ESS suddenly started oscillating after 2 years of stable operation

Really interesting timing on this thread — I've been half-expecting something similar to happen with my own setup.

Been running a Victron Multiplus-II with ESS for about 18 months now in my motorhome's home base system, and it's been rock solid. But I've heard of a few cases where seemingly stable ESS installs start hunting between grid and inverter mode out of nowhere, and it always seems to baffle people because nothing obvious has changed.

A few questions worth exploring:

• Has the grid quality at your property shifted? Grid impedance can change seasonally or if your DNO has done work nearby. ESS is quite sensitive to grid frequency and voltage stability.
• Any recent VenusOS or firmware updates? Victron pushed a few updates in the last year that tweaked ESS control loops — worth checking if an automatic update coincided with when the oscillation started.
• Battery state of health degradation? After two years, internal resistance may have crept up enough to affect how the system responds under load transitions, even if capacity seems fine.
• AC loads changed at all? New appliances — particularly inductive ones like heat pumps or fridges — can introduce instability if ESS grid setpoint is set too aggressively.

Has anyone on here actually managed to diagnose this properly through VRM portal data? I'd be curious whether the oscillation shows up clearly in the graphs as a regular pattern or more erratic.

Would love to understand the root cause before it potentially happens to me — my setup is grid-tied with a Fogstar Drift LiFePO4 bank, so any insight from similar builds would be genuinely useful.

Hey @WattLiz, worth checking your ESS minimum SOC and grid setpoint settings first — sometimes a firmware update quietly resets or shifts those values and the inverter ends up hunting between charge and discharge modes trying to satisfy conflicting targets. Had a similar wobble on my own Multiplus-II a while back and that was exactly the culprit. Also, are you on shore power or purely solar input? If your AC input voltage is dipping at all, the ESS can get a bit confused about whether to pull from grid or battery, which causes that characteristic oscillation. Pop into VRM and have a look at the AC input voltage graph alongside your SOC — if they're both fluctuating together, that'll tell you a lot. What firmware version are you running currently?

Good shout from @WrongFuse83 on the firmware angle — I'd add that oscillation after a long stable period is classic grid setpoint drift territory.

On my van build I noticed similar behaviour when my battery internal resistance had crept up enough that the Multiplus was essentially chasing its own tail — it'd see a load spike, pull from grid, overshoot, push back, repeat.

Worth checking in VRM:

• DVCC current limits — if your BMS is reporting slightly degraded cells, it may be throttling charge current unpredictably
• Grid setpoint — try nudging it from 0W to somewhere like 50-100W to give the system breathing room
• AC input current limit — too tight and it hunts

After 18 months of motorhome use, your battery's internal resistance profile will have changed. That alone can completely destabilise a previously rock-solid ESS tune.

Seen this exact pattern twice — once on my own Multiplus-II and once helping a mate with his narrowboat setup.

Both times the culprit was the grid setpoint drifting too close to zero, which causes the inverter to constantly hunt between charge and discharge modes. Victron's ESS algorithm gets twitchy when it's trying to hold that knife-edge.

Worth pulling up VRM portal → Advanced → ESS reason codes during the oscillation window — you'll see it flip between states in real time. That alone tells you whether it's a setpoint issue or something deeper like AC sensor noise.

@WattLiz — motorhome setups are particularly prone to this if you're frequently connecting to different EHU supplies with variable quality. The grid voltage tolerance settings in ESS may need tightening.

Check ESS → Grid setpoint and nudge it to 50W or above as a starting point.

Great detective work from @MarinePhil and @Drift_Geek already.

One thing nobody's mentioned yet — check your AC input current limit setting. Had mine mysteriously drop to 2A after a VRM firmware push last year. Multiplus was basically arm-wrestling with itself trying to decide whether to import or export every 30 seconds.