Victron Power-In and the location of the Battery Cutoff Switches

Been mulling this one over myself actually, as I'm mid-way through a similar-ish upgrade on my motorhome — swapping out to a pair of Fogstar Drift 200Ah cells and adding a Victron SmartShunt alongside my existing Cerbo GX setup.

The bit that's been keeping me up at night is where exactly to position the battery isolation switch relative to the SmartShunt. Get it wrong and your shunt readings are meaningless — or worse, you've got an unprotected cable run somewhere you don't want one.

My current thinking:

• Battery cutoff switch between battery positive terminal and the busbar
• SmartShunt sitting on the negative side, as close to the battery as practically possible
• Everything downstream of the shunt, so all current — charge and discharge — passes through it

The Victron Power-In complicates things slightly because it's essentially acting as a distribution hub, and you want to make sure the isolation switch is genuinely isolating the whole system, not just part of it.

Anyone running a Lynx Distributor alongside a Power-In in a UK motorhome or van conversion context? I'd love to know how you've physically laid it out — especially if space is tight. My rear locker is not exactly generous.

Also curious whether anyone's found the Victron manual guidance on this actually matches real-world installs, because in my experience those diagrams assume you've got the interior space of a Winnebago rather than a Transit Custom. 😄

What's everyone's approach?

Welcome to the forum @AndyRobinson — great first topic to dive into, the SmartShunt placement question trips up a lot of people initially!

One thing worth keeping in mind with the Fogstar Drift cells specifically: make sure your battery cutoff switch sits between the battery and the SmartShunt, not after it. The shunt needs to see all current flowing in and out to keep the state-of-charge accurate — any load or charger bypassing it will throw the readings off completely.

On my narrowboat setup I've got the main isolator on the negative before the shunt, which keeps everything tidy and measurable.

The Victron documentation is actually pretty clear on this — worth a read if you haven't already. Stick around, there's loads of collective knowledge here for motorhome builds too.

@AndyRobinson the SmartShunt placement is one of those things that seems obvious until you actually sit down with the wiring diagram and realise it very much isn't.

The cardinal rule — which Victron's own documentation states with surprising clarity if you can be bothered to read past page three — is that the SmartShunt must see all current flowing in and out of the battery. Every last milliamp.

On my narrowboat I made the classic error of putting the battery cutoff between the shunt and the battery. Result: the shunt counted phantom charge cycles every time I switched off. The state-of-charge reading became essentially creative fiction.

Cutoff switch goes on the load side of the shunt, not between shunt and battery. The shunt needs to be the first thing connected to your negative terminal, full stop.

With the SmartShunt, the golden rule I kept reminding myself when I wired up my boat setup was: everything — and I mean everything — goes through that shunt on the negative side. One rogue direct connection and your SOC readings become fiction.

For the cutoff switch placement, I'd put it between the shunt and the battery negative terminal, not between the shunt and your loads. Otherwise you're potentially interrupting the shunt's own reference ground, which causes all sorts of grief.

@AndyRobinson the Fogstar Drift cells are a solid choice — I ran a similar pair in a garden office build and they've been rock solid. Just make sure your BMS negative also routes through the shunt or Victron's SOC calculations will drift on you faster than you'd expect.

@CallumHobbs is absolutely right, and it's worth spelling out why that rule exists — the SmartShunt measures current by sitting between negative busbar and battery negative terminal, so anything wired "outside" that loop becomes invisible to it. Your SOC readings will drift increasingly wrong over time as phantom loads accumulate.

Specific gotcha with the Fogstar Drift cells: if your BMS has its own low-side disconnect (many do), double-check whether that disconnect sits inside or outside your SmartShunt measurement loop. Got caught out myself doing exactly this — BMS was cutting load current the shunt couldn't see, and Victron's SOC calculation was wandering off into fantasy territory within a week.

The battery cutoff switch should ideally sit on the positive rail entirely, leaving your negative measurement path clean and uninterrupted.