Victron Multiplus keeps throwing a low battery alarm but voltages look fine to me

Had something similar on my motorhome build last spring. The Multiplus was screaming low battery but my BMV-712 was showing 24.8v which seemed perfectly healthy to me.

Turned out the issue wasn't the resting voltage at all — it was voltage sag under load. The moment the inverter tried to push any serious wattage through, the voltage at the battery terminals was dropping way below what the BMV was reading at its sense wires. In my case I had a dodgy connection on one of the busbars that was creating resistance and causing a localised drop.

A few things worth checking:

• Cable sizing — are your cables adequately rated for the load? Undersized cables cause voltage drop under load
• Connection quality — torque check every terminal, look for corrosion or loose crimps
• Battery internal resistance — if you're running AGMs that have done some mileage, they can look fine at rest but sag badly under load
• Low battery cut-off settings — worth double-checking what threshold you've set in VictronConnect, sometimes it's configured more conservatively than you'd expect

What battery chemistry are you running and what loads are you putting through it when the alarm triggers? Also, are you monitoring with a Victron BMV or shunt, or just going off the Multiplus display itself?

Would be useful to know if the alarm happens instantly under load or after a sustained period — that changes the diagnosis quite a bit. Others on here will have more experience with specific Multiplus configurations than me so hopefully someone can chip in.

@Moorey44 ooh don't leave us hanging like that, mate — "turned out the issue wasn't the" [post cuts off dramatically] 🎭

Classic cliffhanger on a Tuesday morning.

I'll throw my tuppence in while we wait for the conclusion. Had almost identical grief on my Ducato last winter — Multiplus throwing a wobbler, BMV looking perfectly content.

The culprit in my case was the battery temperature sensor. Without it connected properly, the Multiplus was using its own internal temperature as reference, which in a freezing motorhome was making it wildly pessimistic about what the batteries could actually deliver.

Worth checking:

• Is your temp sensor actually seated properly on the battery terminal?
• What's your low battery disconnect voltage set to in VE.Configure?

Sometimes the default settings are calibrated for a toasty garage, not a January layby in Derbyshire.

LochSeeker | Posts: 847 | Location: Scottish Borders

Ha, classic forum cliffhanger from @Moorey44! While we wait for the dramatic conclusion...

@DucatoDream this is actually a fairly common gotcha with the Multiplus. Resting voltage can look perfectly healthy whilst the battery is under load — especially if you've got undersized cabling causing a voltage drop between the bank and the inverter's sensing terminals. The Multiplus is reacting to what it sees at its input terminals, not necessarily what the BMV is measuring at the battery posts.

Worth checking your cable runs and connection quality if anyone else lands on this thread with the same symptoms. A properly calibrated BMV placed at the battery versus what arrives at the inverter can differ enough to trigger alarms, particularly on a 24v system where the thresholds are tighter than you might expect.

@Moorey44 finish your post!! 😄

Had something vaguely similar on my static last year — Victron was throwing fits but voltages looked reasonable. Turned out my battery cable connections had a dodgy crimp causing a voltage drop under load. Looked fine at rest but the Multiplus was seeing something completely different at its terminals compared to what the BMV was reporting.

Worth checking:

• cable connections at both battery and inverter ends
• whether your BMV shunt is wired correctly (sense wires in the right place?)
• any corrosion on terminals

The 24.8v reading is only useful if your BMV is actually measuring at the right point. Classic gotcha that one.

@Moorey44 the suspense is real — but I'd wager the culprit was voltage sag under load rather than resting voltage. Classic trap that one.

My Multiplus on the cabin setup did exactly this. BMV-712 sitting pretty at 24.7v, alarm screaming the moment the inverter kicked a decent load. Turned out my Fogstar cells were fine but I'd a dodgy crimp on one of the busbars creating resistance. Under load, voltage at the Multiplus terminals specifically was dropping well below what the BMV was reading at the battery.

Worth sticking a multimeter directly on the Multiplus input terminals while running something heavy — kettle, drill, whatever. If that reading diverges from your BMV by more than a few tenths, you're losing voltage somewhere in your cabling or connections before it even reaches the unit.

CrafterSolar | Posts: 312 | Location: Array

@OffGridTerry is almost certainly on the right track. The BMV-712 resting voltage means very little without knowing what load was running at the time of the alarm.

Worth checking a couple of things while we wait for @Moorey44 to finish their post:

• Battery internal resistance — older or unbalanced cells sag badly under load even when resting voltage looks fine
• Cable sizing and connections — a dodgy crimp or undersized cable between battery and Multiplus creates a voltage drop the BMV never sees
• Configured low battery cutoff in VictronConnect — sometimes people inherit settings from a previous owner that are set oddly high

My Fogstar Drift cells did something similar before I realised one of my busbars had a loose bolt. Resting voltage fine, loaded voltage dropping a full volt below what the Multiplus expected.