THE USE OF A BATTERY MANAGEMENT SYSTEM

FOR DUAL BATTERY SYSTEMS

PROBLEM No.1: 

An increasing number of serious 4x4 users are choosing to install second batteries to help run

their accessories -primarily due to the lack of capacity offered by the original starter battery. 

Most of these users are not satisfied with the performance of their auxiliary batteries because

they frequently run down during the course of a trip.  

REASON FOR PROBLEM: 

Most of the dual battery systems fitted  into 4x4s appear to charge both batteries 'in line' which means that one battery frequently is not charged fully - as the alternator shuts off when the first battery reaches optimum voltage.    The other disadvantage to such a system is that the main vehicle battery is frequently not isolated from the auxiliary battery.  Thus, when accessories are used over long periods (e.g. fridges), or if needing to winch a vehicle out of mud, etc. both batteries are employed, often leading to the vehicle's starter battery going below critical voltage for restarting the vehicle.

Owners often complain about the lack of apparent storage capacity of their auxiliary battery and tend to perceive the problem to be a faulty battery or  inadequate alternator.  The problem does not necessarily lie with either, but rather with the 'system' that is being employed to charge the batteries in question.  

SOLUTION TO PROBLEM:

 It is evident that in order for the dual battery system to be efficient, both batteries need to be fully charged before the alternator goes into 'float' stage.  This requires the alternator to be switched from one battery to the other - depending upon the level of battery voltage (equating to approximate storage level).  Some owners tackle this by installing a mechanical 'marine' type switch and relying upon their diligence in ensuring that both batteries are equally charged.  Unfortunately, the human factor can lead to 'errors and omissions' - depending upon the individual and his concentration levels at any given time!! 

Other systems offer 'timer charging' whereby each battery gets an allocated amount of 'charging time'.  This technique does not allow for the different charge and discharge characteristics, or variants in size, of the various batteries employed, and is also not the  ideal solution. 

The best system is an automatic, electronic Battery Management System that senses voltage in both batteries and always charges the battery with the lower voltage first.  By this method, both the Auxiliary and Starter battery will always be fully charged prior to the alternator going into 'float' mode.  The Starter battery is still isolated under normal conditions but an over-ride switch can be installed upon demand for those users wishing to bring in their Starter battery to assist in accessory operations such as winching.

  When selecting a Battery Management System be sure to check that there is no voltage drop across diodes and that it is automatic - thereby eliminating human error!! 

PROBLEM No.2: 

Despite fitting a battery management system, (dual battery system) the Auxiliary, and sometimes, the Starter battery, still appear to 'go flat'. 

REASON FOR PROBLEM: 

If a 4x4 owner has several large batteries installed, does short trips and/or a lot of mileage at low RPM (e.g. 'Bush' driving) , then his alternator frequently cannot keep pace with the amperage being drawn.

 Let's take the example of a user with a 90 amp. Starter battery and 2 x 100 amp. deep cycle batteries.  Let us assume the vehicle is fitted with a 55 amp. alternator as factory standard.  Running a vehicle at low RPM does not allow the alternator to supply optimum output.  Couple this with a vehicle driving at night, in the rain, with a 'Fridge' running or Inverter usage and you would probably be pulling a lot more amps than the alternator can feed back in!  The result is that your batteries run down and eventually go flat.  Every time you start the vehicle more amperage is being used for this function and this needs to be replaced by the alternator as well.   

SOLUTIONS: (high output alternator)

Some 4x4 users get their stators rewound to a higher specification (output) but beware!, this also has its pitfalls.  Merely adding a few extra 'turns' onto the stator takes away valuable cooling space in the alternator, couple this with higher heat - due to increased outputs, and you have a reason why rewound stators frequently burn out.

If you decide to have a stator rewound ensure that you use a specialist company that caters for the cooling aspect or, go for a bigger alternator (high charge alternator)  - if you have the room. 

Another option is the recent introduction of  High Charge Regulators.  These increase output by either maintaining the charge rate for longer periods, or by increasing the voltage to the rotor and thus increasing the alternator output (Boost mode).  The 'top-of-the-line' systems offer both facilities and come with 3 stage charging characteristics (Boost/Normal/Float). 

 Couple one of these with an effective Battery Management System and  your problems will be solved!

  SOME HINTS:

 The 4x4 users are also increasingly employing a different type of battery for the powering of 12v accessories - the deep cycle batteries, which are much better suited to this task.  Unfortunately, one of the characteristics of these batteries is the slower charge rate they accept in comparison to the high-cranking, starter batteries.  

 Just another good reason to fit a voltage-sensitive system and not a 'timer'  type which doesn't 'read' battery voltage!!

 Dr Gawie van der Merwe & Geoff Vickerstaff

Content of this paragraph on dual battery systems

dual battery systems, dual battery management system, battery isolators, battery isolator, auxiliary battery, high charge alternator, high charge alternator regulator

For More Information Contact:

Plan-My-Power (PTY) ltd

Dr Gawie van der Merwe
112 First Avenue, Fairlands, Johannesburg ,2195
Tel: +27 11 678 7520
FAX: +27 11 678 6067
Internet: info@planmypower.biz

Copyright © 2005 Plan-My-Power (PTY) ltd