How to Choose Victron Isolators and Protection

This Victron isolators and protection guide covers the small parts that keep a mobile or marine system safe. Cyrix battery combiners and Argo FET isolators share charging between batteries. The Smart BatteryProtect cuts loads before a battery is ruined. Each one solves a different problem, and many systems need one of each. Specs come from 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.

The Victron Isolators and Protection Lineup

Product	Job	Ratings	Settings
Cyrix-ct	Joins starter and house batteries while charging	120A or 230A, 12/24V auto	None
Cyrix-Li series	Same job for Victron lithium with a VE.Bus BMS	Li-ct, Li-load, Li-Charge	BMS-controlled
Argofet	Splits one charge source to 2–3 batteries, never joins them	100A or 200A, 12/24V	None
Smart BatteryProtect	Low-voltage load disconnect	65A, 100A or 220A, 6–35V	Bluetooth

Sources: Victron Cyrix-ct, Cyrix Li-ion, Argofet and Smart BatteryProtect datasheets.

Cyrix Battery Combiners

A Cyrix-ct is a microprocessor-controlled relay. It connects the batteries at 13–13.8V and disconnects at 11–12.8V (12V system), using the voltage trend so it doesn’t chatter. It draws under 4mA when open and has a Start Assist mode that stays engaged for 30 seconds to help start the engine. Best for lead-acid systems on a conventional alternator.

Argo FET Battery Isolators

An Argofet keeps batteries separate at all times, with a voltage loss below 0.02V at low current and about 0.1V at higher current. It has an energize input for alternators that need voltage on B+. Best for boats with two or three lead-acid banks. Size it to the alternator’s full output.

Smart BatteryProtect

The Smart BatteryProtect disconnects DC loads at 10.5V by default on 12V, and reconnects at 12V. It uses MOSFETs, so there are no sparks. It has three modes (buzzer/LED, relay and Li-ion), and in Li-ion mode a BMS can switch it. Never run an inverter’s DC input through it. Victron says to use the inverter’s remote on/off instead.

When You Need a DC-DC Charger Instead

None of the combiners or isolators controls charge voltage or limits current. For a lithium house bank, a smart alternator or batteries of different chemistries, use an Orion XS (50A, 98.5% efficient) or an isolated Orion-Tr Smart.

Quick Picks

Lead-acid van or older truck: Cyrix-ct 120A plus a Smart BatteryProtect 65A on the DC loads.
Boat with starter, house and thruster batteries: Argofet 200-3.
Lithium van with smart alternator: Orion XS, plus a Smart BatteryProtect in Li-ion mode or a BMS-controlled disconnect.
Victron lithium with VE.Bus BMS: Cyrix-Li-ct for the starter-to-house link.

Datasheets and manuals for the Victron battery isolators and combiners and the Smart BatteryProtect are on Victron’s website.

Worked Example: Protecting a Lead-Acid Van’s DC Loads

A van with a 12V 110Ah AGM house battery, a fridge, lights and a water pump:

A Smart BatteryProtect 65A sits between the house battery and the DC fuse box.
At the default 10.5V, it disconnects the loads before the battery is deeply discharged, then reconnects at 12V once charging lifts the voltage.
Many AGM makers prefer a higher cut-off for longer life. In VictronConnect, you can raise the disconnect voltage, for example to 11.8V (example setting; follow your battery maker).
A Cyrix-ct 120A shares alternator charge with the house battery while driving.

Worked Example: A Lithium Van With BMS Control

A van with a lithium battery, an external BMS and an Orion XS:

Set the Smart BatteryProtect to Li-ion mode, and wire the BMS load-disconnect output to its remote input.
When a cell goes low, the BMS signals the BatteryProtect, which switches the DC loads off cleanly with no sparks.
The inverter is wired straight to the battery through its own fuse, and the BMS switches it through its remote on/off, as Victron recommends.
The Orion XS handles alternator charging, with its remote input wired to the BMS charge-disconnect.

Worked Example: A Boat With Three Banks

A sailboat with a 120A alternator, a starter battery, a house bank and a bow-thruster battery:

An Argofet 200-3 splits the alternator’s output across all three banks, sized above the alternator’s full output.
The very low voltage loss, under 0.02V at low current, keeps charging voltage close to the alternator’s output.
A Smart BatteryProtect 100A on the house bank’s DC panel protects the house bank from deep discharge.

Sizing a Smart BatteryProtect

Add up the maximum continuous current of the loads it will switch.
Pick the next model up: 65A, 100A or 220A.
Make sure the loads don’t include an inverter’s DC input.
Fuse the cable between the battery and the BatteryProtect at the battery end.

Setting Up the Smart BatteryProtect

Choose the system voltage. It works from 6 to 35V.
Select a mode: buzzer/LED, relay, or Li-ion for BMS control.
Set disconnect and reconnect voltages, or use a preset.
Watch status and history in VictronConnect over Bluetooth.

Matching Device to Problem

Problem	Device
—	—
Starter battery drained by camping loads	Cyrix-ct or Orion
House battery over-discharged by DC loads	Smart BatteryProtect
Several lead-acid banks on one alternator	Argofet
Lithium house bank, controlled charge	Orion XS or Orion-Tr Smart
Victron lithium with VE.Bus BMS, starter link	Cyrix-Li-ct

Installation Notes

Mount combiners and isolators close to the batteries to keep cable runs short.
Use cable sized for the full current and fuse every positive at the battery.
Keep the Smart BatteryProtect in a dry, ventilated spot, and route its ground wire as the manual shows.

Checking It All Works

After installation, test each device once. Start the engine and confirm the Cyrix clicks in or the Orion starts charging. Switch off the charge sources and run the DC loads until the Smart BatteryProtect reaches its disconnect voltage, or simulate it by lowering the setting temporarily in VictronConnect. A quick test now confirms the protection will work when it’s needed.

FAQ

Do I need a BatteryProtect if my battery has a BMS?

A lithium BMS may already cut loads. A BatteryProtect switches loads cleanly before the BMS has to act.

Can a Cyrix charge lithium?

Only the Cyrix-Li versions in systems with a VE.Bus BMS.

Isolator or combiner?

Combiners join batteries during charging. Isolators never join them.

Does the Smart BatteryProtect draw power itself?

Very little. It’s designed for continuous use, so it can stay in the circuit while the vehicle or boat sits.

Why not run an inverter through a BatteryProtect?

An inverter’s input capacitors draw a large inrush current that can damage it. Use the inverter’s remote on/off instead.

Sources: Victron Cyrix-ct 120A–230A and Cyrix Li-ion datasheets, Argofet datasheet, Smart BatteryProtect datasheet and manual, Orion XS datasheet (victronenergy.com, accessed Oct 2026).