Camper Trailers Tech Tips

setting up a 12volt system for your needs





12volt system
to suit your own needs

One question that crops up from members in the Australian CamperTrailers Yahoo Group, Forum & Facebook Group relates to what is needed in setting up a 12volt battery system in a camper trailer, the type of battery to use, the size of battery & the size of the solar panel.

Most will start with a deep cycle battery & a 240volt multistage charger to maintain the battery at home between trips, then add a solar panel to charge the battery in camp & perhaps a DC to DC charger for touring holidays at a later stage. It is a good idea to have your final set up in mind which will save you money in the long run.  

I hope this article gives a little insight in helping to set up a 12volt system to suit your own needs without being too technical. 


A deep cycle battery, preferably an AGM or absorbed glass matt, is the best battery type suited for camping. These batteries can be discharged more deeply & recharged more readily than a starter battery used in a car.

As you can see on the graph below left, discharging only 10% of the battery capacity may give you 5000 cycles, compared to discharging to 90% which may give you only 400 cycles.

Checking your camper trailers deep cycle battery for state of charge while it is charging using a voltmeter will not show its true state of charge as all you are reading is the surface 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.

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

battery size

A basic 12volt battery system can be a portable AGM battery in a battery box to cover your power needs over a weekend.

So lets say  if your compressor fridge run as a fridge uses 20 amp-hours over a 24 hour period & you are camping for two days, you will use 40 amp hours in that time. Add to that a couple of hours running LED lighting, four hours at 0.5 amps, that would be 2 amp-hours. Total power needs over a two day weekend equals 42 amp-hours. Therefore a 100 amp-hour battery would be suitable.

appliances & accessories

The most common appliances & accessories people use while camping is a 12volt compressor fridge such as an Engel or Waeco etc, recharge mobile phone, laptop or camera batteries, perhaps running a Cpap machine & LED lighting at night.

It is hard calculating the power usage of a compressor fridge as there are so many factors involved such as the fridges size, the  rated power consumption of the motor, the ambient temperature, where the fridge is placed, how many items at ambient temperature is placed in the fridge, the fridges preset temperature, to name a few.

The fridge will not run all the time, but runs & then turns off or cycles to keep the preset temperature. By using a Watts Up Meter I was able to see exactly what the draw of our 40 litre Engel fridge when run as a fridge, while the graph in Jeff's article will let you see how it all works maintaining his 35 litre Engel's internal temperature.

 battery charging at home

After running the fridge from a suitably sized deep cycle battery over the weekend & keeping the battery capacity above 50%, you will need to recharge the battery at home to maintain it in a state of good health. A 240volt multistage charger that is designed to charge an AMG battery is the most suitable. These chargers usually have three or more modes in the charging process, ie bulk, absorption & float to fully charge the battery.

It is recommended a 240 volt multistage charger of around 10% to 15% of the battery capacity be used, that is a 10 to 15 amp charger is suitable to charge a 100 amp-hour deep cycle battery. A smaller size charger will take longer to do the job, but will not knock the sulphate off the plates.

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. 

solar in camp

A solar panel is the most convenient way to charge the battery when camped. The size of the panel will be governed by your power usage & the availability of the sun depending on the time of year. It is best to size your solar panel so the battery is fully charged just after lunch.

A typical solar panel will have an output of around 18 volts which is way too high to charge a battery. A solar regulator is needed to regulate the power going from the solar panels to the battery. Overcharging will significantly reduce battery life & at worst damage the batteries to the point where they are unusable.

A portable solar panel kit has a solar regulator on the back of the panel. There are two different types of solar regulators used, a PWM & a MPPT.

PWM or Pulse Width Modulation slowly lowers the amount of power applied to the battery as the battery get closer to fully charged.

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.

calculating a system

So how do you go about sorting everything to make a 12volt system to suit your needs.

Redarc have an easy to use 12volt system calculator

Richard's Solar Spreadsheet is a more accurate way to see what size battery & solar panel you require by calculating the power usage of various appliances & accessories you wish to use at different locations & times of the year

If you run the fridge as a fridge you could offer a rough average power consumption per day to the spreadsheet by entering one-third of the rated amps. This figure can be found printed on the back of the fridge in A or amps as 'rated DC current' or 'DC power consumption'. If the fridge is rated at 4.5 amps then over an hour with the motor running 33% of the time, the power consumption would be 1.5 amp-hours.

This is the power draw of our 40 litre Engel

On the spreadsheet info page you will also find a table for Peak Solar Sun Hours which will allow you to see how a solar panel will charge the battery for a select location & month. You can also throw in an overcast day with little or no solar input & see how the battery recovers.


Voltage drop is the biggest downfall in charging a remotely located battery in a camper trailer which is a long way from the vehicles alternator, sometimes around ten metres or more. Recent advancements in battery charging technology such as DC to DC chargers, now sees batteries charged to 100% capacity while driving. This gives extra battery capacity when stopping for a few days at a time in different campsites on a touring holiday, rather than having to chase the sun with a solar panel on arrival after a day on the road.

further reading

Bill Darden battery facts
Collyn Rivers articles 
Smart Gauge
wiring diagrams
voltage drop calculator

cable conversion chart
cable size calculator 

Redarc BCDC1225 install
Redarc BCDC1225 feedback

Enerdrive battery monitor
watts up meter
35 litre Engel fridge draw
40 litre Engel fridge draw



by Rob Sanderson




december 2015