MultiPlus-II 24/3000/70 – Low Battery Trip at ~23.6 V / 62 A (AGM, 24 V, 400 Ah)

Q&A and troubleshooting
, , ,
1

I am investigating repeatable “Low battery” trips on a MultiPlus-II 24/3000/70-32 and would appreciate clarification on the protection behaviour.

System Details

  • Inverter/charger: MultiPlus-II 24/3000/70-32

  • Battery bank: 4 × Leoch LDC6-400-L16 AGM (6 V, 400 Ah each) in series

    • 24 V nominal

    • 400 Ah total capacity

  • Battery monitor: SmartShunt 500 A (all battery current passes through shunt)

  • Temperature: ~8 °C in battery room

  • DC cabling: Short runs, 125 A DC fuse, connections checked tight

  • No grid interaction during test (inverter-only island mode)

Relevant VE.Configure Settings

  • DC input low shut-down: 18.6 V

  • DC input low restart: 21.8 V

  • DC input low pre-alarm: 21.8 V

  • Charger absorption: 28.8 V

  • Float: 27.6 V

  • Charge current limit: 70 A

No Assistants beyond standard ESS. Scheduled charge disabled during tests.

Observed Behaviour

During inverter-only testing (AC-in disconnected):

Case 1 – ~1350 VA load

  • AC output: ~1352 VA

  • DC current: ~−62 A

  • DC voltage (VE.Bus): ~23.6 V

  • Udc ripple: ~0.5 V

→ MultiPlus asserts “Low battery” alarm and trips.

Case 2 – ~1950 VA load

  • AC output: ~1959 VA

  • DC current: ~−94 A

  • DC voltage (VE.Bus): ~23.0 V

  • Udc ripple: ~0.6 V

→ Also trips “Low battery”.

In both cases:

  • Measured voltage at MultiPlus DC studs with DMM matches VE.Bus reading (~24.7 V at rest, ~23.6–23.0 V under load).

  • Per-battery measurements under ~1250 W load show balanced sag (~5.93 V per 6 V block).

  • No significant voltage drop across fuse or isolator.

  • SmartShunt minimum voltage does not show values anywhere near 21 V.

Given the configured shutdown of 18.6 V, and restart 21.8 V, the observed trips at ~23–23.6 V appear inconsistent with the static thresholds.

Questions

  1. Is there a dynamic low-voltage protection curve in the MultiPlus-II firmware that trips above the configured static shutdown voltage at certain current levels?

  2. Is there a separate VE.Bus low battery alarm threshold (distinct from the Inverter tab “DC input low shut-down”) that could be active?

  3. Does the firmware implement a current-dependent protection mechanism that could trip around ~23 V at ~60–90 A discharge on a 24 V unit?

  4. Are there any known issues or firmware revisions addressing premature low-battery trips on 24 V MultiPlus-II systems with AGM batteries?

  5. Is there any additional diagnostic logging available to capture sub-second DC voltage dips not visible in the VE.Bus monitor display?

Additional Context

  • Batteries are ~2 years old AGM.

  • Balanced under load.

  • Temperature ~8 °C (cold but not extreme).

  • System is stable below ~1.0–1.2 kW but trips above that level.

Before redesigning the system around lower discharge limits, I would like to confirm whether this behaviour is expected firmware protection or indicative of a configuration or hardware issue.