Split Charge Relay Wiring Guide (UK Campervan Installations)
A split charge relay allows your leisure battery to charge automatically from the alternator while driving. This guide explains how split charge relay wiring works, fuse placement and how to choose the correct cable size for UK campervan installations.
Split charge relay wiring diagram
How a split charge relay works
A split charge relay connects the starter battery and leisure battery together when the engine is running. When the engine stops, the relay disconnects them again to prevent draining the starter battery.
- Engine running → relay connects batteries
- Alternator charges the leisure battery
- Engine stopped → relay disconnects batteries
Typical split charge relay wiring layout
- Starter battery positive → fuse → relay input
- Relay output → fuse → leisure battery positive
- Starter battery negative → chassis ground
- Leisure battery negative → chassis ground
Choosing cable size for split charge relay wiring
Cable size depends mainly on alternator charging current and cable length between batteries.
Fuse placement for split charge relay systems
- Starter battery fuse protects the cable to the relay
- Leisure battery fuse protects the cable from the relay
Estimate fuse size using: Fuse Size Calculator
Voltage drop and charging performance
High voltage drop reduces alternator charging efficiency, especially in longer campervans.
Check expected voltage loss using: Voltage Drop Calculator
Alternator trigger wire example
Split charge relay vs DC-DC charger
Modern campervans increasingly use DC-DC chargers instead of traditional split charge relays. They provide better charging performance with smart alternators.
Compare cable sizing requirements using: DC-DC Charger Cable Size Calculator
Recommended cable types
Useful calculators for campervan charging systems
Calculate your cable size before installation
Correct cable sizing improves alternator charging performance and protects your wiring.
Open Cable Size Calculator