Victron Cyrix-ct Installation: How to Wire a Cyrix

A Victron Cyrix-ct installation is one of the simplest ways to charge a house battery from your engine. The Cyrix links the two batteries when charging voltage appears and separates them when it’s gone. Here’s how it works and how to wire it, based on Victron’s Cyrix-ct datasheet. One caution first: for a lithium house battery, Victron offers the Cyrix-Li and Orion DC-DC chargers instead.

Electrical work can be dangerous: follow the manual, fuse and size every cable correctly, and use a licensed electrician for anything connected to household wiring.

Victron Cyrix-ct Installation: Ratings

	Cyrix-ct 12/24-120	Cyrix-ct 12/24-230
Continuous current	120A	230A
Cranking rating (5s)	180A	500A
Draw when open	<4mA	<4mA
Draw when closed (12V)	220mA	320mA
Stud size	M6	M8

Source: Victron Cyrix-ct 120A/230A datasheet.

How It Decides to Connect

The Cyrix senses both batteries and draws power from either one. So it also connects when the house battery is being charged, from shore power or solar for example, and shares that charge back to the starter battery. For a 12V system, the datasheet gives connect thresholds from 13.0 to 13.8V and disconnect thresholds from 11V to 12.8V, with time delays:

Voltage (12V system)	Connects after
13.0–13.2V	10 min
13.2–13.4V	5 min
13.4–13.6V	1 min
13.6–13.8V	4 s

Above 12.8V it stays connected. It disconnects within seconds as the voltage drops toward 11V, and right away above 16V (overvoltage).

The delays keep it from cycling during short, heavy loads, a problem Victron says simpler combiners have.

Wiring Steps

Disconnect both batteries before you start.
Mount the Cyrix close to the batteries.
Cable from the starter battery positive to one main stud, with a fuse near the battery.
Cable from the house battery positive to the other main stud, with a fuse near that battery.
Connect the ground wire to the negative.
Optional: wire a push button for Start Assist, which closes the Cyrix for 30 seconds to help start the engine from the house battery.

Size the fuses and cable to the Cyrix rating and cable length.

Safety Lockout

The Cyrix-ct won’t close if either battery reads under 2V (12V systems) or 4V (24V systems). That keeps it from operating during installation or when a battery is disconnected.

LED Codes

Solid: connected
10s flash: disconnected
Fast blink (0.25s): alarm, such as overheating, over 16V, both batteries under 10V, or one under 2V

Full specifications for the Cyrix battery combiners are on Victron’s product page.

24V Systems

The Cyrix-ct 12/24 models detect 12V or 24V automatically. For 24V, the datasheet says to multiply the 12V thresholds by two:

Voltage (24V system)	Connects after
26.0–26.4V	10 min
26.4–26.8V	5 min
26.8–27.2V	1 min
27.2–27.6V	4 s

Derived from the datasheet’s 12V table using its “multiply by two for 24V” note.

Current draw while closed is also lower at 24V: 120mA for the 120A model and 180mA for the 230A model.

Choosing 120A or 230A

Match the Cyrix to the current it may have to carry:

Alternator size. A typical 90A or 120A alternator fits the Cyrix-ct 120A. Larger alternators, such as 150A and up, call for the 230A.
Start Assist. Cranking current runs through the Cyrix while Start Assist holds it closed. The 230A model handles 500A for 5 seconds, against 180A for the 120A model, so pick the 230A for larger engines.
Cable and studs. The 230A uses M8 studs for heavier cable lugs. The 120A uses M6.

Worked Example: What Happens on a Drive

Here’s a typical sequence on a 12V van with a lead-acid house battery:

Engine starts. The alternator lifts the starter battery to about 14V.
Cyrix connects within about 4 seconds, since the voltage is in the 13.6–13.8V band or above.
Both batteries charge. The house battery, at a lower voltage, takes most of the current at first.
Engine stops. Voltage falls toward resting levels, and the Cyrix separates the batteries so camp loads can’t drain the starter battery.
Solar or shore power later raises the house battery above the connect threshold, and the Cyrix connects again to top up the starter battery.

Illustrative sequence based on the datasheet thresholds.

Worked Example: Self-Consumption

While connected, the 12V 120A model draws 220mA. Over an 8-hour driving day that’s about 220mA × 8h ≈ 1.8Ah, supplied by the alternator. When the batteries are separated, the datasheet lists under 4mA, or less than 0.1Ah a day (4mA × 24h ≈ 96mAh), so a vehicle can sit parked for weeks without the Cyrix noticeably drawing down either battery.

Worked estimate from datasheet ratings.

Checking the Install

Measure both batteries with a multimeter before starting the engine.
Start the engine and watch the LED. It should go solid once the connect delay passes.
Measure the house battery again. It should now rise along with the starter battery.
Stop the engine. After the voltage falls, the LED should return to its 10-second flash, meaning disconnected.
Test Start Assist if you fitted the button. The Cyrix should close for 30 seconds.

FAQ

Is a Cyrix-ct OK with lithium?

Use the Cyrix-Li models or an Orion DC-DC charger for lithium house batteries.

Does the Cyrix work on 24V?

Yes. The Cyrix-ct 12/24 models work in both 12V and 24V systems.

Do I need a separate charger with a Cyrix-ct?

No. The Cyrix shares whatever charge source is present, alternator, solar or shore charger, between the two batteries.

Why does it wait before connecting?

The time delays keep it from switching on and off during short voltage swings, so it only connects when real charging is under way.

Can I use it with two lead-acid batteries of different sizes?

Yes. The Cyrix only connects and separates the batteries, so the starter and house batteries can differ in capacity, as long as both suit the same charging voltage.

Where should the fuses go?

One at each battery positive, as close to the battery as practical, sized for the cable.

Sources: Victron Cyrix-ct 12/24V 120A and 230A datasheet; Cyrix battery combiners product page, victronenergy.com, accessed Oct 2026.