Intelligent 120A Battery Combiner for Dual Battery Systems Trend-Based Switching, No Voltage Loss, 12/24V Auto-Ranging.

Product Overview

The Victron Cyrix-ct 12/24V-120A (CYR010120011) is an intelligent battery combiner designed to replace traditional diode isolators and basic voltage-controlled relays in dual battery systems. Unlike simple voltage-threshold combiners that connect and disconnect on a fixed time delay, the Cyrix-ct monitors the battery voltage trend whether voltage is rising or falling and only reverses a previous connect or disconnect decision once that trend has genuinely reversed over a sustained period. This prevents unnecessary disconnections during momentary high-current loads and ensures a large, deeply discharged battery bank is not incorrectly rejected when first connected.

The Cyrix-ct operates with virtually no voltage loss unlike diode isolators which cause a 0.6V to 0.8V drop that must be compensated for making it a direct, drop-in alternative without requiring alternator output voltage adjustment. Automatic 12/24V ranging, overtemperature protection, bidirectional voltage sensing, and a push-button start assist for emergency parallel connection round out a combiner engineered for the reliability demands of marine, 4WD, and commercial vehicle dual battery installations.

Key Specifications

Continuous current rating: 120A
Voltage: 12V / 24V (auto-ranging — automatic system voltage detection)
Switching logic: Intelligent trend-based (not fixed threshold/fixed delay)
Voltage loss: Virtually none (replaces diode isolators without voltage compensation)
Bolt size: M6 (13mm long — suits single or multiple cable connections per terminal)
Overtemperature protection: Yes — disengages on excessive contact temperature, re-engages after cooling
Bidirectional voltage sensing: Yes — senses both connected batteries
Dual power supply: Yes — closes even if one battery voltage is too low to operate the Cyrix alone
Low voltage lockout: Will not close if either battery is below 2V (12V system) or 4V (24V system)
Start assist (manual parallel): Yes — push button (30 seconds) or switch
LED status indication: Not on this model (120A) — LED available on 230A model only
Operating temperature: Not specified — confirm with Victron
Dimensions: Not specified — confirm with Victron
Weight: Not specified — confirm with Victron
Warranty: Not specified — confirm with Victron

Features & Benefits

Intelligent Trend-Based Switching — Rather than connecting and disconnecting on a fixed voltage threshold and time delay, the Cyrix-ct monitors the direction of voltage change over time. It will only reverse a previous action if the trend has genuinely changed for a sustained period. The time delay before action is proportional to the magnitude of the voltage deviation — preventing nuisance disconnections during momentary high-current loads (engine cranking, winch operation) and ensuring a large discharged auxiliary battery is connected correctly rather than rejected.
No Voltage Loss — Direct Diode Isolator Replacement — Traditional diode isolators cause a 0.6V to 0.8V drop across the device, requiring alternator output voltage to be increased to compensate. The Cyrix-ct has virtually no voltage loss — making it a direct replacement for diode isolators without any need to adjust alternator or charger voltage settings.
Bidirectional Voltage Sensing — The Cyrix-ct monitors the voltage of both connected batteries simultaneously. It will engage regardless of which battery is being charged — whether the alternator is charging the starter battery or an auxiliary charger is charging the accessory battery. Either charging event will trigger the combiner to connect the batteries in parallel.
Dual Power Supply — The Cyrix-ct can draw its operating power from either connected battery. This means the unit will still close correctly even if one battery's voltage is too low to operate the combiner on its own — a practical advantage when recovering deeply discharged batteries.
Low Voltage Lockout — Safety During Installation — The Cyrix-ct will not close if either battery connection reads below 2V (12V system) or 4V (24V system). This prevents unexpected and potentially dangerous parallel connection during installation or when one battery has been physically disconnected.
Overtemperature Protection — If sustained overload causes the contact temperature to exceed safe limits, the Cyrix-ct disengages automatically. Once the unit cools down, it re-engages — protecting both the combiner and the batteries from thermal damage without requiring manual reset.
Start Assist — Emergency Parallel Connection — A push button or switch input can be used to manually force the Cyrix-ct closed, connecting both batteries in parallel for 30 seconds. Essential for emergency jump-starting from the auxiliary battery when the starter battery is discharged or damaged — without requiring additional wiring or jumper cables.
12/24V Auto-Ranging — Automatically detects the system voltage on startup — no manual voltage configuration required. A single SKU covers both 12V and 24V installations.
M6 Long Bolts (13mm) — The extended M6 bolts allow multiple power cables to be stacked on a single terminal, accommodating parallel cable runs and more complex wiring configurations without additional hardware.
Prioritises the Starter Battery — In the standard dual battery setup, the alternator charges the starter battery first. When the starter battery reaches the connect voltage, the Cyrix-ct engages to allow the auxiliary battery (and any additional batteries) to charge in parallel. The starter battery is not disadvantaged by the auxiliary battery drawing from the alternator output.

Technical Specs

Specification	Detail
Brand	Victron Energy
Part Number	CYR010120011
Model	Cyrix-ct 12/24V-120A
Continuous Current	120A
Voltage	12V / 24V (auto-ranging)
Switching Logic	Intelligent trend-based monitoring
Voltage Loss	Virtually none
Bolt Size	M6 (13mm long)
Overtemperature Protection	Yes — auto disengage and re-engage
Bidirectional Voltage Sensing	Yes
Dual Power Supply	Yes
Low Voltage Lockout	<2V (12V system) / <4V (24V system)
Start Assist	Yes — push button (30s) or switch
LED Status Indication	No (available on 230A model only)
Operating Temperature	Not specified
Dimensions	Not specified
Weight	Not specified
Warranty	Not specified — confirm with Victron

Compatible With / Goes Well With

Suitable applications:

4WD and caravan dual battery systems — alternator-based auxiliary battery charging
Marine vessel dual battery and bow thruster battery management
Motorhome and RV dual battery systems
Commercial vehicle and truck secondary battery charging
Any application where a diode isolator is currently installed and a lossless alternative is desired

Typical wiring configuration:

Alternator → Starter battery (direct connection)
Starter battery → Cyrix-ct 120A → Auxiliary / accessory battery
Additional batteries each connected via separate Cyrix units in parallel

Pairs well with:

Victron Energy SmartShunt 300A (for auxiliary battery monitoring)
Victron Energy Blue Smart IP65/IP67 battery chargers (for shore power charging)
Victron Energy Battery Protect 12/24V-65A (for load protection on auxiliary battery)
Victron Energy Orion-Tr DC-DC converters (for regulated isolated battery-to-battery charging)

Frequently Asked Questions

How is the Cyrix-ct different from a standard voltage-controlled relay or split charge relay?

A basic relay connects and disconnects on a fixed voltage threshold with a fixed time delay. The Cyrix-ct monitors the voltage trend — rising or decreasing — over time and only acts once a genuine trend change is confirmed. This prevents nuisance disconnections from momentary voltage dips during high-current loads, and correctly handles the voltage drop that occurs when connecting a large discharged battery.

Does the Cyrix-ct cause a voltage drop like a diode isolator?

No. The Cyrix-ct has virtually no voltage loss — making it a direct replacement for diode isolators without needing to increase alternator or charger output voltage to compensate. This is one of its primary advantages over diode-based isolation.

What does the start assist function do?

A connected push button or switch can force the Cyrix-ct to close, connecting both batteries in parallel for 30 seconds. This allows the auxiliary battery to assist with starting if the starter battery is discharged or damaged — an emergency jump-start capability without jumper cables.

Is the Cyrix-ct suitable for a system with solar charging on the auxiliary battery?

Yes. The Cyrix-ct senses voltage from both connected batteries. If a solar charger raises the auxiliary battery voltage to the connect threshold, the Cyrix-ct will engage and allow the starter battery to benefit from the solar charge. Bidirectional sensing means either battery being charged will trigger the parallel connection.

What is the difference between the Cyrix-ct 120A and the 230A model?

The primary practical difference is current rating — 120A vs 230A continuous. The 230A model also adds LED status indication (on/off/connecting/disconnecting/alarm states) which is absent on the 120A model. For systems with multiple engage/disengage profiles, the Cyrix-i 400 is the appropriate choice.

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Victron CYR010120011 Cyrix-ct 12/24V 120A Intelligent Battery Combiner