people look at installing a dual-battery system either because they want to
run winches, portable refrigeration,
lighting and a mobile telephone charger while camping and travelling in
batteries are charged by the vehicle’s alternator whilst the engine is
running so when the engine is shut down the charging of the battery ceases.
Running portable 12V appliances will draw power and reduce the power supply
stored in the battery. This often results in flattening the starting
battery which can then leave you stranded. Installation of a dual-battery
system will help you overcome this.
winches can drain significant amps from the vehicle battery quicker than an
alternator can replace them so a dual-battery system can be set up so you
can run the winch off both batteries.
you should consider what 12V electrical items you expect to run from the
dual-battery system and calculate how much power source (Watt hours) is
required to run all the items and how long you want to run the items between
recharges. It is recommended you then factor in about a 25-30% safety
margin. This calculation will assist in deciding the amount of amp hour
battery you need.
What parts are there to dual-battery system?
dual-battery system consists of the two batteries
with one termed as the ‘primary battery’ and the
other the ‘auxiliary battery’, battery cradles,
wiring for the discharging/charging system which
integrates the vehicle’s electrical system and a
battery isolator. An optional visual read-out
device can also be placed in the cab of the vehicle
for you to check on the battery charging or battery
power storage level.
Some larger 4WD vehicles come with two starter
batteries that are electronically paralleled for
starting the vehicle and this should not be confused
with a dual-battery system. Starter batteries can
have CCA (Cold Cranking Amps) marked on them
and are called ‘shallow cycle’ batteries.
The size of the cable linking
the batteries in a dual battery system should be at
least 13.5mm square.
Batteries and what is the best battery for a
There are two main types of batteries. When you
buy your vehicle from the manufacturer it will
likely have a ‘shallow cycle’ single high-current
discharge (HCD) battery. These HCD batteries or
cranking batteries provide a brief high amp charge
to start the engine.
Deep-cycle batteries are preferred as the
‘auxiliary’ battery for a dual-battery system as
they provide sustained power over long periods of
time and are designed for repeat charge and
discharge. These deep-cycle batteries can be larger
in size so the one you purchase will obviously need
to fit somewhere whether it is in the vehicle engine
bay or behind the seat in the vehicle cab.
Batteries come in various sizes and weights. The
number of internal lead plates can vary in number
ie: 7, 9, 13, 15, 17 & 19 and thickness so the amp
hour capacity also varies. Battery terminals can
also range from standard, standard/wingnut to dual
fit. Deep cell batteries also cost more.
Starting batteries have thinner plates and
will accept a greater amount of charge more quickly. Deep
cycle batteries have thicker plates so they only accept a
small amount of faster charging before the resistance of the
plates begin to reduce the amount of charge uptake. So,
different batteries reach full charge at different voltages
and with a dual battery system you need to consider if your
vehicle alternator and battery isolator are capable of
charging both batteries.
There are wet cell and dry cell
batteries. Wet cell batteries give off hydrogen gas
which is an explosive vapour so you would not want
to place this type of battery behind the seat inside
a vehicle cab or near inverters, battery chargers or
any source of a likely spark. Care should be taken
when handling lead-acid batteries as any acid spill
can cause serious burns.
These types of batteries also known as ‘wet cell’ as
the lead plates are suspended in wet acid and there
are caps on the battery for you to check the
electrolyte levels which require periodic topping
up. Wet cell batteries should be housed in a well
vented area and kept upright.
Gel-cell batteries are called ‘gel’ batteries as the
electrolyte is held in a jelly-like state and the
batteries are sealed and spill proof. A three stage
smart charger is required to charge these gel
electrolyte batteries. Gel-cell batteries hold
their charge longer than ‘wet cell’ batteries but
will suffer permanent damage if over charged. A
rest period will be necessary after rapid charging
to make sure the true state of charge is known.
Absorbed Glass Matt (AGM) batteries
so far have been considered to be the best or
superior for 4WD or RV use especially where the main
source of charge is from the vehicle’s alternator as
they are able to be recharged faster. AGM batteries
have a positive and negative plate separated by a
glass matt and the acid is also a gel. Reports
state AGM batteries are less sensitive to charge
rates than the gel-cell batteries. AGM batteries
are sealed, spill proof and maintenance-free.
For example, ‘Full River’ have a AGM ‘HGL Series’ which is hybrid type
battery best for use when charging from the alternator only. These
batteries have thicker plates and more active internal materials so they are
easier to charge when flat.
Spiral batteries are a type of AGM battery with the
lead plates tightly compressed into spiral wound
cells. Spiral batteries are smaller in size,
lighter, sealed and maintenance-free. Spiral
batteries are reported to have a high cranking
performance, a longer shelf life (when not in use),
a higher charge acceptance rate during recharging
and more robust to withstand vibrations. Some
spiral batteries can be installed in any position
making for more flexible installation.
There is also a battery called Allrounder
which is built for starting and deep-cycle
Good sturdy battery traysor battery
cradles are required to secure the battery in
the vehicle. Battery tray joins should be MIG
welded and those that are powdercoated will resist
abrasion and corrosion.
will be extended if you do not run the battery down
more than 50% of their capacity.
It would also be a good idea to know how many hours
it will take for your vehicle’s alternator to charge
the auxiliary battery. A standard vehicle
alternator is not designed to recharge a deeply
discharged battery so it could take up to 5-6 hours
to charge an 85 Ah deep cycle. The vehicle’s
alternator can be modified with a specialised
charging alternator and a smart multi stage
A few brand names for batteries are Century
Batteries, Trojan Batteries, Exide Endurance Deep
Cycle batteries, Optima Batteries, Federal Batteries
Deep Cycle Series and Concorde Lifeline AGM deep
Isolators and what they do
do what the name suggests. They ‘isolate’ the
battery used to start the vehicle leaving it to
maintain its charge after you have turned off the
When you start a vehicle’s engine there is a surge
of power to the starter then the alternator starts
producing current. The isolator responds by firstly
providing power to the primary battery until it is
fully recharged then when the primary battery is
full the isolator switches the current over to
charge the auxiliary battery. 12V electrical items
are then run off the auxiliary battery.
Batteries can be isolated by a manual switch, a fuse
or an automatic switch. With manual switches to
turn the isolator on and off, human nature is that
you will forget to do so at one time or another. So
that leaves you with a battery management system or
automatic system. Automatic battery management
systems link batteries for charging and isolate
batteries for discharging.
To avoid any damage to a vehicle’s on board computer
or electronic fuel injection (EFI) system select an
isolator that has in built spike/surge protection.
The manufacturer of dual battery isolator Smart
Start, Redarc Electronics, recommend fuses be fitted
in the cable close to the positive terminals of each
battery for safety and to reduce the risk of fire.
Redarc website states:-
‘Fitting a fuse to one battery will protect that
battery but will not protect the other one and will
not prevent the fire risk. A fuse should also be
fitted in the line to the 12V equipment to protect
the auxiliary battery in the event of a short on
that line. Please note that if you are wiring the
vehicle with an emergency jump start switch via the
blue override wire from our Smart Start then the
fuses or circuit breakers must be suitably rated.
This is due to the high starting currents
experienced, which could result in blown fuses. In
this case battery starter cables must be installed
between the start battery and the auxiliary battery.
The same risks apply as described above therefore if
suitable fuses are not obtainable please ensure the
cables are well protected’.
With a diode isolator each battery circuit is
completely isolated from the other as diode isolators act as
a one-way valve between the batteries. Current flow is
prevented from flowing from one battery to the other so each
battery is an independent power source. These are not used
much these days but can be found on older vehicles.
For a good basic ‘parallel’ charging dual
battery system it is recommended both batteries be identical
(ie: age, size, capacity and design). The batteries are set
up to be charged in parallel and once the ignition is turned
on the solenoid is automatically switched on and this allows
the power to flow between the batteries. Once you turn off
the ignition the solenoid opens thus isolating the main
Then there is
which are essentially a smarter version of continuous duty
solenoids. Smart solenoids have a voltage sensor that
allows automatic operation so parallel charging does not
commence charging the auxilliary battery until the main
battery has reached a predetermined voltage – around 13.5V.
Once the main battery has reached that state
of charge both batteries can equalise. The batteries remain
in parallel, after you turn off the ignition, until the
starting battery draws down to a predetermined voltage
(around 12.8V) and then the solenoid contact opens and
auxilliary battery is disconnected. This action prevents
flattening of the starting battery.
Voltage Sensing Relays
(VSRs) are not a ‘parallel’ charging system as such but they
operate similar to a smart solenoid via a relay (and not a
solenoid) with the two batteries electrically separated. The
auxilliary battery is disconnected from charging if the main
battery voltage drops below a predetermined level. On the
downside, the capacity for a large current draw can tend to
be limited given their lighter construction.
are the best of all dual battery charging systems. With
electronic isolators you can use dissimilar batteries as the
electronics are specifically designed to recognise the
difference and adjust the rate of charge however, the main
battery is always given charge priority.
There are dual battery specialists who
advocate the electronic isolators as, in their opinion,
basic parallel charging systems can ‘mask’ the fact that if
the main battery is flat or damaged the auxilliary battery
will do most of the starting/cranking then when the charge
in the auxilliary battery is depleated the vehicle will not
start as both batteries are in a discharged state.
If you are a keen 4WD enthusiast make sure
the system you select is one that has dust and water
Installation of a dual battery system
Installation of a dual battery system is best done
by an expert. It requires connecting into the
vehicle’s electrical system and this must take into
account the vehicle’s auto electrics. A certain
amount of frustration can also be experienced in
anchoring the sturdy battery cradle to accommodate
the weight of the deep cycle battery and finding
space for it in the vehicle engine bay. Care also
needs to be taken with the installation of the
isolator so it will not vibrate too much or melt
near exhaust heat. The type of wiring and cables
for a dual-battery system need careful consideration
to ensure a high charging rate to the auxiliary
battery and to avoid voltage drop.
Names of some of the products for dual-battery
systems and suppliers are, Redarc Electronics, ARRID
Advanced dual-battery controller, DBS Management
System, IBS Management System, Piranha Off Road
Products, TJM, ARB and eBay.
If you are technically minded dual battery
management kits are available from the above
suppliers and eBay.
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