WORDS JOSEPH VAN WOERKOM PHOTOGRAPHY BY ROB CAMERIERE AND CARAVAN & MOTORHOME
EVEN A MODEST 50AH HOUSE BATTERY CAN PROVIDE SUFFICIENT 12V POWER TO FREE CAMP FOR A FEW NIGHTS
People often ask what 12V equipment they really need to have in their RV, which is a bit like asking how long is a piece of string. The answer is that RVers that always use caravan parks need very little while those that free camp for extended periods need a lot more. Let’s have a look at three different scenarios to see how these needs change.
USING CARAVAN PARKS
RVers who only camp at caravan parks don’t need a comprehensive 12V power system. In fact they don’t even need a ‘house’ battery as the only time they’ll need 12V power in the van is when they are actually on the move in order to keep the fridge operating.
For caravanners this 12V power is supplied from the tug’s alternator via a ‘hot’ wire, which can be wired into the 7-pin or 12-pin trailer plug or via a separate high current connector usually called an Andersen plug.
When camped all the RV power needs are supplied by the 240V mains provided by the caravan park. Note that this is true even if the RV is equipped with 12V lighting and other 12V devices like water pumps as the RV manufacturer will also have installed a 240V/12V power supply.
Many RV fridges can be powered from 12V DC, 240V AC and LPG. While travelling, 240V AC is not available and LPG should not be used for safety reasons so these fridges are usually run from 12V DC when on the move. Unfortunately they draw a lot of current when running on 12V and can quickly flatten even a large battery when the engine is not running.
To prevent this from happening, either disconnect the trailer plug when stopping for any length of time such as a lunch break or fit a motion-sensing Fridge Switch. This device disconnects the fridge when the RV stops moving and automatically reconnects it when it senses that the RV is moving again.
OCCASIONAL FREE CAMPING
RVers who want to be able to have the occasional stop over outside of caravan parks need to include a ‘house’ battery in their 12V power system. The ‘house’ battery is simply a battery in the RV that is not used for starting the engine and is there solely for the purposes of providing 12V power for camping purposes.
To keep the house battery charged while travelling caravanners will need a ‘hot’ wire between the tug and the van as previously discussed. However, even if heavy gauge wire is used in conjunction with an Andersen plug, the vehicle’s alternator will not be able to fully recharge the house battery due to excessive voltage drop along the long length of cable between the alternator and the house battery.
This voltage drop problem can be overcome by installing a DC/DC charger close to the house battery. These devices are in fact multistage smart chargers that can step up the low DC voltage at the charger’s input to the higher voltages needed to properly recharge the battery.
If the RV is equipped with LED lighting and other power demands are small, even a modest 50Ah house battery can provide sufficient 12V power to free camp for a few nights without having to recharge the battery.
EXTENSIVE FREE CAMPING
RVers that want to independently free camp for extensive periods will need a much more elaborate 12V power system. This usually includes a high capacity 12V or 24V battery system, DC/DC charger, smart mains charger, solar panels, solar regulator and an inverter.
The required capacity of the house battery depends upon how much energy will be extracted upon it on a daily basis and how long it needs to supply this energy without being recharged. In general these range from 50Ah to 500Ah and can cost anything from $150 to over $2,000.
To charge the house batteries three different energy sources can be used; the tug’s alternator, the sun and mains power. Many RVers elect to charge the house batteries from both the tug and from solar panels at the same time when travelling as DC/DC chargers and solar regulators are usually designed to work together.
In fact, some manufacturers now market combined DC/DC chargers and solar regulators in the one device. These smart chargers automatically select which energy source to use and provide smart charging to boot.
LOOKING AFTER YOUR BATTERIES
RV batteries are expensive items and so it pays to look after them to ensure that you get the longest life possible. The best way to do this is to ensure that they are never discharged below 50% State of Charge (SOC) on a regular basis.
Unfortunately SOC is not an easy thing to determine. While SOC can be determined by measuring the battery’s terminal voltage, to get a precise reading the battery needs to have been ‘at rest’ for a number of hours.
An accurate reading requires a sophisticated microprocessor controlled battery monitor that constantly monitors the current going into and out from the battery. By adjusting for charging and discharging losses, it can accurately determine how much of the battery capacity is left and display this to the user as a percentage of charge remaining, just like a fuel gauge. A SOC reading of 100% means that the battery is completely full and a reading of 50% means that only half of the battery’s capacity is left.
A good battery monitor will also be able to raise an alarm if the battery voltage goes too high or too low and even disconnect the battery via an external relay if the battery’s SOC or voltage falls too low.
Another device that can save the battery from excessive discharge is a Low Voltage Disconnect. The devices are typically located in the positive lead from the battery and will disconnect the load, and thereby save the battery if its voltage falls below a preset minimum.
|Amp hour (Ah)||An amount of current flow equivalent to 1 amp for 1 hour.|
|Andersen plug||A heavy current, low resistance plug/socket.|
|Battery||A chemical device that stores and delivers electrical energy.|
|Current shunt||Converts current into a small voltage.|
|DC/DC charger||A smart charger that is powered by 12V or 24V DC instead of mains power.|
|Deep cycle battery||A battery specifically designed for many cycles of low power delivery followed by recharging.|
|Hot wire||A wire between the tug and the caravan used to supply 12V power while travelling|
|House battery||The battery used to supply 12V power for camping purposes.|
|Inverter||A device that converts 12V or 24V DC into 240V AC mains power.|
|Resistance||Causes a wire to heat up as current passes through it. Smaller gauge wire has higher resistance. Doubling the length of a wire will double its resistance.|
|Solar panel||A device that converts sunlight into DC power.|
|Solar regulator||A device that ensures that the attached battery is not overcharged or damaged by the high voltage output by the solar panel.|
|Smart charger||A battery charger that changes its output voltage and current according to the state of charge of the battery.|
|Starter battery||A battery dedicated to starting the engine.|
|State of Charge (SOC)||The percentage amount of electrical power currently in a battery compared to its rated capacity in amp-hours.|
|Voltage drop||The reduction in voltage at the load end of a cable compared to the supply end due to the resistance of the wire carrying the current.|
AN EXAMPLE INSTALLATION
To give you an idea of what’s involved in a full blown free camping electrical setup, I recently upgraded my own caravan’s electrics courtesy of the guys at Bainbridge Technologies.
They supplied me with a combined DC/DC and solar charger, a 15A mains charger, a battery monitor, a 200A low voltage disconnect and a dual USB 12V/5V charger for charging devices like iPhones or toy helicopters.
The DC/DC charger can push as much as 25A into my 105Ah house battery from the tug’s alternator or via a portable solar panel. Since I will be bypassing the solar regulator on my portable solar panels to use the D250S, I have installed a red Andersen plug on the caravan and will do the same on my solar panels. The red Andersen plug will not mate with the grey one used for the tug’s hot wire so I can be sure that no one can accidently plug the solar panel directly into the tug and fry the battery.
The dropout voltage of the low voltage disconnect is programmable from 9.5V to 12V, defaulting to 10.5V. It also has a remote On/Off input which I wired up to the alarm output of the battery monitor to disconnect the battery if the battery’s SOC falls below 50%.