Best Cable for Split Charge Relay
This guide explains the best cable for split charge relay installations in 12V vehicles, campervans, motorhomes, 4x4 builds and auxiliary battery systems. It covers common cable sizes, voltage drop, fuse protection and when to move up from 6mm² to 10mm² or larger battery cable.
What is the best cable for a split charge relay?
For many basic split charge relay setups, 6mm² automotive cable is often used as a starting point. However, the best cable choice depends on current, cable run length, alternator output and how quickly you want the leisure battery to charge.
In real 12V installations, the best result usually comes from choosing cable based on voltage drop as well as amp rating. That is why many installers prefer 10mm² or 16mm² battery cable for longer runs or higher charging currents, especially between the starter battery, relay and leisure battery.
- 6mm² – common for shorter, lower demand split charge relay installs
- 10mm² – better for reduced voltage drop and improved charging performance
- 16mm² – often preferred for longer runs or heavier duty battery charging circuits
Quick cable guide for split charge relay systems
| Typical Setup | Suggested Cable Size | Best For | Notes |
|---|---|---|---|
| Small basic split charge relay install | 6mm² | Short cable runs, modest charging current | Common entry-level choice, but check voltage drop carefully |
| General campervan or 4x4 relay install | 10mm² | Better charging performance on medium runs | Often a stronger all-round choice than 6mm² |
| Longer run to rear leisure battery | 16mm² | Reduced voltage loss over distance | Helps maintain better charging voltage at the leisure battery |
| High current battery charging setup | 16mm² to 25mm² | Heavy-duty systems and large auxiliary batteries | Often used where cable length or current demand is high |
Why cable size matters in split charge relay wiring
A split charge relay connects the starter battery and leisure battery while the engine is running, allowing the alternator to charge both batteries. If the cable is too small, resistance increases and charging voltage at the leisure battery can fall.
- Smaller cable means higher resistance
- Higher resistance increases voltage drop
- Voltage drop reduces charging efficiency
- Poor charging performance can leave the leisure battery undercharged
That is why the best cable for split charge relay systems is usually chosen with both current capacity and cable run length in mind.
Typical cable route in a split charge relay installation
In most installs, current flows from the starter battery area through the relay and then to the leisure battery. On many vans and 4x4 conversions, the leisure battery may be several metres away, which makes voltage drop more important.
6mm² vs 10mm² vs 16mm² for split charge relay wiring
6mm² cable
6mm² automotive cable is commonly used in basic split charge relay kits. It can be suitable on short runs with moderate charging current, but it can become restrictive where the leisure battery is mounted farther from the engine bay.
10mm² cable
10mm² cable is often the best balance between cost, flexibility and lower voltage drop. For many general campervan conversions, it is a more robust option than 6mm² and can improve charging performance noticeably.
16mm² cable
16mm² battery cable is a strong choice where charging cable runs are longer, battery capacity is larger or the system is designed for heavier duty use. It helps keep voltage loss lower and supports more efficient charging.
Best cable type for split charge relay installs
For split charge relay wiring, many installers use flexible automotive battery cable rather than smaller thin wall cable, especially on the main charging path between batteries.
- Flexible battery cable is easier to route in vehicle installations
- Heavier cable sizes are available for charging circuits
- Suitable for starter battery to leisure battery connections
- Commonly used with crimp lugs, MIDI fuse holders and battery terminals
Thin wall cable is excellent for many automotive circuits, but on the main split charge feed, larger battery cable is often the more suitable choice.
Fuse protection for split charge relay cable
Every main positive cable in a split charge relay system should be fused close to the power source. This helps protect the wiring in the event of a short circuit or installation fault.
- Fuse near the starter battery end
- Fuse near the leisure battery end where appropriate
- Select fuse size to protect the cable, not just the device
- Use quality fuse holders and secure terminations
Common split charge relay cable size examples
| Vehicle / Use Case | Typical Cable Choice | Reason |
|---|---|---|
| Small van conversion with short run | 6mm² to 10mm² | Basic charging circuit with limited distance |
| Campervan with rear leisure battery | 10mm² to 16mm² | Helps reduce voltage drop over a longer route |
| 4x4 auxiliary battery setup | 10mm² to 16mm² | Good balance of flexibility and performance |
| Motorhome or larger auxiliary battery bank | 16mm² to 25mm² | Better suited to heavier charging circuits |
When a split charge relay may need heavier cable
You may want to step up to a larger cable size when:
- The leisure battery is mounted far from the starter battery
- You are charging larger battery capacity banks
- You want faster and more effective charging
- The alternator and installation can support higher charging current
- You want to future-proof the system for upgrades
In many real-world installations, upgrading from 6mm² to 10mm² or 16mm² gives better charging results than staying with the minimum workable size.
Split charge relay vs DC-DC charger cable choice
A basic split charge relay system and a DC-DC charger do not always use the same cable strategy. DC-DC chargers often have defined current ratings and manufacturer cable recommendations, while simple relay systems rely more heavily on installation design and voltage drop control.
- Split charge relay systems often benefit from oversized cable to reduce voltage loss
- DC-DC chargers usually have specific cable guidance from the charger manufacturer
- Long runs can justify larger cable in both types of system
Helpful tools for choosing split charge relay cable
Related automotive cable guides
Check your split charge relay cable size before installation
Using the correct cable size helps reduce voltage drop, improve leisure battery charging and protect your 12V wiring system.
Open Voltage Drop CalculatorElectrical installation safety notice
This guide is provided as a general reference for split charge relay cable selection in 12V vehicle and leisure battery systems.
Correct cable size depends on current, route length, cable type, installation method, fuse protection, ambient temperature and the specific equipment used in the vehicle.
Always confirm relay, battery, alternator and cable manufacturer recommendations before final installation.