A Victron Cyrix battery combiner is a microprocessor-controlled heavy-duty relay. It connects a starter battery and a house battery in parallel when one of them is being charged, and separates them again when charging stops. Victron pitches the Cyrix as a replacement for diode isolators, because it has virtually no voltage loss, so you don’t need to raise alternator or charger voltage. The family includes the Cyrix-ct (120A and 230A) for lead-acid systems and the Cyrix-Li series built for Victron LiFePO4 systems run by a VE.Bus BMS.
Victron Cyrix Battery Combiner Specs (Cyrix-ct)
| Spec | Cyrix-ct 12/24-120 | Cyrix-ct 12/24-230 |
|---|---|---|
| Continuous current | 120A | 230A |
| Cranking rating (5 s) | 180A | 500A |
| Connect voltage | 13–13.8V (26–27.6V) | 13–13.8V (26–27.6V) |
| Disconnect voltage | 11–12.8V (22–25.7V) | 11–12.8V (22–25.7V) |
| Draw when open | Under 4mA | Under 4mA |
| Draw when closed (12V) | 220mA | 320mA |
| LED status | No | Yes |
| Protection | IP54 | IP54 |
| Operating temperature | −40 to +60°C | −40 to +60°C |
| Bolt size | M6 | M8 |
Source: Victron Cyrix-ct 120A–230A datasheet. Voltages auto-range for 12V or 24V.
How the Cyrix-ct Decides When to Connect
Cheap voltage-sensitive relays can chatter: a big load drops the voltage and they disconnect, or a flat house bank pulls the voltage down the moment they connect. Victron says the Cyrix-ct watches the voltage trend, not just a fixed voltage, and only reverses a decision if the trend has reversed for a while. For example, between 13.6V and 13.8V it connects after about 4 seconds, while between 13.0V and 13.2V it waits about 10 minutes. Above 16V it disconnects for overvoltage.
Other features from the datasheet:
- Bidirectional sensing. It also engages when a shore charger is charging the house battery.
- Start Assist. A push button keeps it engaged for 30 seconds to help start an engine from the house bank.
- Over-temperature disconnect if the contacts overheat.
Cyrix-Li: For Victron Lithium Systems
The Cyrix-Li models (Li-load, Li-Charge and Li-ct) take a control signal from a VE.Bus BMS. The Cyrix-Li-ct connects a lead-acid starter battery to a LiFePO4 bank when it senses 13.4V or more, and disconnects below 13.2V or when the BMS signals a cell problem.
Who It’s For
- Boats and older vehicles with a conventional alternator and lead-acid house batteries.
- Victron LiFePO4 systems with a VE.Bus BMS (Cyrix-Li versions).
- If you have a smart alternator or a lithium house bank without a Victron BMS, a DC-DC charger such as the Orion XS is usually the better tool, because it controls current and charge voltage.
LED Codes (230A Model)
| LED | Meaning |
|---|---|
| On | Engaged |
| 10s flash | Disengaged |
| 2s flash | Connecting |
| 2s blink | Disconnecting |
| 0.25s blink | Alarm: over temperature, voltage above 16V, both batteries below 10V, or one battery below 2V (double these for 24V) |
Setup and Troubleshooting Tips
- Won’t connect after install. The Cyrix won’t close if either battery reads under 2V (12V systems) or 4V (24V). Victron designed this to prevent surprises during installation. Check that both batteries are connected.
- Takes a while to connect. That’s by design. Victron says the Cyrix watches the voltage trend and adjusts its delay to how far the voltage deviates. It doesn’t switch on a fixed timer, so it avoids chattering on and off.
- Disconnects on a hot day or a long heavy charge. It disengages if the contacts overheat and reconnects after cooling.
- Fast blinking LED. Read the alarm list above. Over 16V usually points to a charging fault.
- Lithium house battery. A Cyrix-ct doesn’t control charge voltage or current. With LiFePO4, Victron offers Cyrix-Li models controlled by a VE.Bus BMS. Most builds with a smart alternator use a DC-DC charger such as the Orion XS instead.
Choosing Between the 120A and 230A Models
Size the Cyrix-ct by the alternator’s output. A 100A alternator fits the 120A model with margin; a 150A or larger alternator calls for the 230A model.
Closed-state draw matters on long engine runs: 220mA is about 5Ah over a 24-hour passage on the 120A model, and 320mA about 7.7Ah on the 230A model. Both are small compared to the charge the alternator is delivering. When open, the draw is under 4mA.
Worked examples are arithmetic estimates from the published ratings above, not test results. Real numbers depend on temperature, cable runs, battery state and load.
Worked Example: A Day of Driving
With the engine running, the alternator lifts the starter battery into the 13–13.8V connect window and the Cyrix-ct joins the two batteries so the house bank charges too. When the engine stops and loads pull the voltage below the 11–12.8V disconnect window, the Cyrix opens and the starter battery is kept separate for the next start. On 24V systems the same logic applies at 26–27.6V and 22–25.7V.
Victron’s Cyrix battery combiners product page has the full datasheet and manual downloads.
FAQ
Is a Cyrix the same as a battery isolator?
It does the same job as a diode isolator but uses a relay, so there’s almost no voltage drop.
Can a Cyrix-ct charge a lithium battery from my alternator?
Victron makes the Cyrix-Li-ct for that job in systems with its VE.Bus BMS. For other lithium setups, use a DC-DC charger.
120A or 230A?
Match it to your alternator output and cable size. Choose the 230A if you want the LED status and the control cable included.
Does a Cyrix drain the battery?
Under 4mA when open; 220mA or 320mA when closed on 12V.
Can I use one on 24V?
Yes. The Cyrix-ct auto-ranges between 12V and 24V.
Cyrix or Argo FET?
The Argo FET is a solid-state isolator. The Cyrix is a relay that parallels the batteries.
Do I need fuses on both sides?
Yes. Fuse each battery connection close to its battery, sized for the cable, so a fault in the combiner wiring is protected from either direction.
Sources: Victron Cyrix Battery Combiners product page; Cyrix-ct 120A–230A and Cyrix Li-ion 230A datasheets (victronenergy.com).
Sources: Victron Cyrix-ct 120A–230A and Cyrix-Li datasheets (victronenergy.com, accessed Oct 2026).
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