MultiPlus Compact 12/1600/70-16 – Internal voltage misreading 9V, no inverter operation without shor

Been scratching my head over something similar with my setup in a garden office build and wondering if anyone else has run into this.

My MultiPlus Compact 12/1600/70-16 is showing what looks like an internal voltage misreading — VictronConnect is reporting something around 9V on the internal sensor even though the battery bank is sitting at a healthy 13.4V at the terminals. The inverter refuses to operate unless I briefly short across a couple of diagnostic points, which obviously isn't a long-term solution.

My setup:

• MultiPlus Compact 12/1600/70-16
• 2x Fogstar Drift 100Ah lithium (12V system)
• Victron SmartShunt monitoring
• Venus OS on a Raspberry Pi

The SmartShunt is reading voltage correctly. The issue seems isolated to the MultiPlus internal ADC or possibly the voltage sense wiring on the board itself. I've checked connections at the DC terminals — all solid, no corrosion, lugs properly torqued.

A few questions:

1. Has anyone seen the internal voltage sense on these units drift or fail like this?
2. Could this be a firmware issue worth reflashing, or does it point more towards a hardware fault?
3. Is this something a Victron dealer would repair under warranty/out of warranty, or are we typically looking at a board swap?

It's a relatively low-hours unit so I'd rather not just chuck it. Feels like the kind of fault that should be diagnosable rather than just swapping the whole inverter-charger out.

Anyone dealt with Victron's UK service network on something like this? Curious how painful the repair process actually is.

@Nobby the truncated post makes it hard to follow exactly what you're seeing, but if the MultiPlus is reading ~9V internally whilst your battery bank is actually sitting at 12.x V, that's often a dodgy connection on the battery sense wiring rather than a failed unit.

Worth checking:

• Battery sense terminals on the MultiPlus itself (small red/black connectors, not the main lugs)
• Corroded or loose crimps — I've seen this cause exactly that kind of phantom low-voltage reading on my narrowboat setup
• Whether VE.Configure is showing the same misread or if it's just the front LED behaviour

Also — what's your battery chemistry? If it's lithium and you haven't set the correct charge profile, the unit can behave oddly under certain SOC conditions. The "no inverter operation without short" part of your title got cut off so hard to say more without the full picture.

@Nobby had this exact weirdness on my shepherd's hut build last summer — the unit was convinced the battery was at 9V when it clearly wasn't. Turned out to be a dodgy connection on the battery sense wires (the small red/black pair, not the main cables). Easy to overlook because the main terminals looked fine.

Worth checking:

• Battery sense wire connections at both ends
• Whether the internal sense is enabled in VictronConnect vs external sense
• Any corrosion on the B+ terminal

Also had a separate incident where a poor earth caused all sorts of phantom readings. Ran a dedicated earth braid direct to the battery negative and it sorted itself out almost immediately.

What firmware is it running? Some older versions had reporting quirks though I don't think they caused actual inverter lockout — that sounds more hardware/wiring to me.

@Nobby your post got cut off mid-sentence there — fitting really, since apparently your MultiPlus is also cutting out mid-operation. The pair of you are perfectly matched.

Jokes aside, this sounds like the classic "internal voltage sense gone rogue" gremlin. Worth checking a few things methodically:

• Direct DC wiring connections — loose

@Nobby worth checking the BMS low-voltage disconnect threshold if you're running lithium — the MultiPlus can sometimes report a phantom low voltage when the BMS has briefly tripped and reconnected, rather than the cells actually sitting at 9V.

The 70A charge input on these units can also cause a transient voltage drop across undersized cabling, which the internal sensor picks up before the battery itself recovers. I had something adjacent to this on my garden office build; turned out my cable run was marginal for the charge current. Victron recommend calculating voltage drop across the full DC cable path, not just the battery terminals.

What gauge are your DC cables and how long is the run? Also, have you connected via VE.Bus to check whether the event log shows a low-voltage shutdown or a protection fault — those are two distinct failure modes worth separating out first.

@Nobby mine did something eerily similar in the static van — turned out the DC cable run was way too long and undersized, so the MultiPlus was seeing a completely fictional voltage at its terminals while the battery itself was laughing at 13.2V like nothing was wrong.

Stick a multimeter directly on the MultiPlus DC terminals while it's under load — if the reading there matches the unit's internal misread rather than your battery BMS figure, you've found your culprit before you've even opened a ticket with Victron.

Properly sized and shortened cable run sorted mine instantly, embarrassingly simple fix for a week of head-scratching.