Victron Multiplus keeps throwing a low battery alarm but cells are reading fine

Had almost exactly this happen to me last winter during a three-day power cut — the Multiplus was screaming low battery while my Fogstar cells were sitting at a perfectly respectable voltage under no load.

The culprit for me was voltage sag under load. The Multiplus samples voltage while drawing current, so even if your cells look fine at rest, a momentary dip when something kicks in (fridge compressor, kettle, whatever) can trip the alarm threshold before the BMS even blinks.

A few things worth checking:

• Cable sizing and connection resistance — I had a slightly loose terminal on my busbar that was causing enough resistance to exaggerate the sag
• Multiplus low battery voltage setting — in VE.Configure, check what threshold you've actually set. Mine had defaulted to something conservative that didn't suit my LiFePO4 chemistry
• BMS communication — if you're not running DVCC through a Cerbo or similar, the Multiplus is essentially flying blind and relying purely on voltage

Once I tightened everything up and adjusted the low battery cutoff to something sensible for lithium, it stopped entirely.

What's your current low battery voltage threshold set to? And are you running any communication between the BMS and the Multiplus, or standalone? That detail tends to change the diagnosis quite a bit. Others here will likely have seen this too — it's more common than the Victron docs let on.

@SolarRachel did you check what your battery monitor was reporting versus what the Multiplus was actually measuring at its own terminals? I had something similar with my motorhome setup and it turned out to be a significant voltage drop across a dodgy crimped connection on the negative busbar — the Multiplus was seeing a lower voltage than the cells were actually sitting at under load.

Worth checking:

• Cable cross-section — are your battery cables adequately sized for your inverter's rated current?
• Connection resistance — torque on all terminals, any signs of oxidation?
• Low battery cutoff setting in VictronConnect — is it set appropriately for your cell chemistry?

What chemistry are your Fogstar cells and what's the low battery threshold currently configured to in the Multiplus settings?

@SolarRachel this is almost certainly a voltage drop issue under load rather than a genuine low state of charge. The Multiplus measures voltage at its own terminals, and if your cable run has any meaningful resistance, you'll see a significant sag the moment it draws serious current.

Check your cable cross-section and connection quality end-to-end. I've seen 10mm² cable cause exactly this on my narrowboat setup when it should've been 25mm² minimum for the run length involved.

Also worth checking in VictronConnect: what are your low battery shutdown and warning voltage thresholds set to? Fogstar cells have a relatively flat discharge curve, so if your thresholds were calibrated for lead-acid, they'll trigger far too early on lithium chemistry.

@BevJackson64 raises a good point — comparing the Multiplus's reported voltage against a proper battery monitor like a Victron BMV-712 will immediately tell you whether it's a measurement or a genuine cell issue.