Dual battery system vs. charge booster

Dual battery system vs. charge booster

23. June 2020 Off By Peter

There are two main reasons for installing a second battery. When traveling with the vehicle, the additional battery is used to power appliances such as the refrigerator and other electrical accessories such as lamps, chargers, etc. This prevents the additional consumers from discharging the starter battery to the point where it is no longer possible to start the vehicle.

For those who work often and regularly with a winch, a second battery can sometimes provide additional “juice” when needed.

With Land Rover Discovery and other modern vehicles with a large number of electrical consumers, there is another point to consider. If the doors are opened and closed frequently and unlocked and locked, the vehicle repeatedly switches from sleep mode to standby mode. During this process control units are activated and the fuel pump works to vent the lines. If this happens again and again without actually starting the engine and recharging the batteries, the starter battery is discharged to such an extent that at some point the vehicle can no longer be started.

With a switchable auxiliary battery, one push of a button is enough to connect the supply battery to the starter battery and the vehicle can be started again.


Dual battery system or charge booster

There are various systems on the market that have been specially developed for use in vehicles and that serve the purpose of charging a second battery.

Basically there are systems that use the charge voltage of the vehicle generator to charge the second battery and are able to connect the batteries directly to each other (e.g. the DBS02 from Tmax)
and other systems that act as charging boosters and raise the applied voltage to a higher level by an electronic circuit (boosting, e.g. Ctek d250).

One of our recommendations from the first mentioned category is the dual battery system DBS02 from T-Max. The DBS provides an electronically switchable connection possibility (relay) between the starter battery and the supply battery and simultaneously measures the battery voltage in both batteries and displays it to the user. As soon as the vehicle’s alternator provides a voltage above 13.3V, the DBS connects the starter battery to the supply battery via the relay, so that both batteries are charged simultaneously. Otherwise the batteries are separated from each other so that no mutual discharge can take place. The DBS also makes it possible to use different types of batteries without any problems. In addition to the standard lead acid battery, an AGM battery or a particularly powerful wraparound battery (e.g. Optima yellow) can be installed as an additional battery. On Discovery 4 models without a start/stop battery, the empty box on the driver’s side in the engine compartment is ideal for installing the second battery.

The advantages of such a system are these:
– Both batteries can be connected directly to each other, for example, additional power can be provided when working with the winch.
– The switchable connection means that the vehicle can also be started with the supply battery in an emergency without the need for a cable tangle.
– In comparison, the systems are often cheaper than charging boosters

But everything that has advantages has of course also disadvantages:
– There are certain situations where the DBS does not charge the supply battery – for example when the starter battery is fully charged and the supply battery is very empty. Then the vehicle does not switch on the alternator and the voltage does not rise above the necessary 13.3V. So the supply battery is not charged either. But if this situation is known, the connection can be made by manually connecting the two batteries (by pressing a button) and thus the system can be tricked out.
– In vehicles with modern battery management systems (like the Discovery 4) it happens more often that the alternator charges only with a lower voltage than 13.3V. In this case the supply battery is also not charged and you have to use the above mentioned trick of manual connection.


Where does a charge booster have its application?

As an alternative to such systems, a charging booster can also be used. The charge booster is usually much more expensive than a DBS system, but in some situations it offers certain advantages (but also disadvantages). With a charge booster, the voltage applied to the charge booster (on-board voltage) is raised to the optimum charge voltage level of 14.5V by an electronic circuit. At the same time, the charge booster limits the current to a defined value, so that, for example, the maximum charge is 5A or 10A. Thus the connection of a charge booster is often also possible via a cigarette lighter socket in the trunk or in a caravan via the trailer socket. So as soon as the charge booster is supplied with voltage, it starts to charge the supply battery – without the risk of discharging the starter battery by the connected consumers.
A disadvantage of a charge booster is obvious. There is no direct connection between supply battery and starter battery possible. Therefore no starting of the vehicle by pressing a button or the provision of additional energy when using the winch.

In the end both devices have advantages and disadvantages. Which one is chosen depends on the goal to be achieved.


Occasionally there is the question whether “overloading” can occur:

The DBS itself cannot influence overcharging – the DBS does not switch off the charging process or disconnect the battery if the voltage is too high. The charging voltage comes from the alternator and is regulated by the car itself – depending on whether it is a Disco 3 or Disco 4 either via a charge regulator or via the Battery Monitoring System.

When both batteries are connected together by the DBS, the system voltage drops to a lower voltage. Then the car charges the two batteries until the voltage becomes so high that the BMS switches off the charging process, which also causes the charging voltage to drop, which in turn causes the batteries to be separated again (below 13.3V). During the charging process, the current flows, so to speak, primarily to where there is the least resistance, primarily into the empty battery.

So overcharging cannot happen as long as everything is working normally.

With your Disco 4 you have to consider something else: The Disco 4 has a BMS. The BMS controls the charging process and also has the ability to charge the batteries, when they are quite full, only with very low voltage (e.g. 12.9V). In such a case, when the starter battery is very full and the supply battery is very empty, the DBS would not connect the two batteries together. This can be remedied by connecting the two batteries manually. Then the system voltage drops again and the BMS provides a higher charging voltage. So this is a small workaround, which you should know as a Disco 4 driver if you use a DBS with a BMS.

If you pay attention to this everything is good. Basically you can say anyway, a double battery system should be checked from time to time, because nothing is more annoying than to assume that you are self-sufficient, but it is not…

We have also written a few things about BMS here:

Batterie, BMS und Batterie laden – Discovery 3 / 4 (5)


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All information as always to the best of our knowledge and belief, but without guarantee!