Hi Victron Community & Support,
I am experiencing unexplained periodic AC current spikes on a MultiPlus-II 12/3000/120-50 120V (firmware v558) that started exactly yesterday after I downgraded the Cerbo GX firmware from 3.72 to 3.55.
The system had been running perfectly for the last 4 years with zero similar behaviour. The Cerbo GX firmware 3.55 was used from the date of its release, followed by a short upgrade to 3.72 before downgrading back to 3.55.
System details:
- Shore power → isolation transformer → MultiPlus AC input
- MultiPlus set to Charger Only mode
- “Limit internal charge to prioritize other energy sources” is enabled with Sustain voltage = 13.24 V (Advanced tab in VEConfigure)
- Battery is a fully charged 12 V AGM (SOC remains 100 %, battery current stays ~0 A during spikes)
- Two MPPT solar chargers stay in Float and handle all charging independently
- No DC loads at all
- Only AC loads on the MultiPlus AC output: Starlink + one dehumidifier. I am 100 % confident there are no other/unknown loads on the AC-output circuit.
What is happening (clearly visible in the logs):
There are now exactly four distinct AC current spike events in the VRM logs. Every ~8 hours the AC input current (and AC output current) jumps from ~1–2 A to ~13.9–14.3 A and stays elevated for 8–15 minutes (≈ 1 650–1 700 W).
During every spike:
- VE.Bus state = Sustain
- Battery current ≈ 0 A
- VE.Bus charge power/current ≈ 0 W
- Solar chargers continue independently (no interaction)
- It is pure shore-power pass-through to the AC loads
The pattern is clockwork-like. My VRM CSV export (2026-04-11 00:00 to 2026-04-14 19:20) shows all four spike periods where Input current phase 1 / Output current phase 1 reach ~14 A. Clear examples are the spikes at approximately 08:21 and 16:34 on 2026-04-14; the other two events are also plainly visible in the same data set.
Additional analysis of the full log (2026-04-11 00:00 to 2026-04-14 19:20):
- Exactly four spikes, spaced ~8 hours apart, with no spikes present in the earlier part of the log.
- No VE.Bus errors, no alarms (low/high battery, overload, temperature, etc.), no BMS lost, no DVCC issues, no GX Error #48.
- Battery SOC stays at 100 %, charge state remains Float/Sustain, MPPTs operate independently.
- No other parameters change at the moment of the spikes (no sudden shifts in frequency, voltage, temperature, tank levels, or any other logged value).
- The only system change was the Cerbo GX firmware downgrade.
Graphs:
The VRM graphs for AC Input Voltage/Current, AC Output Voltage/Current, and AC Output Power clearly show all four spikes over the period.
I understand that a Cerbo GX firmware downgrade is an unlikely root cause, but the timing is exact and this behaviour has never occurred before. Is there any known interaction between Cerbo GX 3.55 and MultiPlus Sustain / Charger-Only logic? Could the isolation transformer somehow be involved when the MultiPlus is in Sustain mode under load?
Has anyone else seen similar periodic high-current pass-through spikes in Charger Only + Sustain? Any ideas on what could trigger a repeating ~8-hour cycle, or any further diagnostics I should run?
Thank you very much for any help or insight!
Best regards!
multiplus settings.pdf (288.6 KB)
177064_0_Vela_log_20260411-0000_to_20260414-1920.csv (3.3 MB)