two 120 amp hour Ritar AGM deep cycle batteries are housed in a box each side of the camper trailer & wired in parallel, that is positive to positive & negative to negative, making them one big battery of 240 amp hours at 12 volt.

The positive of the chargers are wired to the positive terminal on one battery while the negative of the chargers are wired to the negative terminal on the opposite battery. The same method is used with wires from the both battery terminals going to the fuse block. Connecting, charging & discharging batteries in this manner has proved very successful for battery longtivity as the batteries are now eight years old & show 12.8 volts at rest.

While at home the batteries are maintained using a 25 amp 240 volt multistage battery charger which is portable as there is no room in the battery boxes. The battery chargers charging outlet has an Anderson connector so it can be easily connected to the two camper trailer batteries when needed. I can also use the charger on the vehicles starter or auxiliary battery.  

Between campsites the 25 amp output of the Redarc BCDC1225 3-stage charger sees the batteries fully charged in most circumstances from the vehicles alternator while we drive during the day. When camped the batteries are charged via the MPPT solar regulator inbuilt into the Redarc BCDC1225 from the unregulated output of two 65 watt Kyocera folding panels. This is the Redarc install

The Plasmatronics PL1210 solar regulator was left in place on the back of the solar panels & bypassed to allow unregulated output to the Redarc BCDC1225 MPPT solar regulator. The solar panels can be used to charge our vehicles starter battery if needed via the Plasmatronics regulator. The two Anderson connectors on the solar panel are simple to use, unregulated output is red, while the grey Anderson has regulated output. Different coloured Anderson connectors cannot be joined together, however to keep it simple I have doctored the red Anderson. Alternately you could paint the unregulated Anderson with red nail polish or label it.

There is very little voltage drop from a ten metre length of 6mm twin core sheathed auto cable running from the solar panel to the MPPT regulator in the Redarc BCDC1225 next to the batteries. We have found it is a good length to chase the sun around the shadows of trees during the day.

An Enerdrive battery monitor is used to give us a accurate indication of the batteries state of charge. It does require periodical synchronizing so the monitor displays the correct figures 

red unregulated & grey regulated anderson connectors six blade fuse block & cigarette socket outlets

We have an average daily power consumption of around 40 amp hours over a twenty-four hour period depending on the season for the fridge power draw. On most days in camp the batteries are fully charged by after lunch thanks to the Redarc BCDC1225 MPPT solar regulator. The average battery draw broken down is..... 
* 20 amp hours average for the 40 litre Engel fridge running as a fridge
* 10 amp hours for the 12 volt Cpap machine running for 8 hours
* perhaps 10 amp hours average depending on what we are charging including.....
* laptop via the laptop 12 volt power supply 
* Canon camera batteries via a 12volt battery charger
* iphone & ipad via a USB port cigarette socket adaptor
* 12 volt charger for rechargeable AAA batteries used in hand held torches & head torches
* hand held UHF 12 volt battery charger

We have 12 volt chargers for all accessories as we did not want to go down the 240 volt inverter road. That may have been different if the Cpap was a 240 volt model. It can get complicated with the different 12 volt chargers & leads, but everything is labelled to avoid confusion. Bar clamps are useful to stop leads from tangling too.

laptop 12 volt power supply 12volt chargers for UHF, Canon Camera battery, AA/AAA batteries & USB adaptors for iPhone & iPad

further reading

Bill Darden battery facts
Collyn Rivers articles
Smart Gauge
wiring diagrams
voltage drop calculator 
Collyn Rivers page
Redarc BCDC1225 install
Redarc BCDC1225 feedback
Enerdrive battery monitor install 
Watts Up meter
35 litre Engel fridge draw

info by Rob


january 2018