Vito Split Charge Relay Installation

The Mercedes-Benz Vito is a versatile workhorse, often pressed into service as a campervan conversion, a mobile workshop, or a vehicle carrying a significant amount of electrical equipment. For many of these applications, powering a secondary, leisure battery is essential. This is where a split charge relay comes into play. But can you put a split charge relay on your Vito? The straightforward answer is yes, you absolutely can, and it's a very common and highly recommended modification for anyone looking to effectively manage power between their vehicle's starter battery and an auxiliary battery system.

What is a Split Charge Relay?

At its core, a split charge relay acts as an intelligent switch. When your vehicle's engine is running and the alternator is charging the starter battery, the relay detects this voltage increase. Once the starter battery reaches a certain voltage threshold (typically around 13.2-13.8 volts), the relay closes, connecting the starter battery to the auxiliary battery. This allows the alternator to charge both batteries simultaneously. When the engine is turned off and the voltage drops, the relay opens, isolating the two batteries. This prevents the auxiliary battery from draining the starter battery, ensuring you can always start your Vito.

Why Install a Split Charge Relay in Your Vito?

The benefits of installing a split charge relay are numerous, especially for Vito owners who utilise their vehicles for more than just basic transport:

• Powering Ancillary Equipment: Whether it's lights, fridges, inverters, charging points, or even specialised tools, an auxiliary battery system powered via a split charge relay provides a dedicated power source that won't leave you stranded with a flat starter battery.
• Campervan Conversions: This is arguably the most popular application. A split charge relay allows you to run your camping fridge, lights, and charge devices without draining the main engine battery.
• Mobile Workshops: Tradespeople using their Vito as a mobile workshop can power essential tools, diagnostic equipment, and lighting reliably.
• Protecting Your Starter Battery: The primary function is to safeguard your starter battery, ensuring your Vito always has enough power to start its engine.
• Simplicity and Cost-Effectiveness: Compared to more complex DC-DC charging systems, a basic split charge relay is a relatively simple and cost-effective way to establish a dual-battery setup.

Types of Split Charge Relays

There are a few main types of split charge relays you might consider for your Vito:

1. Voltage-Sensing Relay (VSR)

These are the most common and easiest to install. They automatically connect and disconnect the batteries based on the voltage of the starter battery. They are generally reliable and a good starting point for most users.

2. Manual Split Charge Relay

These require a manual switch to connect or disconnect the batteries. While offering more control, they also introduce the possibility of forgetting to engage or disengage them, potentially leading to issues.

3. Smart Alternator Compatible Relays

Modern vehicles, including some Vito models, are equipped with "smart" or variable voltage alternators. These alternators don't always maintain a consistent high voltage output. Standard VSRs might not interpret this correctly, leading to delayed or inconsistent charging. For these vehicles, a smart split charge relay or a DC-DC charger is a better option. These are designed to work with the variable output of smart alternators.

Installing a Split Charge Relay on Your Vito: A General Guide

While specific wiring can vary slightly depending on your Vito model and the exact relay you purchase, the general principle remains the same. It's crucial to consult your relay's specific instructions and, if unsure, seek professional advice.

What You'll Need:

• Split charge relay (appropriate type for your Vito)
• Appropriate gauge wiring (consult a wiring gauge chart based on current and distance)
• In-line fuses or circuit breakers (sized correctly for the wire and battery)
• Battery terminals and connectors
• Crimping tool
• Wire strippers
• Multimeter
• Basic hand tools (spanners, screwdrivers)
• Optional: Cable ties, conduit for neatness and protection

Step-by-Step Installation (General):

1. Safety First: Disconnect the negative terminal of the starter battery. Ensure all electrical loads in the vehicle are switched off.
2. Locate Batteries: Identify the location of your starter battery and your auxiliary battery. In a Vito, the starter battery is typically under the driver's seat or in the engine bay. The auxiliary battery is often fitted in the rear cargo area or a custom-built compartment.
3. Mount the Relay: Mount the split charge relay in a suitable location, ideally close to the auxiliary battery or in a central position, ensuring it's protected from moisture and vibration.
4. Wire the Starter Battery Side: Run a positive cable from the starter battery's positive terminal to one of the main terminals on the split charge relay. Fit an appropriate fuse or circuit breaker on this cable, as close to the starter battery as possible.
5. Wire the Auxiliary Battery Side: Run a positive cable from the auxiliary battery's positive terminal to the other main terminal on the split charge relay. Fit another fuse or circuit breaker on this cable, as close to the auxiliary battery as possible.
6. Connect the Trigger/Sense Wire: This is where different relays vary. Most VSRs have a small terminal that needs to be connected to a power source that is only live when the engine is running. This could be:
   • Directly to the starter battery's positive terminal (if the relay has its own internal voltage sensing).
   • To an ignition-switched live feed (e.g., a fuse tap on a circuit that is only powered when the ignition is on).
   • Some relays have a specific connection point designed to sense the alternator's output. Always refer to your specific relay's wiring diagram.
7. Connect the Grounds: Ensure both the starter battery and the auxiliary battery have robust ground connections to the vehicle's chassis. The relay itself may also require a ground connection, depending on the model.
8. Double-Check Wiring: Before connecting the battery, meticulously check all connections against the relay's diagram. Ensure correct polarity and that all connections are secure.
9. Connect Battery Terminals: Reconnect the negative terminal of the starter battery.
10. Test the System: Start the Vito's engine. Use a multimeter to check if the voltage across the auxiliary battery terminals begins to rise once the engine is running and the starter battery is sufficiently charged. The relay should click as it engages. Turn off the engine and verify that the voltage between the batteries is no longer connected.

Important Considerations for Your Vito

When installing a split charge system on your Vito, keep these points in mind:

Wiring Gauge:

Using the correct wire gauge is critical. Undersized wires can overheat, cause voltage drop, and lead to inefficient charging or even fire. Use a wiring gauge calculator or chart based on the maximum current expected and the length of the cable run.

Fusing:

Fuses or circuit breakers are non-negotiable safety devices. They protect your wiring and batteries from overcurrents and short circuits. Always fuse both positive connections, as close to the power source as possible.

Smart Alternators:

As mentioned, if your Vito has a "smart" alternator, a standard VSR might not be ideal. A DC-DC charger offers superior control and charging efficiency for modern vehicles. It isolates the batteries and uses the vehicle's electrical system to intelligently charge the auxiliary battery, often with specific charging profiles for different battery types (like AGM or Lithium).

Battery Type:

Ensure your auxiliary battery is suitable for deep cycling. Standard car starter batteries are not designed for repeated deep discharges and will have a significantly reduced lifespan. Deep cycle, leisure batteries, AGM, or Lithium batteries are recommended.

Cable Runs:

Plan your cable runs carefully. Keep them away from hot engine components, sharp edges, and moving parts. Use conduit and cable ties for a tidy and safe installation.

Common Problems and Troubleshooting

If your split charge system isn't working as expected, consider these common issues:

Is a DC-DC Charger Better?

While a split charge relay is a good solution for many, a DC-DC charger offers a more advanced and often superior method of charging an auxiliary battery, especially in modern vehicles:

• Smart Alternator Compatibility: Designed to work with variable voltage outputs.
• Battery Type Specific Charging: Can be programmed for optimal charging of different battery chemistries (AGM, Gel, Lithium).
• Voltage Drop Compensation: Actively compensates for voltage drops over long cable runs.
• Isolation: Provides complete electrical isolation between the starter and auxiliary batteries.
• Higher Efficiency: Often more efficient in transferring power.

However, DC-DC chargers are typically more expensive and can be more complex to install than a simple split charge relay.

Frequently Asked Questions

Q1: Can I use a standard car relay?

No, a standard automotive relay (like a 30A or 40A relay) is not designed for the continuous high current and voltage sensing required for a split charge system. You need a relay specifically designed for dual-battery applications.

Q2: What size fuse should I use?

The fuse size depends on the gauge of your wire and the expected maximum current draw of your auxiliary system. Always consult a wiring chart and choose a fuse that protects the wire, not just the battery. A common starting point for many leisure setups is a 50A or 70A fuse, but this must be determined by your specific wiring and expected load.

Q3: Do I need to connect the relay's ground?

Check your specific relay's manual. Some relays have a dedicated ground terminal that needs to be connected to the vehicle chassis for the voltage sensing to work correctly. Others are designed to sense voltage through the main battery connections.

Q4: How do I know if my Vito has a smart alternator?

Identifying a smart alternator can be tricky without specific vehicle diagnostic tools. Generally, vehicles with start-stop technology or advanced battery management systems are more likely to have them. If in doubt, consult your Vito's manual or a Mercedes-Benz specialist. If you experience inconsistent charging voltages or your standard VSR doesn't seem to engage reliably, it's a strong indicator.

Conclusion

Installing a split charge relay on your Mercedes-Benz Vito is a practical and achievable modification that opens up a world of possibilities for powering your accessories and creating a more functional vehicle. By understanding the principles, choosing the right type of relay, and following a careful installation process, you can effectively manage your dual-battery system and ensure your Vito is always ready to go. Always prioritise safety and consult the specific instructions for your chosen components.