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Charging RV batteries

Battery State of Charge

Caution:  Any electrical work or maintenance of battery chargers or inverters must be carried out by qualified personnel.  Always switch off any AC power supply and disconnect the battery before any maintenance is carried out and let the electrician know if there is an inverter. 

Always comply with battery manufacturer’s safety instructions and ensure the required system voltage does not exceed the product’s capacity.  Protective devices such as fuses, circuit breakers and safety switches are required to meet relevant safety standards.

240v SymbolPower Voltage in Australia - 240V or 230V?

Australians have commonly referred to AC mains power as 240V. However, in 1983 the International Electrotechnical Commission set out to achieve an international standard 50Hz supply voltage of 230/400 volts by 2003.  On 23 February 2000, a new voltage standard, AS60038, was published in Australia to replace the previous 240V standard. Mains voltage in Australia is now 230V 50 Hz.


There are several ways to obtain power to charge or maintain the charge of your RV batteries
Anderson Plug

Charging the Caravan/RV Battery While Driving - Tow Vehicle Alternator – as you are driving your vehicle, with a caravan, or 5th wheeler or camper trailer under tow, the wiring from the tow vehicle can be linked to the RV for the vehicle to charge the RV batteries.  Heavy duty cables are run from the tow vehicle’s alternator to the tow bar where there will be a connector such as an Anderson plug.  Heavy duty wiring will also be run from the RV batteries along the ‘A’ frame or draw bar with another Anderson plug which connects to the one from the tow vehicle. 

RV three way refrigerators as they are commonly known (12V/LPG gas/230V AC) run on 12V power sourced from the vehicle alternator whilst the RV is under tow then on LPG gas or mains power when on site. 

A tow vehicle's standard battery does not have the voltage capacity, nor is it designed to, power the number of accessories a traveller wants such as fridges, camp lights and inverters. The ability to charge an in-house battery in your caravan from a vehicle alternator will be limited to any spare capacity of the alternator left over from what the vehicle needs to run itself and any portable chest refrigerator you may have in the vehicle itself . You should seek specialist advice if you want to have a set-up to do this as there are a lot of factors that come into play such as a specialty alternator, the cabling, the type of battery, the type of vehicle you have and how long you drive each day.

DC-DC Chargers

DC-DC Chargers can also be installed to transform the output of the alternator for faster and more complete charging of the battery bank. Redarc produce a DC/DC multi stage DC-DC battery charger that allows charging of the auxiliary battery of different chemical characteristics to the source battery, act as a dual battery isolator and protect the start battery if the voltage is too low. Redarc also have the BMS1215S2 Battery Management System for multistage AC-DC and DC-DC designed to charge batteries to their optimal charge from either AC mains power, DC vehicle power or solar power should your RV have solar panels installed.


Generators – generators convert mechanical energy into electrical energy so when a generator is plugged into your RV you are able to run 240V (230V AC) appliances as well as the lighting.  Generators can also be used to charge deep-cycle batteries if batteries have been installed in the RV.  Obviously, you must have fuel to run the generator.  Generators can be portable or located in built-in compartments in RVs, such as those in larger motorhomes.

The generators used for charging batteries must be capable of supplying a ‘constant’ power supply and also be capable of supply for the AH size of the charger.  To avoid any variation to constant power make sure you have filled your generator with fuel before charging the batteries because as it begins to get low on fuel the output is likely to fluctuate. 

Honda 2kva Generator

The downside of using a generator is the noise, the ongoing cost of fuel and disturbing the serenity of fellow campers so please be considerate.

Battery ChargerIf you want to use a generator to run 230V AC appliances such as a micro wave oven or air conditioner you may need to turn off the RV battery charger as it is likely to cause an overload on the generator unless the generator is suitable sized to run the battery charger and appliance(s) at once.

To charge a deep cycle battery in your 4WD using a generator you will need a suitable mains powered charger.  Look into a 3 stage charger for this purpose.

Battery Chargers

All batteries will slowly discharge over time so a battery charger is required to recharge them.  Wikipedia defines a battery charger as ‘a device used to put energy into a secondary cell or rechargeable battery by forcing an electric current through it’.

Xantrex Battery Charger For three or four stage chargers you will hear terms such as ‘bulk’ or ‘boost’, ‘absorption’, ‘float’ and ‘storage’ phases.

Boost phase = is the first stage of the charging process. Essentially it is the stage where the charger puts in as much current it is capable of and as the battery storage voltage increases the charge rate decreases until the battery is about 75% full.

Absorption phase = this is the second stage of the charging process where the charge going in is reduced to about half of the ‘boost’ rate or a constant rate for several hours (unless a heavy load is drawn and it will then cycle back to the ‘boost’ stage).

Float phase = at this stage the battery charge is decreased again to about 97% of charge.

Storage phase = or ‘maintenance’ stage is where there is constant voltage to maintain the battery at full capacity whilst maintaining any DC operating loads.

Digital battery monitors can be installed to enable you to monitor the level of charge in your batteries.

There are various types of battery chargers for various types of application. There are those battery chargers designed for lead acid batteries, sealed lead cell and others for AGM gel cell batteries and the charging output depends on the battery manufacturer’s recommendation. 

Battery chargers range from the simple charger that supplies constant DC current to the battery and this type often take a long time to charge with a risk of over-charging the battery. Then there are time based chargers which really are simple chargers with a timer, configured for a particular battery, and then left to charge but these types of battery chargers still run a risk of over-charging.  Other battery charges use a pulse width modulation or pulse technology where a DC pulse is fed into the batteries but it is not recommended to leave AGM or Gel battery types connected to the charger for extended period of time.

The new ‘smart battery chargers’ are designed to recharge multiple battery banks simultaneously, they are designed to know when to stop the charge to avoid battery cell damage and provide good charging performance from inconsistent line voltage.  Xantrex supply a PRO and 'Freedom' series of inverter/chargers (in different sizes and power ranges) that is a combination of an inverter, battery charger and transfer switch in one complete system.  When mains power (AC) is available it charges the house batteries and allows any surplus AC power to pass through to power other AC loads such as the microwave and TV. When the AC power is turned off, the unit inverts DC battery power into AC electric power.

Suitably rated fuses or circuit breakers must be fitted close to the battery to protect the wiring and minimise the risk of fire.  When a circuit breaker is tripped it can prevent destruction from an overloaded circuit and may prevent a fire occurring.

Charging Time for Batteries

The time it takes to charge a battery or batteries depends on several things:-

1.      the size of the battery charger

2.      (CAP = capacity) - the battery capacity in amp hours (Ah)

3.      (DOD = depth of discharge in %) how deeply the battery is discharged and

4.      (CC = charge current) the rated current output of the charger in amperes

and an approximate charging time calculation may be:-

  CC x 80

For example:-

Xantrex’s user manual has a calculation table and let’s say you have a Xantrex TrueCharge2 40amp and –

- one battery with a rated capacity of 120 Ah (CAP) and it is 50% discharged (DOD) then the approximate charging time with a TrueCharge2 40 would be:-

                             120 x 50                   = 1.875 or say 2 hours
                              40 x 80

- a battery bank of say 4 batteries in parallel with the rated capacity of 120 Ah each and the battery bank is discharged 50% then the approximate charging time would be:-

(4 x 120) x 50%       = 7.5 hours
     40 x 80

The above calculation however, is a rough guide only as a ‘bulk’ or ‘boost’ charge will go to 75% after which a charger will back off and the next phase will take longer at a lower current until ‘float’ phase which could take 12-24 hours to do a full top up.  It does however, indicate that charging batteries may take considerable time.




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