Victron sells three different ways to charge a house battery from a vehicle's alternator. The Cyrix battery combiner, the Argofet battery isolator and the Orion DC-DC charger all go between the starter battery and the house battery, but they work very differently. This guide explains what each one does and which one fits an RV, van or truck camper, especially if the house battery is lithium.
Three ways to share an alternator
| Cyrix-ct (combiner) | Argofet (isolator) | Orion-Tr Smart / Orion XS (DC-DC charger) | |
|---|---|---|---|
| How it works | A relay that connects the two batteries in parallel when either one is being charged | Solid-state (FET) splitter that lets one alternator charge 2 or 3 batteries without connecting them together | Converts the alternator's voltage into a controlled charge for the house battery |
| Charge profile | None: the house battery sees the alternator's voltage | None: same as the alternator | Its own 3-stage (Orion-Tr Smart) or 4-stage (Orion XS) profile, set in the app |
| Limits current from the alternator | No | No | Yes |
| Ratings we stock | 120 A and 230 A | 100 A or 200 A, for 2 or 3 batteries | 18 A and 30 A (Orion-Tr Smart 12/12), 50 A and 70 A (Orion XS 12/12) |
| Best fit | Lead-acid house battery, standard alternator | Several lead-acid banks on one alternator, for example on a boat | Lithium house batteries, smart alternators, long cable runs |
Cyrix-ct: the simple combiner
The Cyrix-ct watches the voltage on both batteries. When one of them is being charged and rises to between 13.0 and 13.8 V, the Cyrix closes and the two batteries are charged in parallel. When the voltage falls again, it opens, so running the fridge overnight cannot flatten the starter battery. Victron's connect table shows the delay shrinking as the voltage rises: 10 minutes at 13.0 to 13.2 V, and 4 seconds at 13.6 to 13.8 V. Below 13 V it stays open.
Other details from Victron's datasheet:
- The 120 A model carries 120 A continuous (180 A for 5 seconds of cranking). The 230 A model carries 230 A continuous (500 A cranking) and adds a status LED.
- It senses both directions, so it also closes when a shore-power charger is charging the house battery.
- Start Assist: a push button closes it for 30 seconds to help start the engine from the house battery.
- It auto-detects 12 or 24 V and has "virtually no voltage loss".
The catch is that last row in the connect table. Victron's DC-DC datasheets note that Euro 5 and Euro 6 smart alternators "often" supply too low a charging voltage even with the engine running. If the voltage never reaches 13.0 V, a Cyrix-ct stays open and the house battery gets nothing.
Argofet: the isolator for several banks
The Argofet lets one alternator, or one single-output charger, charge two or three batteries at the same time without ever connecting them together. Victron says the voltage drop is under 0.02 V at low current and averages 0.1 V at higher current, so the alternator's voltage does not need raising. Models are the 100-2, 100-3, 200-2 and 200-3: 100 A or 200 A, for two or three batteries, on 12 or 24 V.
It also has an alternator energize input. Some alternators need voltage on their B+ output before they start charging, and an isolator would otherwise block it. Victron also points out that cable runs must be short and thick enough. In its example, 50 A through 10 mm² (AWG 7) cable over 5 m drops 0.35 V. Like the Cyrix, the Argofet passes the alternator's own voltage straight through, with no charge profile of its own.
Orion DC-DC: a charger in its own right
An Orion takes whatever the alternator supplies and charges the house battery at a set current and voltage. Victron's datasheets name three situations it is built for: smart alternators that run too low, long cable runs that drop voltage, and protecting the starter battery from being drained. It has engine-running detection, so it only charges while the alternator is supplying power. You set the charge current, the battery type and the input lock-out voltage in the VictronConnect app.
- Orion-Tr Smart isolated: 12/12-18 A and 12/12-30 A, with galvanic isolation (200 V DC between input, output and case).
- Orion-Tr Smart non-isolated: 12/12-30 A, sharing a common negative.
- Orion XS: 12/12-50 A (IP65) and 12/12-70 A (IP20, dry locations only), adjustable in 0.1 A steps, with a VE.Direct port for a GX device.
Choosing between the Orion models is covered in our Orion XS vs Orion-Tr Smart guide. For wiring, see how to install a Victron Orion DC-DC charger.
Lithium house battery? Victron's own rules
This is where the choice gets clear. Victron's manual for its Lithium NG batteries says lithium "accept[s] a much higher charging current" than lead-acid, and lists three precautions to avoid overloading the alternator:
- An alternator rated at least twice the battery's capacity. Victron's example is a 400 A alternator for a 200 Ah battery.
- A temperature-controlled alternator regulator.
- A current-limiting device, such as a DC-DC charger.
Check your alternator's rating and regulator against the first two. If it does not meet them, the third is the one that applies. Victron's SuperPack NG manual says the same thing directly: use a DC-DC charger between the alternator and the battery, and it names the Orion-Tr Smart and Orion XS.
What about the Cyrix-Li models? The Cyrix-Li-ct parallels a lead-acid starter battery with a LiFePO4 battery. Victron's datasheet describes it working together with the VE.Bus BMS, which opens it on cell over-voltage or over-temperature. It still adds no current limit, so the alternator rules above still apply. The Cyrix-Li-load and Cyrix-Li-Charge are switches that a BMS uses to cut loads or chargers. They are not alternator chargers. If you have a newer NG BMS, check its manual for which signals it offers before pairing it with a Cyrix-Li.
Sizing the Orion against your alternator
A DC-DC charger draws more from the alternator than it delivers, because of conversion losses. Victron's Orion-Tr Smart manual gives the 12/12-30 as drawing about 35 A from the input at full output. The same manual warns that "at idle speed, alternator output can drop to roughly 50% of the rating", and that a hot alternator gives less still. Victron's advice is to compare the alternator's continuous rating, at idle and when hot, with the total input current of all chargers plus the vehicle's own loads. If the alternator can't keep up, use a smaller model or fewer units in parallel.
Victron's fuse table for the isolated Orion-Tr Smart lists a 60 A fuse on the 12 V side. Minimum cable is 6 mm² at 0.5 m, 10 mm² at 1 to 2 m and 16 mm² at 5 to 10 m.
Quick picks
- Lithium house battery, any alternator: Orion-Tr Smart or Orion XS.
- Smart alternator (newer vans and pickups): an Orion, set up for engine-running detection.
- Travel trailer charged through the 7-pin: an Orion. The long cable makes voltage drop the main problem, and the Orion can boost.
- Lead-acid house battery, older standard alternator, short cable run: a Cyrix-ct is the simplest option.
- Two or three lead-acid banks on one alternator: an Argofet.
Not sure yet whether you need alternator charging at all? Our Victron Orion DC-DC charger guide covers who can skip it. Browse every option in our DC-DC converters and chargers. Found it cheaper? We'll beat it.
FAQ
Can I use a Cyrix with a lithium house battery?
A standard Cyrix-ct adds no current limit, and Victron's lithium manuals call for one: the SuperPack NG manual says to use a DC-DC charger between the alternator and the battery. The Cyrix-Li-ct is designed for systems with a Victron VE.Bus BMS.
Will a Cyrix work with a smart alternator?
Only when the voltage reaches its 13.0 V connect level. Victron's DC-DC datasheets say smart alternators often run lower than that, which is the case the Orion is built for.
Do I need a Cyrix or Argofet as well as an Orion?
No. The Orion connects the starter battery to the house battery on its own and stops charging when the engine is off.
Isolated or non-isolated Orion?
Isolated versions keep the input and output negatives separate. Our Orion guide explains when that matters.
How fast will it charge?
At most its rated current: 30 A from a 12/12-30 is 30 Ah for each hour of driving, if the alternator keeps up. That 30 A only lasts while the battery is in the bulk stage.