Load shed with GX device

Been thinking about this a lot lately for my boat setup. I've got a Cerbo GX running the show and I'm wondering whether there's a clean way to do automatic load shedding before the BMS pulls the plug entirely.

The scenario I keep running into: batteries get low, BMS disconnects everything hard, and then I've got fridge, inverter, nav gear all dropping off at once. Not ideal when you're underway.

What I'd like is something that cuts non-essential loads at say 20% SOC — fridge and inverter first — while keeping nav and instruments alive. Then if it keeps dropping, pull more. Staged shedding rather than one big cliff edge.

Has anyone managed this with Victron kit? A few things I've been looking at:

• Cerbo relay outputs — can these be triggered by SOC thresholds via Node-RED or the built-in relay settings?
• Carlo Gavazzi meters with some kind of switching logic?
• Ve.Bus Smart Dongle controlling the MultiPlus directly?

I know the Cerbo has two relay outputs and you can set conditions on them, but I'm not sure if the logic is flexible enough for multi-stage shedding without writing custom Node-RED flows.

Running Fogstar Drift 100Ah lithiums if that matters — they have their own BMS but no external comms as far as I can tell, so I can't get SOC data directly from them into the GX.

Anyone done something similar on a boat or narrowboat where you've got mixed critical and non-critical loads? Curious what approach actually works in practice rather than just theory.

@EwanCole58 yeah the Cerbo handles this dead easy via the relay outputs. Set up a virtual switch in VRM tied to SOC thresholds — mine cuts non-essentials at 30% and everything bar the fridge at 20%.

Node-RED on the Cerbo is where it gets properly useful though. Can do time-based rules, voltage and SOC combined, all sorts. Bit of a rabbit hole but worth it.

Main thing is getting it sorted before the BMS disconnects — a hard BMS cutoff is rough on inverters. Much cleaner to shed loads progressively.

What BMS are you running? Some play nicer with the Cerbo than others via DVCC.

@PeakVanLifer cut off there mate — but yeah, the relay approach works until your SOC readings are lying to you because your Fogstar cells haven't been properly characterised, which on a narrowboat in winter is basically always 😅

The cleaner method IMO is using Node-RED on the Cerbo — you can shed loads in stages based on voltage AND SOC together, so you're not binning your inverter the second the BMS gets jumpy. Done this on my own boat, saved me from the "everything off at 3am in a lock" situation more than once.

Key thing people miss: set your shed thresholds a good 5–10% SOC above your BMS low-voltage disconnect. Give yourself actual breathing room rather than relying on the BMS as your first line of defence — it really isn't designed for that.

@EwanCole58 worth noting the Cerbo's relay approach has limits — you only get two physical relays, so plan your load priority carefully before committing. In my static caravan setup I use a combination of Cerbo relay + a Victron ESS assistant to stage the shedding rather than doing it all at once. Prevents a big voltage sag triggering false SOC readings, which is exactly the problem @OffGridGeek was hinting at before getting cut off.

Also consider whether you're trusting SOC or voltage thresholds as your trigger. On a boat with variable charge sources (engine alt, solar, shore) your SOC estimation can drift. Voltage with a short time delay is often more reliable as a final backstop, with SOC handling the earlier shed stages.

Good timing on this thread — I had almost exactly this scenario play out in my motorhome last winter when the BMS hard-tripped and killed everything including the heating. Not ideal at 2am in a layby.

What sorted it for me was using the Cerbo's ESS assistant alongside the relay outputs. You can stagger your load shedding across multiple SOC thresholds rather than one all-or-nothing cutoff. Non-essentials drop at say 30%, critical loads survive down to 15%, BMS never gets triggered in normal use.

@BayJason is right about the two-relay limit — I got around this by adding a Victron Venus GX relay expander and running secondary contactors for the bigger loads.

Also worth setting a separate low cell voltage alarm in the Cerbo independent of SOC — caught a dodgy cell in my Fogstar pack before it caused grief.

Good thread this. One thing I'd add that nobody's mentioned yet — have a look at the Node-RED flows you can run directly on the Cerbo. You can set up logic that watches battery voltage and SOC together, so if either drops below your thresholds it triggers the relay in sequence. Means you can shed non-critical loads first (inverter, water pump, whatever) and give yourself a proper buffer before the BMS even thinks about disconnecting.

@EwanCole58 on a boat especially I'd prioritise what stays on during shedding — nav lights, VHF, bilge pump obviously need to survive whatever else you cut. Map that out before you start building the logic.

The Victron community forum has some decent example flows if you search Node-RED load shedding — saved me a lot of head-scratching when I set mine up.