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Dual Battery system 4WD

9 June 2011

4WD Dual Battery System Charging

Many theories and practical advice have been written on this subject but the one thing common is that many theories are wrong and not all practical advice suits every one.

Frequently asked questions.
1/ Will my alternator burn out when extra batteries are installed.
. Answer…Alternators are designed to handle all loads when driving at night and these loads include driving lights, spot lights, and instrument lights (total load approximately 300Watts or 25Ahrs). Most towing is done through the daylight hours and an additional battery will add approximately 30Ahrs to the alternator load.
Alternators can handle up to two additional batteries provided a Voltage Sensitive Relay is used. This relay will allow for the starter battery to be charged, and only then, will it parallel other batteries. This protects the alternator from having a flat battery coming on line before the starter battery is charged.

If an Alternator is overloaded for long periods of time then you can be sure that the lifetime will be reduced. This overload condition comes into play when 500Watt sound systems and the like draw a high continuous power for Base bin speakers and other high current equipment. Alternatively a near flat battery will only draw high current for a small amount of time as it comes up to charge.

2/ What type of battery should be used.
Answer…The new AGM deep cycle batteries are excellent under the bonnet and the caravan boot as they not only charge faster than the wet cell deep cycles but are maintenance free and come to full capacity with the standard output of 14.2 Volts from Alternators.

Caution must be taken though when selecting a deep cycle battery.
The ‘Full River’ brand has a DC range and an HGL range. The HGL range should be selected when charging from the Alternator only. This is a hybrid battery and is much easier to charge when flat. The DC range is much heavier plated and is quite stubborn to charge from flat thus putting a greater load on the alternator.
This advice changes though when using Solar and multi stage 240V chargers.

3/ What cable size is best.

Answer…Cable sizing for minimum voltage drop depends on the length of cable run. ‘Springers’ seems to be the only retailer selling cross sectional area cable. This factor needs to be known when assessing voltage drop. 6mm cable sold at Springers’ is 6mm cross sectional area. When purchased at an Automotive or Marine shop it is in fact only 4.59mm cross sectional area. This is the cause of lots of Fridge run problems. ..but getting back to under the bonnet….the cable size should be minimum 13.5mm cross sectional area between batteries so as to avoid voltage drop and therefore under charging and sulphating batteries.
Written by Brian Springer 

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