Glossary

Split-charge relay

Also Combiner relay, VSR

What is Split-charge relay?

A switch that connects starter and house battery during charging and then disconnects them again. It is controlled by the voltage or by the alternator's signal.

What matters about Split-charge relay in practice?

Unlike the isolating diode, it costs no voltage loss. In return, both battery banks get the same charging voltage — which only works if they are the same battery type.

With different technologies, especially lead-acid at the front and lithium behind, there is no way around a DC-DC charger: otherwise the charging profiles cannot do justice to both.

How does a split charge relay switch?

Either via the alternator’s D+ signal or by voltage sensing: when the voltage at the starter battery rises above a threshold because the alternator is charging, the relay connects both batteries. When it drops again, it disconnects. The exact thresholds are in the data sheet.

Split charge relay, isolation diode or DC-DC charger?

  • Split charge relay: no voltage loss, but both batteries get the same voltage. Suitable for the same battery type.
  • Isolation diode: simple decoupling, but voltage loss across the diode.
  • DC-DC charger: its own charging profile for the service battery, needed for lithium or a smart alternator.

What to watch when installing

Choose the relay for the charging current the alternator can deliver and fuse both sides close to the battery. More on charging from the engine on the page boat alternator.

Related terms

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Still unclear?

We are happy to explain it for your specific case.

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