Victron Cyrix-Li-ct: Intelligent Battery Combiner for LiFePO4 and Lead Acid Systems

Seamless Parallel Connection Between Starter and Lithium Batteries

The Victron Cyrix-Li-ct 12/24V-120A is a microprocessor-controlled battery combiner engineered to parallel a lead acid starter battery with a LiFePO4 (lithium iron phosphate) service battery. It integrates directly with the Victron VE.Bus BMS, providing a critical layer of protection against cell undervoltage, overvoltage, and overtemperature events. This ensures your lithium battery bank remains within safe operating limits while maintaining the starter battery's charge.

Unlike generic voltage sensing relays (VSRs), the Cyrix-Li-ct responds to BMS control signals rather than voltage alone. This prevents premature disconnection or unintended cycling, a common issue with standard combiners in lithium systems. The result is a more stable, reliable dual-battery setup for caravans, motorhomes, marine vessels, and off-grid installations.

How the Cyrix-Li-ct Protects Your Lithium Investment

Lithium batteries require precise charge and discharge management. The Cyrix-Li-ct acts as a gatekeeper, engaging only when the BMS confirms safe conditions. It disengages instantly if the BMS signals a fault, such as cell overvoltage or high temperature, isolating the lithium battery from the alternator or charger. This rapid response helps prevent irreversible cell damage and extends battery service life.

Key protective functions include:

• BMS-controlled engagement: The relay closes only when the VE.Bus BMS Charge Disconnect output is high, ensuring charging only occurs under safe conditions.
• Immediate disengagement: If the BMS control input becomes free floating (indicating a fault), the Cyrix opens without delay, halting current flow.
• Undervoltage lockout: The relay disengages when battery voltage drops below 13.2V (12V system) or 26.4V (24V system), preventing deep discharge of the starter battery.
• Transient voltage suppression: A built-in suppressor limits voltage spikes during sudden disconnection, protecting sensitive electronics.

Start Assist: Emergency Power When You Need It

A standout feature is the start assist function. A short negative pulse on the control input closes the relay for 30 seconds, allowing the lithium service battery to assist a depleted starter battery. This can be a lifesaver in remote locations, providing a temporary boost without manual jump leads. The function is easily activated via a momentary switch connected to the BMS interface.

Engineered for Demanding Environments

The Cyrix-Li-ct is built to withstand the rigours of mobile and marine applications. Its compact, lightweight design (46 x 46 x 80 mm, 0.11 kg) simplifies installation in tight spaces. The IP54 protection rating guards against dust ingress and water splashes, while the robust 3/8"-16 (Approx. M10) stud terminals accommodate heavy-duty lugs up to 4/0 AWG (approx. 95 mm²), ensuring low-resistance connections for 120 A continuous current.

An integrated LED provides clear visual feedback on system status, aiding diagnostics and troubleshooting. The front-facing wiring diagram and technical reference further streamline setup, reducing installation time and potential errors.

Typical Applications

• Caravans and motorhomes: Automatically charge the house lithium battery from the alternator while protecting the starter battery.
• Marine systems: Combine a lead acid engine battery with a LiFePO4 house bank, with BMS-coordinated safety.
• Off-grid solar: Integrate with Victron inverter/chargers and MPPT controllers for a fully managed energy system.
• Commercial vehicles: Maintain auxiliary lithium batteries for lifts, refrigeration, or onboard equipment.

Installation and Compatibility Notes

The Cyrix-Li-ct requires a Victron VE.Bus BMS for operation; it is not a standalone voltage sensing relay. For systems without a VE.Bus BMS, consider the Victron Cyrix-ct or BatteryProtect range. Always refer to the Victron wiring guidelines and ensure cable sizing matches the 120 A rating. Note typical de-rating factors apply for continuous high-current use in elevated temperatures.

Take Control of Your Dual-Battery System

Upgrade your energy management with the Victron Cyrix-Li-ct 12/24V-120A. Designed for seamless integration with Victron lithium batteries and BMS, it delivers the precision and protection your system demands. Order today to ensure reliable, automated battery combining on your next adventure.