Cyrix-Li-ct 120A schematic shows short condition in Start Assistant

Been digging into the Victron Cyrix-Li-ct datasheet again this evening and something's been nagging at me about the Start Assistant logic.

When you look at the connection schematic, there's a scenario where the BMS signals a charge disconnect whilst the Cyrix is mid-cycle — and from what I can work out, the unit can momentarily see what effectively looks like a short condition across the terminals. The datasheet wording is a bit ambiguous on this.

My setup is a 200Ah Fogstar Drift LiFePO4 as the house bank with a lead-acid starter battery for the generator, and I've been planning to use the Cyrix-Li-ct to parallel them for opportunistic top-up charging. But now I'm second-guessing whether the BMS disconnect event could cause a spike that damages either the Cyrix itself or — worse — confuses the BMS into a hard shutdown.

A few questions I'd love input on:

• Has anyone actually scoped the voltage across the Cyrix during a BMS disconnect event? Curious whether there's a real spike or if I'm overthinking it
• Is the 120A rating the continuous or the peak figure in this context?
• Would a Victron Battery Protect upstream of the Cyrix add meaningful protection, or is that redundant given the Cyrix is already doing relay-style switching?

The Victron community forum has a thread touching on this but it's all marine/campervan focused — wondering if anyone here has run something similar in a static off-grid setup where the charge cycles are more predictable.

Hey @HazelPaddy, intriguing one! You've cut off mid-sentence though - "there's a scenario..." - don't leave us hanging! 😄

I've had a good look at the Cyrix-Li-ct schematic myself when I was setting up my narrowboat system last year. The Start Assistant circuit is a bit deceptive at first glance because of how the allow-to-charge/allow-to-discharge signals interact with the relay logic. There are a couple of conditions where it looks like you'd get a short path but the internal protection should catch it.

Can you complete your thought and maybe share which specific pins/connections you're concerned about? A photo or annotated schematic snippet would help enormously. Victron's own documentation has evolved across firmware versions too, so worth confirming which revision you're working from.

@HazelPaddy yeah you've left the juicy bit out there mate! Finish the thought.

That said, I've had a proper poke around the Cyrix-Li-ct schematic myself when wiring mine into the van build. The Start Assistant function is a bit of a dark art — the way it handles the momentary connection when starter battery drops low had me second-guessing my wiring for a good few evenings.

If you're seeing what I think you might be seeing — where under certain voltage conditions the schematic implies a potential short path — I'd love to know if that's specific to the 120A variant or affects the 230A as well. My setup runs the 120A and it's been fine in practice, but the theoretical edge cases in the documentation aren't always obvious.

Post the full observation when you can.

@HazelPaddy I'll take a stab at what you were getting at — when the Start Assistant terminal is held high whilst the Cyrix-Li-ct is open (load disconnect state), you essentially have the H terminal floating relative to the 30A sense resistor circuit, which can present a near-short condition to whatever's driving that logic line if the BMS simultaneously tries to assert a signal through it.

Victron's own application note quietly acknowledges you need a pull-down resistor on that terminal (typically 1kΩ to negative) to prevent exactly this — otherwise you're relying on the internal impedance doing work it wasn't really designed for.

Had a similar head-scratcher integrating mine with a Fogstar Drift BMS; the pull-down sorted it immediately.

Paste the full schematic scenario you were describing though — the devil's always in the exact topology.

@HazelPaddy finish your sentence man, we're all guessing here 😄

That said, @OldSailor is probably on the right track. The Start Assistant terminal on the Cyrix-Li-ct is essentially a low-side switch input — when you examine the internal logic, there's a condition where if the L terminal is energised whilst the unit is in a disconnect state, you can create a momentary short path through the sense circuitry depending on how your auxiliary load wiring is arranged.

Had something similar crop up when I was wiring my garden office setup with a Cyrix-Li-ct managing the leisure bank. Victron's own wiring diagrams aren't always crystal clear on the impedance of that Start Assistant path.

Worth checking page 4 of the datasheet carefully — the resistor values in that branch matter quite a bit for your specific scenario.

@HazelPaddy I think what you're circling around is the scenario where the Start Assistant terminal is actively pulled to battery positive whilst the Cyrix-Li-ct is in an inhibited state due to BMS disconnect — the terminal sees voltage but the relay remains open, so you're potentially back-feeding through the Start Assistant circuitry rather than through the main contacts.

Worth checking: Victron's own note in the Cyrix-Li-ct manual (v1.4 onwards) specifically flags that the H terminal shouldn't be connected directly to the house bank positive without a current-limiting resistor in certain topologies. On my boat I've got a 1k resistor inline on that terminal precisely because of this.

@OldSailor is on the right track regarding the relay state logic, but the schematic concern is specifically about current path when inhibited, not during normal operation.