There are lots of claims that having a SMART ALTERNATOR means you need a DC/DC device to be able to properly recharge your auxiliary battery. With the Discovery 3 and 4 ( and many other new vehicles like Ford Rangers and VW Amaroks ), this is nothing more than deliberately misleading advertising hype.

No other D3 or D4 Dual Battery System offers the many features or advantages like our systems provide and here is a comparison between our Discovery 3 Dual Battery Systems and DC/DC setup.

PLEASE NOTE the following info is based on the feedback we have received from many of the more than 4,500 D3 and D4 owners using our kits both here in Australia and around the world.

Because of the way our isolators allow up to 50% of the cranking battery’s capacity to be used to help power your accessories, which adds an additional 45Ah to the auxiliary battery’s 55Ah. This gives you a total of 100Ah of battery capacity for running your accessories while camping.

With a DC/DC setup, because an Optima 55Ah Yellowtop battery is the largest that will fit under the bonnet of a D3 or D4, you only have a total of 55Ah available.

With a 20 or 25 amp DC/DC setup, if you use all the 55Ah capacity while camping, the auxiliary battery will be discharged down to 0% SoC ( 10.5v ) and you will now need need to drive for at least 3 hours to allow the DC/DC device enough time to be able to replace 95% of that used capacity. See the explanation relating to 95% State oCharge ( SoC ) below.

With one of our systems, if you use the same 55Ah capacity while camping, your cranking battery and auxiliary battery will only be discharged down to a bit over 60% SoC ( 12.2v ). Because of the large alternators in D3s and D4s, it will take less than an hours driving to have both batteries back over 95%

This means our systems replace the same amount of used battery capacity, 3 to 4 times quicker than a DC/DC device can.

But even if you use the full 100Ah capacity that is available with one of our systems, you will charge the auxiliary battery back over 95% in about 45 minutes to less than an hour, and charge your cranking battery to over 95% in less than an hour and a half.

So in reality, our dual battery system paired up with your large alternator, will replace around double the used battery capacity in less than half the time that a DC/DC device will take to replace just half of that capacity.

And the advantages of fitting one of our systems does not end there!

Because of the way our isolators keep the cranking and the auxiliary batteries connected together after the motor is turned off, and because the Optima Yellowtop battery is a rapid charging battery, there are a number of special advantages only available with our systems.

When driving around town, every time you turn off your motor, because the Optima charges quicker than any other battery, the Optima is always going to be in a higher state of charge than the cranking battery, and because the optima battery is always in a higher state of charge, it will now slowly back discharge into the cranking battery, keeping the cranking battery in a higher state of charge, and this helps to keep the cranking battery in a better condition and help extend the operating life of the cranking battery, while having no detrimental effect on the auxiliary battery.

Furthermore, again because our isolator keeps the batteries connected, when you go to start your motor, while the bulk of the energy needed to start your motor will still come from the cranking battery, but the auxiliary battery now assists the cranking battery every time you start your motor.

This makes it easier for the motor to start and also, because less energy is drawn from the cranking battery, while starting the motor, the cranking battery is recharged in a shorter drive time.

This one feature alone, of sharing the load while starting the motor, makes a huge difference in vehicles that are regularly driven for short distances around town, the “Shopping Trolley Syndrome".

None of these features are available when a DC/DC setup is used.

So the next time someone tries to tell you that a DC/DC device is the only way to charge your D3’s or D4’s auxiliary battery, you know they either haven’t got a clue how the alternator works in your D3 or D4, or they just want to sell you something you do not need.


The following is basically what happens when charging Lead Acid type batteries to 95% SoC

First off, the label "Lead Acid" battery is usually used for Wet Cell or Flooded type batteries. While this is correct, but AGM and Gel Batteries are also Lead Acid Batteries and when I state Lead Acid Batteries on this website, I am referring to all types of Lead Acid Batteries.

When charging a lead acid battery with a DC/DC device, or from Solar Panel Regulator or with a Battery Charger and from an Alternator, the charging current, while charging low to moderately discharge lead acid batteries, will be controlled by the charging device and this is known as the “BULK” stage of a charging cycle.

DC/DC devices, Solar Panel Regulators and Battery Chargers all have a LIMITED charge current while Bulk charging. This means these devices limit the charge current during the Bulk stage of the charge cycle.

These devises, because they have a set limited charge current, are known as Constant CURRENT Charging devices.

An Alternator, because most have a very high charge current capability, which is usually far greater than most batteries will require while charging, are known as a Constant VOLTAGE Charging device.

Optima Yellowtop batteries are a special type of battery that will safely accept “unlimited” charge current, according to their manufacturer. This means they can take everything available from the alternator.

All lead acid batteries will charge at the charge current being applied to them until the battery’s terminal voltage reaches the charging devices maximum charger VOLTAGE. From this point in the charge cycle, while the charge VOLTAGE remains at a constant maximum voltage level, the current will now taper off, and this is what is called the ABSORPTION stage.

The current being draw by the battery while in the ABSORPTION stage is controlled by the battery itself and no longer controlled by the charging device. The charge being ABSORBED by the battery now tapers off from what ever the maximum limited charge current of the devices is at the point where the charger goes from the BULK to ABSORPTION stage and the battery now continues to charge in the ABSORPTION stage.

If the charging device is allowed long enough time, the battery will eventually reach a 100% SoC. At which point the charging device transitions to the FLOAT Stage of the charging cycle.

When charging with a DC/DC device, or from Solar Panel Regulator or with a Battery Charger, the transition from the BULK stage to the next stage in the charging cycle, the ABSORPTION stage, occurs when the battery is at about an 80% SoC. Now a number of factors have to be taken into account before the exact transition point occurs but working on about at “roughly" 80% SoC is accurate enough for what is being covered here.

While the charging current progressively reduces once the charge cycle is in the ABSORPTION stage, using a 20 amp DC/DC device, to charge a battery from a starting SoC of 0% ( 10.5V ) your 55Ah Yellowtop battery will reach 95% SoC after about 3 or more hours of driving.

The last 5% of the charge will now take at least another 3 hours of driving.

With the alternator directly charging the auxiliary battery ( via one of our isolators ), your 55Ah Yellowtop will go into ABSORPTION stage very quickly. In this case, because of the high charge voltage and high charge current, the Yellowtop will be at as low as 30% SoC but the battery will still be drawing 70 or 80 amps at the beginning of the ABSORPTION stage.

The yellow top will reach an SoC of 95% in under an hours driving, three times sooner than a DC/DC device will take, BUT, just like a DC/DC device, the last 5% of the charge will now take at least another 3 hours of driving.

This is why a realistic common reference point in the state of charge of the battery is set 95% is used as an example of the way both types of systems can charge the auxiliary battery. And from this reference point, you can clearly see that there is a huge advantage for using one of our systems over the restricted charging a DC/DC device offers, and our systems are heaps cheaper to set up.