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Split Charge Relay
VSR, voltage sensitive relay, battery isolator relay, dual battery relay, leisure battery charging from alternator, B2B relay, smart relay, automatic charging relay
A split charge relay (VSR) is an automatic switch that connects the starter battery to the leisure battery when alternator voltage rises above ~13.3 V, letting both batteries charge while driving, and disconnects them when the engine stops to protect the starter battery.
VSRs are simple and inexpensive (~£15–40), reacting to voltage alone without managing the charge curve. Not suitable for vehicles with smart alternators (Euro 6+, start-stop) or for LiFePO4 leisure batteries — use a DC-DC charger in those cases. Popular brands: Sterling, Ring, Victron Cyrix.
FAQ
What's the difference between a split charge relay and a DC-DC charger?
A VSR is a simple voltage switch: it connects both batteries when alternator voltage reaches ~13.3 V. A DC-DC (B2B) charger actively regulates current and voltage for a proper multi-stage charge. VSRs are cheaper but don't work with smart alternators and can't give LiFePO4 the correct charge profile.
Can I use a split charge relay with a LiFePO4 battery?
Technically yes, but it is not recommended. LiFePO4 needs a multi-stage charge (bulk/absorption/float) that a VSR cannot provide — it simply passes alternator voltage through. Use a DC-DC charger with a LiFePO4 profile for correct charging and maximum battery life.