Simple Dual Battery Systems            by M Lauterbach

There are numerous dual battery management systems, using different methods and specifications, many of these system maintaining their superiority. Following is info gathered from many of these sources, and should not be taken as the absolute gospel, but as a guide, as all the different battery types have different characteristics and requirements .

Most of these systems connect the auxiliary battery in parallel with the starting battery after a predetermined time, so that the main battery starter drain can be recharged before recharging the auxiliary battery.  Straight away we are at a disadvantage, as the starting battery will be of a different type, and most likely, a different size, and require different charging requirements that the second installed deep cycle battery, which is powering all your auxiliaries.  Ideally, each battery needs to be charged on its own, so that the alternator can "read" the exact state of charge in that battery, and deliver the right voltage (and charging current).  Some expensive systems do this.  Then there are also other systems which utilise boost charging at higher voltages (and currents), to reduce charging time.

The auxiliary battery should be a deep cycle battery, which can be discharged more often than a normal starting battery.  A starting battery can be expected to fail after 30 completed discharges.  A deep cycle battery will last much longer, even if often discharged to 80%.  A deep cycle battery can be expected to last 4 to 8 times longer.

The basic differences between the two batteries is that the starting battery has numerous thin plates for maximum surface area, to facilitate large starting currents.  Some describe the setup as a "lead sponge", which gets easily damaged by deep cycling.

The deep cycle batteries are not designed for huge current draws, but rather for high discharges.  As a result they comprise of thick lead plates, which can buckle and short out when exposed to huge current draws, such as when running winches off them. 

There are many more battery designs, such as the sealed gel batteries.  Their characteristics with respect to charging/discharging voltages/currents, capacities and longevity, are all different.

The following figures represent a rough guide to a battery state of charge at 20 degrees C:

Ideally, one should try not to discharge the battery below 40%.  Additionally, charging rates should not exceed a tenth of the amp hour (AH) rating of a battery for too long a period, ie 10.5A for a 105AH battery.  But, the need to charge the batteries quickly often exists in the bush, when you have been standing for two days

When a battery is discharged, its internal resistance is low, hence the initial charge current is high.  As it gets charged, the internal resistance increases with a resultant drop in charge current, unless the voltage is increased.

In an alternator, the output is regulated by the regulator, by managing the current to the field coils (energising the field coils).  Normally, the alternator will have an initial output of between 13.6V to 14.1V, which will then tapers off to typically 13.6V to 13.8V as the internal resistance of the battery increases to normal ie it tapers off while restoring the charge drained during starting.

Deep cycle batteries take a lot longer to charge than starting batteries.  Contrary to popular belief, a fully drained battery can take longer than 8 hours to recharge at 13.8V!  Hence some companies are marketing special boosters, which raise the alternator voltage, so that the batteries can be charged at a higher rate.  This however can reduce battery life, but allows the batteries to become fully charged during a day's drive, if they have enough capacity for charging, especially where two auxiliary batteries are used.

Some of the better charging systems have three charging stages.  Various manufactures have different voltages for these different stages, which they believe are best.  The first stage, often referred to as the bulk charge,  the voltage is regulated (changed) to keep a maximum safe current rate, until the battery reaches 80 to 90% of full charge.  Voltage ranges are between 10.5V and 15.4V.

During next stage (absorption charge), the voltage is kept constant, while the current gradually tapers off as the internal battery resistance gradually increases.  Typical voltages here are 14.1V to 15.4V

During the last float charge stage, the voltage is reduced to a level, where the battery is just trickle charged at typically 13.8V.

A few charging systems also limit the initial charge current to prevent possible battery damage.

If you alternator charges correctly at between 13.8V and 14.1V when the batteries are not fully charged, then the simple non-boosted battery systems work just fine, provided you have enough time to charge the batteries.  A timer is added, to leave the auxiliary battery disconnected for about 9 minutes, to allow the starting battery to be fully charged first.

When doing an installation, there are a few vital items to observe:

• All connections should be sound, and properly crimped, or soldered, even though a soldered joint is not as sound as a properly crimped joint (the best is crimped and soldered).  Using a vice, or a hammer and punch for "crimping", is not good enough, as these connections will have a volt drop at higher currents.  A minor volt drop of eg 0.3V will drop the charge voltage from 13.8V to 13.5V, preventing the battery from becoming fully charged.
• The charging cable between the alternator and the battery often only consist of two 4mm² wires in parallel, which have an allowable current carrying capacity of about 77 amps.  At 77 amps, the volt drop will be between 0.3V to 0.4V per meter, which is too much.  Solder in an additional 10 to 16mm² welding cable to prevent any volt drop
• Only use quality brass battery terminals and lugs.  Check each joint, eg on the Cole Hersee relay.  If not tightened properly, you can easily experience a 0.2V drop per joint.
• Check your earth cable between the batteries and the chassis.  Often the earth cable is routed from the battery to the chassis, and then from there to the engine or gearbox.  This contains one extra connection, which is exposed to the environment.  It is best if you connect another welding cable (25mm²) directly from an earth on, or near, the alternator, directly to the negative terminal  on the starting battery.
• Connect both the earth and positive terminals of the deep cycle battery to the starting battery (positive via Cole Hersee relay) with at least 16mm² welding cables.

Ask 10 different fundis around a camp fire on what the best battery charging system or dual battery system is, and you might get ten different answers, as there is no consensus as to the best charging voltages and currents.  And there cannot be and exact answer, as there are too many different battery types, which have different charge conditions.  The chances are good that you will never get the ideal battery life while achieving more than 95% charge on average. 

As the top-of-the-range charging systems cost a great deal more than a quality deep cycle 105AH battery, it can be argued that a simple system, such as in the diagram above, could be used to charge your auxiliary 105AH battery.  Similar circuits to the one above are used by many of the popular battery systems, with their own led volt meters for each battery.  The timer is installed so that most charge can be restored to the main battery before the auxiliary batteries are charged.  If your alternator has enough spare charging capacity, like a 100A alternator in a diesel vehicle which only needs 20A max, then a timer circuit is not necessary as the alternator will be able to charge al batteries at the same time. 

Ideally each battery should be isolated and charged separately, as the main and auxiliary battery will be of different type and size in most cases, and also at a different state of charge, and will require different charge conditions.  But, if charged long enough in parallel, a satisfactory state of charge in each battery will be obtained, even if the timer is omitted.

Below is another circuit diagram, which incorporates a locally sold in-line battery charger (BC), which is supposed to speed up the charging of the aux battery.  The BC is a useful tool as it varies the voltage dynamically in the later charging cycles to maintain a higher charge current than the alternator would supply due to the increasing internal battery resistance.  But because the current charge is limited to 12A, it cannot match the initial charge current which the alternator would normally supply a flat battery (more than 25A in some cases).  In the circuit below, a current sensor will only switch the charging over to the BC once the normal direct charge to the aux battery falls below the rated charging current of the BC.  The BC is also useful as it conditions the aux battery by as it switches the charge off for a fraction of a second every 2 seconds or so to measure the battery charge.  This cyclic charging is useful for "de-sulphering" the plates in the battery.

If you have more than one aux battery, the normal 12A BC is obviously not good enough, as it will only deliver 6A max to each battery, and a 60% discharged 105AHr battery will take in excess of 9 hours to achieve full charge, assuming a full 6 Amp charge through all charging cycles.  As the charge is reduced in the last two charging cycles, this charging time will be much longer.

The circuit also ensures that while the alternator is charging (running engine), the power to your auxiliary loads (fridges, radios etc) is fed directly by the main battery/alternator, leaving the full charge of the BC to be used for charging of the aux battery or batteries.  This also isolates the high charging voltage created by the BC from the aux equipment.

But take note that I have removed my BC because it froze often, and cannot deliver a high enough current to charge both my aux batteries in a full day's driving if they are discharged to about 40%.  I have not installed a current sensor yet, which would help the charging rate tremendously.  The distributors have tested my unit and have declared it to be 100%.  I am still waiting from them what could cause the unit to freeze so often.

December 2010 addition:

I have now installed a CTEK charger, which is capable of charging at 20A.  It has three terminals - ground, battery in and battery out.  It is extremely easy to install, as it has a built in timer, and connects/isolates the batteries automatically, so there is no need for external relays and timers, only your fuse board to the external loads (fridges, radios etc).  The best feature using the rugged CTEK charger is that it is an intelligent charger, and will keep the aux batteries in tip top condition, even if your alternator is only pushing out 13.5V

Below is a circuit.  Please note that I have added two options:
1.  The first option is to install a heavy duty relay like a Cole Hersee to bridge the batteries for extra power in times of need, like when using the winch, or when the primary battery is too drained to start the vehicle.

2.  The second option includes a load shedding relay.  This relay is activated as soon as the alternator is charging.  The function of this relay is to switch the power source of your aux equipment directly to the alternator, thereby allowing the CTEK Charger to direct all it's charge to the Aux Batteries, ensuring that they are charged in the quickest possible time.  Also, the aux equipment will not be subjected to the higher charging voltages from the charger, but will receive a regulated voltage from the alternator.  This of course assumes that your alternator is a simple regulated alternator with a constant output, and not one of the more modern "intelligent" alternators, which adjust their voltages and charging rates according to battery conditions.

The normal relays will only handle up to 30A.  If you need more capacity, you can add more relays, but please note that the alternator might not be able to trigger more than one relay, in which case you would have to install another relay which will be activated by the alternator, and which in turn will switch on the load shedding relays.

Please note that no fuses are shown, which should be added between the battery and the load.  All main cables should also be suitably fused.

Normal disclaimers apply and the above circuits are installed entirely at your own risk