Potential Bug / Feature Request: Inverter Low DC Voltage Shutdown Prevents Partial-System Charging

Been mulling this one over after a weird situation with my motorhome setup last summer, and I reckon it's worth a proper discussion here.

Running two Victron Multiplus-II units in parallel, and I noticed that when the DC voltage drops and triggers the low-shutdown threshold on both, you're essentially stuck — neither unit will restart to allow even partial charging from shore power or solar until the battery voltage recovers on its own. Which it obviously can't do if nothing's allowed to run. Classic deadlock.

The low restart voltage setting should theoretically break this cycle, but in a multi-inverter setup it seems like the units aren't always playing nicely together. One kicks in, sees the other hasn't restarted yet, and backs off again. I've reproduced this a couple of times now with my Fogstar LiFePO4 bank — usually after a heavy overnight draw in winter when the solar input the next morning is minimal.

My workaround has been to manually bump the restart threshold up a fair bit, but that feels like a bodge rather than a fix.

A few questions for the forum:

• Has anyone else hit this with parallel Multiplus setups?
• Is there a way to configure VE.Bus or VictronConnect to allow at least one unit to act as a "charger-only" master during recovery?
• Could this be addressed through a firmware update, or is it more of a feature request territory?

Would love to know if this is a known edge case or if I'm just doing something daft with my thresholds. Victron's documentation on multi-unit low-voltage behaviour is a bit thin, in my opinion.

KenMitchell | 847 posts | ⚡ Solar Enthusiast

@DorsetExplorer Cracking topic - I've bumped into something similar with my Multiplus-II 48/3000.

The frustration is real: the inverter shuts down on low DC voltage, fair enough, but then your shore power or solar can't properly charge back through it because the unit's protecting itself. Bit of a chicken-and-egg situation.

Worth checking your DC input low shut-down and DC input low restart thresholds in VEConfigure - there's often a meaningful gap between the two, and widening that restart threshold can help. Also look at whether your Assistants configuration is interfering with the charge path during protection mode.

Have you got a Cerbo GX in the mix? Sometimes DVCC settings complicate things further when running parallel units.

What firmware version are you on? Victron have tweaked some of this behaviour in recent releases.

Interesting scenario. A few questions that might help narrow this down:

1. What's your low voltage cutoff set to on the Multiplus-II units? The default is quite conservative and may not account for the voltage sag under parallel load sharing.
2. Are both units seeing the same DC bus voltage, or could there be a wiring imbalance causing one unit to trip first and dragging the other down?

With my static caravan setup I found that Fogstar cells under heavy load showed a significant voltage drop versus resting voltage — the inverter was reading a "false floor" essentially.

Worth checking in VictronConnect whether the units are genuinely synced via the VE.Bus cable, as a loose connection there can cause exactly this kind of cascading shutdown behaviour.

Is your battery a single bank or split across the parallel units?

Something very similar happened to me on the narrowboat last winter — drained the bank chasing a faulty bilge pump that ran all night without me realising.

What saved me was having a small dedicated "bootstrap" battery (just a cheap 20Ah Fogstar lithium) isolated from the main bank, wired directly to the Multiplus AC-in side via a simple relay. When the main bank hit the low-voltage cutoff, the bootstrap kept just enough alive to let the shore power charger kick in and resurrect the system.

It's a bit agricultural as solutions go, but it works reliably. The Victron DVCC settings can also help here — worth digging into whether your ESS assistant configuration is contributing to the lockout @DorsetExplorer.

Curious what @BlownFuse finds once you've answered those questions — the cutoff threshold will tell us a lot.

My static caravan Multiplus just sits there judging me whenever the voltage drops — basically the inverter equivalent of a disappointed parent refusing to help until you've sorted yourself out first.