VE Bus error 15. and ESS Assist

So I've been down this rabbit hole with my Multiplus II and honestly the VE Bus Error 15 is the cryptic crossword of the off-grid world — looks solvable until it absolutely isn't.

Got my van conversion setup mostly singing — Cerbo GX talking nicely to the Fogstar Drift 24V battery via CAN bus, Victron MPPT doing its thing on the roof, all very wholesome. The moment I tried enabling ESS Assist though, the MP II threw a tantrum worthy of a toddler who's been told the biscuits are all gone.

Few things worth checking if you're hitting similar grief:

• Grid code settings — ESS on a Multiplus II is very particular about this, especially if you're not actually grid-tied (which most of us off-gridders aren't)
• Battery voltage thresholds — if your BMS is reporting 100% but the MP is still seeing "low battery" on the L terminal, there's likely a disconnect between what the BMS is communicating and what the Multiplus actually trusts
• ESS Minimum SOC — worth checking this isn't set higher than your current charge state in VEConfigure

The BMZ batteries are a bit of a dark art with Victron — has anyone actually got a solid DVCC profile working with one, or are we all just winging it and hoping for the best?

Also worth grabbing the latest VEConfigure and checking whether your ESS Assistant version is current — older versions on newer firmware is a lovely way to waste a weekend.

Anyone else found the Cerbo logs useful for diagnosing this, or is it just me staring at graphs hoping for a revelation?

@RiverFinn know this one painfully well from setting up my garden office system.

Error 15 is almost always a phase synchronisation issue — the Multiplus is trying to lock onto grid frequency and something's throwing it off. In my case it turned out to be a dodgy earth-neutral link I'd bodged temporarily and then forgotten about.

Few things worth checking specifically for a van conversion:

• Are you running a generator or shore power as input? Cheap generators with wobbly Hz will trigger this repeatedly
• Check your VE.Bus cable if you're running multiple units — corrosion on those RJ45 pins is silent chaos
• In VEConfig, look at the AC input current limit — set it too low and the Multiplus gets confused during handover

Victron's own KB article on Error 15 is actually decent for once. Worth a read before diving deeper into ESS Assist settings.

@RiverFinn had this exact grief with my garden office Multiplus II last spring. What nobody mentions clearly enough in the Victron docs is that Error 15 can also be triggered by cable length imbalance between your battery and inverter when you've got multiple strings — even a few centimetres difference in your negative/positive runs can cause enough impedance mismatch to throw it.

Check your VE.Bus cable too if you're daisy-chaining anything. Mine turned out to be a slightly loose RJ45 seating that looked perfectly fine visually.

The Victron Community forum has a specific thread on Error 15 with ESS that's worth digging out — more detail than the official manual gives you.

With a tiny house setup I've wrestled with this more than I'd like to admit. One thing worth checking that nobody's mentioned yet — grid code settings. In the UK you need the correct grid code selected (G98 or G99 depending on your install) and if that's misconfigured it can throw Error 15 in ways that look completely unrelated to phase sync.

Also double-check your AC input current limit in VEConfig — if it's set too low relative to your actual loads, the Multiplus can get confused during the handoff between inverter and grid-assist modes, especially under ESS.

Victron's ESS design guide PDF is actually worth reading cover to cover if you haven't — dull as dishwater but it sorted my head out on the logic flow.

@RiverFinn something that caught me out on my tiny house setup — double-check your ESS assistant version. Had Error 15 appearing randomly and it turned out I had a slightly outdated assistant loaded. Pulled it all down in VEConfigure and reinstalled the latest ESS assistant fresh, and it cleared it.

Also worth checking if you've got grid code set correctly for UK (EN 50549-1). Easy to overlook when you're deep in the weeds.

What firmware are you running on the MPII? There were a few dodgy builds a while back that played havoc with ESS stability.

@RiverFinn worth being precise here — Error 15 specifically means "VE.Bus error: Dongle missing or invalid assistant loaded."

So before anything else: which ESS Assistant version did you actually flash? There's a known mismatch issue if your Multiplus II firmware and the assistant aren't on compatible versions. Check VEConfigure → Assistants tab and note the exact build number.

Also — are you on a parallel or single inverter setup? Error 15 behaves differently depending on that. My cabin setup threw the same thing after a Victron firmware update pushed through VRM overnight without me noticing.

Run VE.Bus System Configurator first if you haven't. It'll flag assistant conflicts immediately rather than you chasing ghosts in the settings.