Ciryx CT 120 et régulateur solaire MPPT 75/15

Been mulling this one over as it's relevant to my own tiny house setup.

I've got a Victron SmartSolar MPPT 75/15 feeding into a secondary Fogstar Drift lithium battery, with a Cyrix-CT 120A handling the coupling to my vehicle's starter battery. It mostly works, but I've been scratching my head over how the Cyrix and the MPPT interact when both are doing their thing simultaneously.

My concern is this: when the MPPT is pushing bulk charge into the lithium, the Cyrix sees the voltage rise and interprets it as the starter battery being charged too — so it closes and starts pulling current across to the primary battery. That feels... wrong? Like the solar is inadvertently doing work it's not supposed to be doing.

A few questions for anyone who's run a similar arrangement:

• Has anyone actually measured whether this causes issues long-term with the starter battery, particularly if it's AGM vs lithium?
• Is there a smarter way to wire this so the Cyrix only responds to alternator charging and essentially ignores the MPPT cycle?
• Would a Victron Battery Protect in the circuit help, or is that overcomplicating it?

I'm also wondering if the 75/15 is even the right controller here — it handles my single 200W panel fine, but adding a second panel for EV trickle charging ambitions might push me toward a 100/20.

Anyone running a Cyrix alongside solar who's sorted this cleanly? Would love to see actual wiring diagrams if you've got them.

@OakSoul one thing worth double-checking with the Cyrix-CT 120 in that arrangement — it's a current-sensing relay, not a proper DC-DC isolator. So if your SmartSolar is trying to do absorption/float on the secondary Fogstar Drift, the Cyrix may combine the banks at an awkward voltage point and confuse the charge profile.

On my boat I ran a similar setup briefly and the Drift's BMS got a bit grumpy about it. Ended up putting an Orion-Tr Smart between banks instead — proper isolated DC-DC, much cleaner.

The 75/15 is also fairly modest on current output. What's your panel wattage going into it? Depending on that figure, the 15A ceiling might actually be your limiting factor before the Cyrix even becomes relevant.

@OakSoul had almost identical setup in my motorhome for a while — Victron MPPT 75/15 into a Fogstar Drift with a Cyrix-CT doing the combining.

Main thing I'd flag: make sure your MPPT charge profile is actually set for lithium. The Cyrix-CT will connect the banks when it sees current/voltage rise, but if the controller's still on a AGM profile it'll undercharge the Fogstar and you'll wonder why SOC never looks right.

Also worth checking the Cyrix threshold voltage suits your Drift's resting voltage — default settings can be a bit conservative for LiFePO4.

VictronConnect makes tweaking the MPPT profile dead easy if you've not dug into it already.

Running almost this exact arrangement in my garden office build — one thing that caught me out early on was the absorption voltage mismatch between what the MPPT was broadcasting and what the Cyrix-CT was actually "seeing" at the relay terminals.

The Cyrix doesn't know your Fogstar is lithium unless your MPPT profile is dialled in correctly. I've got mine set to the Fogstar's recommended charge profile (14.2V absorption, 13.5V float) and the Cyrix engages cleanly every time without hunting.

Worth pulling up your Victron Connect history and checking whether the MPPT is actually hitting absorption consistently — if it's frequently dropping back to bulk, the Cyrix behaviour can look erratic when really it's just doing its job faithfully.

@OakSoul what profile are you currently running on the 75/15?

@OakSoul ran something similar on my boat for a bit. One thing nobody's mentioned yet — make sure your Cyrix-CT 120 is seeing the right current threshold to actually trigger. If your loads are light and the alternator or MPPT isn't pushing enough amps through the sense circuit, it just won't close. Caught me out embarrassingly late into my build.

Also worth checking whether your Fogstar Drift's BMS is happy with the charge profile coming through — mine needed the absorption voltage dialling back slightly in VictronConnect before it stopped grumbling.

Had this exact combo on my narrowboat for two seasons — the bit that'll catch you out is the Cyrix-CT 120 seeing the Fogstar's resting voltage after a partial discharge and deciding the batteries are "equal enough" to connect when they really aren't, causing a sneaky backwards current creep that your MPPT never asked for. 🚤

Worth enabling voltage controlled load on the Victron app so you've got a proper backstop — saved my Fogstar from a grumpy morning more than once.