Off grid tesla powerwall, customer want to add additional storage using a multiplus

Really curious about this one because it touches on something I've been mulling over for my boat setup, and I reckon it's relevant to a few tiny house builds too.

Has anyone actually managed to integrate a Victron Multiplus-II alongside a Tesla Powerwall 2 in a proper off-grid configuration? Not grid-tied — fully standalone.

The scenario I keep seeing pop up is where someone already has a Powerwall running their system, then they've got a separate lithium bank (say, some Fogstar cells or similar) sitting unused, and they want to bring it all together rather than waste the capacity.

My questions would be:

• Does the Powerwall's BMS play nicely with the Multiplus-II, or does it cause headaches with voltage/charge curve conflicts?
• If you drop a Cerbo GX into the mix, can you get meaningful visibility over both battery systems, or does the Powerwall side remain a bit of a black box?
• Is AC-coupling the Powerwall on the Multiplus output the cleanest approach here, or are there better ways to tackle it?

I can imagine the frequency-shifting side of things getting complicated when the Powerwall thinks it's in charge but the Multiplus has other ideas...

Anyone done something similar, or seen it done on a customer install? Would love to know whether this ends up being a slick integrated system or just a frustrating bodge. Real-world experience over theory any day!

Seen a variation of this with a static caravan setup. The core problem is the Powerwall wants to be master of its own universe — it doesn't play nicely with external kit trying to manage the same DC bus.

The practical workaround most people land on is treating the Multiplus as an AC-coupled addition rather than trying to merge DC sides. Powerwall handles its thing, Multiplus sits on the AC output with additional batteries behind it.

That said, Victron's ESS assistant gets complicated quickly when there's already a proprietary BMS in the loop. The Powerwall's gateway doesn't expose the signals you'd need for proper coordination.

@RustySkipper for a boat I'd honestly question whether keeping the Powerwall makes sense at all — a clean Victron Multiplus-II with Fogstar or similar lithium from the start avoids the headache entirely.

This is a classic "two kings, one castle" problem. The Powerwall's BMS is deeply proprietary and will absolutely not play nicely with a Multiplus trying to assert grid-forming control over the same bus.

Your realistic options:

1. AC coupling — Put the Multiplus on a separate AC bus, let it handle loads independently, and use the Powerwall purely for its own island. Messy but functional.
2. DC side addition — Forget integrating with the Powerwall's battery entirely. Add a separate Victron lithium bank alongside, managed by a Cerbo GX, and treat the Powerwall as a "dumb" AC source feeding in.

Option 2 is what I'd lean toward for @RustySkipper's boat context — I've got a similar arrangement at my cabin where two storage systems coexist without either knowing the other exists. Ignorance is bliss, apparently.

The Powerwall's warranty situation under either configuration is also worth a very careful read before touching anything.

Really interesting thread for my van/boat research too.

@DailySolar's "two kings" analogy is spot on — but has anyone looked at whether the Multiplus could be configured purely as a dumb AC-coupled inverter on a dedicated circuit, with the Powerwall handling its own DC side completely independently? Essentially keeping them in separate kingdoms rather than forcing integration?

Would something like this even be worth pursuing, or does the Powerwall's AC sensing inevitably create conflicts regardless of how you isolate them?

Also curious — if the customer is set on expanding storage, would ditching the Powerwall altogether and going full Victron with Fogstar Drift lithium banks actually be the more cost-effective route at this point? Sometimes a clean slate beats forcing incompatible kit to cooperate. What are the rough economics looking like for their existing capacity?

Yeah had a similar headache when I was sorting my garden office setup.

@DailySolar is right about the BMS conflict — but the workaround I've seen work is treating the Powerwall as a dumb AC-coupled battery rather than a smart system. Basically let the Multiplus sit upstream and manage everything, then the Powerwall just responds to AC frequency shifts via its inverter (frequency shifting trick).

Key thing though — you lose all the Powerwall's smart features. It's basically lobotomised at that point.

Also worth noting Tesla have been actively patching firmware to prevent exactly this kind of integration, so what worked 18 months ago might not now. Check the firmware version before committing.

Victron's AC coupling docs cover the frequency shifting stuff pretty well if anyone wants the technical side.

Has anyone actually got this working in practice with a Multiplus-II specifically? I'm planning a similar hybrid setup on my narrowboat and the bit that worries me most is the AC coupling side — if the Powerwall is already managing AC, does the Multiplus just see it as another AC source and get confused about frequency shifting?

@QuietTrekker did your garden office solution involve keeping them on completely separate DC buses, or were you trying to get them talking to each other properly?

Wondering whether the cleanest answer here is just to put the Multiplus on a separate battery bank (Fogstar Drift LiFePO4 is what I'm eyeing) and keep the Powerwall entirely isolated for its original purpose — essentially two independent systems sharing the same loads rather than one integrated one.

Messy compromise, but might be the only realistic option without Tesla's blessing?