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What's the purpose of a Victron Orion-Tr DC/DC isolator?

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Krumelur:
I'm in contact with Liontron as I'm planning to replace my aux batteries with the LX LiFePo4.
My current setup is:

* The original charger installed by Bavaria
* A Victron Cyrix-ct battery isolator (thanks Yngmar for recommending! :-))

According to Liontron (and my research confirms this), the LX battery can be used as a 1:1 substitute for the aux battery; to the external world, it acts like a lead-acid battery and is compatible with the charger.

Now what I don't get: the Liontron support wrote: "to deal with the charging current, a Victron Orion-Tr 12/12 would be required".
I don't understand why. If the engine is running and the alternator is delivering energy to the batteries, the Cyrix-ct would connect both starter and aux batteries and charge them. The voltage would not go above 14 volts (or whatever the alternator's limi exactly is). If energy is consumed, the Cyrix-ct would separate the batteries. I fail to see a scenario where the Liontron LX would have to deal with voltage levels outside of its specs?

Yngmar:
Not everything is an isolator! :)

The Cyrix-Ct is a VSR - a voltage sensing relay. Victron calls it a battery combiner, as that is what it does (when certain voltage conditions are reached).

The Orion is a B2B - battery to battery charger, taking DC from one battery bank and stepping it up into a suitable charging voltage for another. A DC to DC charger.

I don't know the LX system, but it sounds like a drop-in-replacement for a lead acid battery, in which case it internally regulates its charging and should work with either. Perhaps the supplier isn't aware of what the VSR does and just always recommends the one solution they know to be working (the B2B charger). Since it sounds like you're already in conversation with them, just ask why they insist on this.

It might be because the lithium battery has a very different discharge curve that may not trigger the VSR to separate until it is discharged. So it will work for charging but not separate batteries and keep the banks combined for too long when discharging. That depends on the exact voltages in play.

Krumelur:
I’m so isolated  ;D

Like you suggested, I asked them, and the reply is really unsatisfying because they sent me an explanation copied from some technical article. Now I know what the device is, but not why it’s recommended. They are unable to give me the information I need like if the battery automatically limits the charging current.

Reading some blog posts and watching videos, I’m getting the impression that my experience with the technical support is representative; Liontron and the businesses they are connected to (Greenakku and Bosswerk) don’t seem to have the best reputation and now I’m looking into alternatives.

elias:
Hi , the amount of current that the Lifepo4 can draw from the alternator is enormous and can even lit it ! Check this
https://www.victronenergy.com/blog/2019/10/07/careful-alternator-charging-lithium/

In order to limit the amount of current that your lifepo4 bank will draw from your alternator I believe the dc dc is suggested .

Krumelur:
Yep, when charging a plain vanilla lithium pack that is always the case.
Here, however, we're talking about a battery advertised as a drop-in replacement for a lead acid 12V pack.
The question is if it should then not limit itself to not fry the world it's being "dropped into" :-)

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