Multiplus 2 initiates pulsing Voltage Output once Batteries reach 95% SOC

Seen something similar with my static caravan setup and thought I'd share in case it helps anyone.

Running a Multiplus-II 48/3000 with a Fogstar Drift 48V pack and grid tied via an ET112 meter. Everything runs smoothly until the batteries hit around 93-95% SOC, then the inverter starts doing this odd pulsing thing — output voltage oscillates noticeably and you can actually hear it in anything sensitive plugged in.

Took me a while to track down but I think what's happening is the Multiplus is hunting for balance between grid and battery once the bulk charge phase ends. It's essentially trying to decide whether to absorb, float, or hand back to grid and can't quite settle. Bit like a thermostat with too tight a differential.

A few things worth checking if you're seeing the same:

• ESS assistant settings — specifically the minimum SOC and grid setpoint values
• Battery charge voltage — if it's slightly mismatched to what the BMS is reporting, the Victron gets confused
• DVCC settings in VictronConnect — make sure CVL is being set by the BMS, not overridden manually

I adjusted my grid setpoint from 0W to about 50W (slight import bias) and it settled things down considerably. Not perfect but much less hunting.

Curious whether others have seen this specifically with lithium packs vs lead acid. My old AGM setup never did this — wonder if the flatter discharge curve of lithium is part of what confuses the algorithm.

Anyone else tinkered with the ESS mode switching voltages to resolve it?

FinnThomas | 847 posts

@RetiredElectrician interesting one this. What you're likely seeing is the Multiplus-II entering absorption phase and the BMS on the Fogstar Drift beginning to throttle charge current as cells approach capacity. The "pulsing" voltage is essentially the inverter responding to the BMS communicating reduced charge acceptance.

Worth checking your DVCC settings in VictronConnect - if you're not running a Venus device or Cerbo, the Multiplus won't have proper BMS communication and may be fighting against the pack's internal protection logic.

Also, what have you set your absorption voltage at? Fogstar recommend 54.4V for the Drift if I recall correctly. Running it higher could trigger the BMS more aggressively at that 95% threshold.

Is this causing any actual problems or purely cosmetic on the monitoring side?

Interesting thread — this lines up with something I've been trying to get my head around for my static caravan setup too.

@FinnThomas I'd be curious whether the BMS on the Fogstar Drift is asserting charge current limits via DVCC as SOC climbs? I've read that if the BMS starts requesting reduced charge current through CAN comms, the Multiplus-II can exhibit exactly this pulsing behaviour as it tries to throttle down rather than switch to a clean absorption phase.

A few questions worth investigating:

• Is DVCC enabled in VenusOS?
• What does the charge current limit from the BMS look like in VRM around the 95% mark?
• Is the ET112 seeing any corresponding grid import spikes during the pulses?

Worth pulling the VRM data logs for that window and checking the BMS charge current limit parameter specifically.

Really crucial detail nobody's picked up on yet — the ET112 placement matters enormously here.

If your ET112 is on the grid feed before your loads rather than after, the Multiplus-II can get confused about net consumption during that absorption/float transition, especially once the Fogstar's BMS starts throttling charge acceptance near 95%. The inverter sees fluctuating demand figures and responds with exactly the pulsing behaviour you're describing.

Worth checking in VE.Configure whether your ESS assistant has "Keep batteries charged" mode active — that interacts badly with a BMS that's already current-limiting.

I had something analogous on my motorhome-to-static conversion last spring. Took me three weekends of logging VRM data before I spotted the pattern. The Victron VRM portal's advanced tab will show you ChargeState toggling rapidly between Absorption and Float — that's your smoking gun.

Good shout from @PanelLouise on the ET112 placement — that's bitten me before.

Worth adding: when my Victron Multiplus-II does this pulsing behaviour near top of charge, I found the absorption voltage setpoint in VEConfigure was the real culprit. Had mine set 0.2V too high for the Fogstar's actual upper limit, so the BMS kept nudging back against it — creating exactly that rhythmic pulse you're describing.

Pull the BMS comms data via DVCC if you haven't already. If the BMS is periodically issuing a charge current limit of zero, that's your smoking gun — it's not a Multiplus fault, it's a negotiation between the two.

Drop your absorption voltage by 0.1V increments until the pulsing settles. Took me three iterations to land on the right figure.