Battery-To-Battery Chargers

What are they & why might I need to use one?

Battery-To-Battery chargers allow you to charge an auxiliary / leisure battery from your alternator whilst driving. They are installed in-line between the positive terminals of two batteries (one of which is connected to an alternator, typically a starter battery) and offer several benefits when compared with traditional split charge systems that utilise a Voltage Sensitive Relay (VSR) or heavy duty alternator-triggered relay.

  • Direct alternator charging used in a traditional split charge system with a VSR will only achieve around an 80% charge state. VSRs 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% re-charge for your leisure battery in a controlled way and typically achieves it around 5x faster than with direct alternator charging.
  • 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.
  • Using a battery-to-battery charger is the only reliable 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. Due to this highly variable voltage VSRs are not suitable for use with 'smart' alternators.

Just as with VSR-based systems, battery-to-battery chargers are simple to install (having only Battery IN, Battery OUT and negative connections) and will engage/disengage automatically on engine start/stop, ensuring the batteries are isolated and cannot discharge one another.


Find out more about Battery-To-Battery chargers in our Knowledge Centre