Auxiliary battery charging and smart alternators

Whilst smart charging and regenerative braking are excellent for the environment and the customer’s pocket with reduced costs for running their vehicles they cause problems when it comes to charging a second battery in an after-market application.

Why? An auxiliary battery will not be charging for significant parts of a journey
During the periods when the smart alternator output is below the charging voltage threshold there will be no charging taking place. These periods can be significant when combined over the full length of a journey meaning that any connected auxiliary battery will not receive as much charge as it would if connected to a system with a traditional alternator.

Gel and AGM batteries

Gel and AGM batteries (which are now popular for use in auxiliary battery applications) are sensitive to over-charging and voltages above 14.4V can cause gas bubbles to form in the electrolyte gel which can permanently damage the battery. Some modern alternators come with very high preset voltages which can range from 14.8V to 15V. So not only do you have to consider your battery not getting enough charge you also have to be concerned that it is getting too much.

Exide Marine 142Ah (ER650) - Deep Cycle Battery

Voltage Sensitive Relays

Voltage sensitive relays rely on voltage thresholds that trigger them to engage anddisengage. Typically they engage to connect the starter and auxiliary battery together at around 13.8V and disengage to separate them at around 12.8V, which in vehicles with traditional alternators coincides with engine start-up and shut-down. This is ideal as it makes sure that the auxiliary battery is charging whenever the engine is running. However, with a smart alternator, when the output voltage drops to below 12.8V a VSR would disengage, meaning that the auxiliary battery would have significant periods where it wouldn’t be charging, even though the engine would be running (some models of VSR may not engage at all, dependent on the on-board software). VSRs can also engage and disengage frequently with the large and rapid voltage changes from a smart alternator and so the high current contacts may fail prematurely. PLEASE NOTE: Voltage sensitive relays are not compatible with SMART alternators. Please select a battery to battery charge instead.

140a 12V Voltage Sensitive Relay

How can you tell if your vehicle has these technologies?

Almost all new vehicles will now have smart alternators fitted as standard and if your vehicle is branded as an eco-efficient model then it is very likely also to have regenerative braking technology. Another indicator is to check the European Emissions Standard to which your vehicles engine complies. If the engine is compliant with the Euro 5 or Euro 6 emissions standards onward then it will almost certainly have a sm

What do you need to use to charge?art alternator. If in doubt you should contact the manufacturer and they should be able to advise.

The $64,000 question. Well it’s not all bad news. If you have a smart alternator then you will need to use a battery-to-battery charger to ensure that you can charge your auxiliary battery effectively. battery-to-battery chargers take the highly variable voltage output from a smart alternator and boost or reduce it to maintain a stable voltage output according to a multi-stage charging profile (in the same way as a mains charger does) providing a safe, controlled and fast charge for your auxiliary battery.

They are easy to install and, just like a voltage sensitive relays they are simply connected between the positive terminals of your starter and auxiliary batteries, together with a negative connection to the vehicle chassis or starter battery’s negative terminal.

 What are the benefits of battery-to-battery?

  1. Batteries charge much faster when using a battery-to-battery charger when compared with direct alternator charging (as found in split charge systems using voltage sensitive , relays, heavy duty relays or diode isolators. This can typically be around 5 times faster.
  2. A much deeper state of charge can be achieved because the battery-to-battery charger uses a multi-stage profile to maximise the depth of charge, something not possible in standard split-charge systems which will typically charge a battery to around 80% of its capacity.
  3. Charging will always take place when the engine is running, regardless of the alternator output voltage, because low voltages are boosted to meet the required charging profile.
  4. As in a split charge relay systems, the starter and auxiliary batteries are electrically isolated when the engine is not running to ensure that one cannot discharge the other.
  5. The charger protects auxiliary batteries from high voltage spikes produced as a result of regenerative braking systems, so preventing damage to sensitive Gel and AGM batteries.
  6. The current reaching the auxiliary battery is determined by the charger rating (rather than by the maximum the alternator can produce), meaning that potentially damaging current in-rush is eliminated. This current in-rush can occur in split charge systems if the auxiliary battery is dead-flat or very low and can overload cables and blow fuses if they are not large enough to cope with the alternator output.
  7. Direct alternator charging used in a traditional split charge system with a voltage sensitive relay will only achieve around an 80% charge state. Voltage sensitive simply allow charge to pass directly from the alternator to the auxiliary battery whereas battery-to-battery chargers take the alternator output and boost or reduce it to provide a stable voltage output according to a multi-stage charging profile. This provides a 100% recharge for your leisure battery in a controlled way.
  8. Current flow is limited by the charger’s current rating, eliminating potentially damaging high in-rush currents and often allowing the use of smaller connecting cable than with VSR-based split charge systems.
  9. Using a battery-to-battery charger is the only solution if you intend to re-charge a second battery in a modern vehicle that has an ECU-controlled ‘smart’ alternator (generally Euro 5/6 compliant engines onward). These have a highly variable voltage output that is often too low to provide a charge or (in the case of regenerative braking systems) so high that it can damage some battery types. As a result voltage sensitive relays not suitable for use with ‘smart’ alternators.
Battery-to-Battery ProBatt Ultra 12V/12V 120A digital charger

We offer a range of battery-to-battery chargers which are reliable and will ensure a stable voltage passes to your auxiliary battery extending it’s life and producing a more reliable power source for your needs. Check out our full range at

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