Victron Cyrix vs Argo FET vs Orion is the standard choice for charging a house battery from an engine. All three keep the starter battery safe, but they work very differently. One is a relay, one is an isolator and one is a charger. Here’s how to choose, using Victron’s datasheets.
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 vs Argo FET vs Orion at a Glance
| Cyrix-ct 120A | Argofet 100-2 | Orion XS 12/12-50A | |
|---|---|---|---|
| What it is | Voltage-sensing relay | FET battery isolator | DC-DC battery charger |
| Batteries connected together? | Yes, when charging | Never | Never |
| Rating | 120A continuous | 100A | 50A charge current |
| Voltage loss | Virtually none | <0.02V to ~0.1V | Regulated output |
| Controls charge voltage? | No | No | Yes, 10–17V |
| Limits current? | No | No | Yes, 1–50A |
| Smart alternators | Poor fit | Poor fit | Designed for it |
| Settings | None | None | VictronConnect |
Sources: Victron Cyrix-ct 120A–230A, Argofet and Orion XS datasheets.
How the Cyrix Works
The Cyrix-ct connects the two batteries in parallel when it sees charging voltage, 13–13.8V on 12V, and separates them when voltage falls. Victron says it watches the voltage trend, so it doesn’t chatter. It’s bidirectional, so it also engages when a shore charger charges the house battery. It draws under 4mA when open.
How the Argofet Works
The Argofet splits one charge source between two or three batteries but never joins them. There’s no relay to wear out. It has an energize input for alternators that need voltage on B+.
How the Orion Works
The Orion XS is a charger. It takes whatever the alternator gives, from 9V to 17V, and delivers a proper bulk, absorption and float charge at the current you set. Its engine-shutdown detection is tuned for smart alternators. The isolated Orion-Tr Smart adds 200V galvanic isolation and voltage conversion.
Which One Should You Buy?
- Lithium house battery: Orion XS or Orion-Tr Smart. Cyrix and Argofet can’t set the lithium charge profile or limit current. Victron’s Cyrix-Li-ct only suits systems with a VE.Bus BMS.
- Smart alternator (Euro 5/6, most newer vehicles): Orion.
- Lead-acid house and starter, conventional alternator: Cyrix-ct is the simplest and cheapest.
- Boat with 2–3 lead-acid banks: Argofet, sized to the alternator.
- Separate negatives or 24V-to-12V: Orion-Tr Smart isolated.
Full specifications for the Cyrix battery combiners are on Victron’s product page.
Worked Example: A Van With a Smart Alternator and Lithium
A 2020 van with a smart alternator and a 12V 200Ah lithium house battery:
- An Orion XS 12/12-50A set to its smart-alternator engine detection charges only while the engine runs.
- At the full 50A, a 90-minute drive puts back about 75Ah, taking the battery from 40% to around 77% (estimate).
- The charge profile is set to the lithium battery maker’s voltages in VictronConnect, and current can be reduced if the alternator is small.
- The starter battery stays separate the whole time, so camping loads never touch it.
A Cyrix or Argofet here would leave charging up to the alternator’s own voltage, which a smart alternator often drops below what lithium needs.
Worked Example: A Lead-Acid Camper With a Conventional Alternator
An older camper with a conventional alternator, a lead-acid starter battery and a 110Ah AGM house battery:
- A Cyrix-ct 120A connects the batteries when the engine starts charging and separates them when it stops.
- The house battery charges at whatever the alternator supplies, with almost no voltage loss across the relay.
- Parked overnight, the Cyrix opens and the house battery runs the lights and fridge alone.
- On shore power, the Cyrix also engages when the shore charger raises house voltage, topping up the starter battery too.
It’s a simple, low-cost setup with no settings to configure.
Worked Example: A Boat With Three Banks
A sailboat with a 90A alternator, a starter battery and two lead-acid house banks:
- An Argofet 100-3 splits the alternator’s output to all three batteries without ever connecting them.
- Choose an Argofet rated at or above the alternator’s maximum output, so the 100A model suits a 90A alternator.
- If the alternator needs voltage on its B+ to start charging, wire the energize input as shown in the manual.
Sizing at a Glance
| Charge source | Good starting point |
|---|---|
| — | — |
| Up to 120A conventional alternator, lead-acid | Cyrix-ct 120A |
| Larger alternator, lead-acid | Cyrix-ct 230A |
| Several lead-acid banks | Argofet sized to the alternator |
| Any alternator, lithium house battery | Orion XS 50A, or several Orion units for more current |
| Different voltages or isolated negatives | Orion-Tr Smart isolated |
Wiring Notes
- Fuse each cable at the battery end, sized to the cable.
- Keep runs short and cable thick, especially for the Orion XS, which reduces current if its input voltage sags.
- Run negatives back to a common point, or use the isolated Orion-Tr Smart where negatives must stay separate.
- Check each device’s manual for the torque values and cable sizes.
Adding Solar to the Mix
Solar charging works alongside all three. A SmartSolar MPPT on the house battery charges it when parked, while the Cyrix, Argofet or Orion handles the drive. With a Cyrix, solar on the house battery can also keep the starter battery topped up when the Cyrix engages.
Standby Draw When Parked
All three are light on the batteries when the vehicle sits. The Cyrix-ct draws under 4mA when open, which is about 0.1Ah a day (estimate). The Argofet has no relay coil to hold. The Orion XS can be switched off with its remote input, or left to detect engine-off on its own. For a van parked for weeks, the solar charger usually matters more than any of these.
Upgrading Later
Starting with a Cyrix on lead-acid and moving to lithium later is a common path. When you upgrade, swap the Cyrix for an Orion XS or Orion-Tr Smart rather than keeping both. The existing fused cables can often be reused if they meet the Orion’s cable size for your run length.
FAQ
Can I combine a Cyrix and an Orion?
You don’t need both for the same job. The Orion already separates the batteries.
Which is most efficient?
Cyrix and Argofet lose very little voltage. The Orion XS peaks at 98.5% efficiency.
Which helps start a dead engine?
The Cyrix-ct’s Start Assist holds it closed for 30 seconds. The Orion XS has emergency reverse charge.
Do I need a Cyrix if I have an Orion XS?
No. The Orion XS keeps the batteries separate and charges the house battery itself.
Which option is lightest on the alternator?
The Orion, because you set its current limit. A Cyrix or Argofet lets the house battery take whatever the alternator can deliver.
Can a Cyrix work with lithium?
The standard Cyrix-ct isn’t designed for lithium charging. Victron’s Cyrix-Li-ct is for systems with a VE.Bus BMS.
Sources: Victron Cyrix-ct and Cyrix-Li datasheets, Argofet datasheet, Orion XS datasheet and manual (victronenergy.com, accessed Oct 2026).
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