Properly Grounding Orion XS in a Four Wheel Camper

Been wrestling with a similar grounding question on my van build and thought it worth raising here for the UK crowd, as I keep seeing conflicting advice across different forums.

I've got a Victron Orion XS DC-DC charger going into a 100Ah Fogstar Drift lithium, and I'm genuinely unsure about the correct grounding arrangement. Specifically:

• Should the Orion's negative output connect directly to the leisure battery negative, or to a common chassis ground point?
• Does it matter whether the vehicle uses a negative chassis earth (most UK vehicles do)?
• Is there any benefit to keeping the leisure system fully isolated from the chassis ground, or does that create more problems than it solves?

I've read Victron's own documentation twice and it's not entirely clear on which approach is preferred when you're running a proper BMS — mine is a Daly 100A. Some sources suggest chassis grounding the negative bus is fine, others say it introduces noise or fault current risks with lithium setups.

My van is currently at the wiring stage so I'd rather get this right first time rather than pull everything apart later.

Has anyone here done a clean Orion XS install in a camper or shed/office setup and settled on a grounding method they're confident in? Particularly interested if anyone's had earth loop issues or fault tracing nightmares caused by getting this wrong early on.

@MuddySkipper the single most important thing with the Orion XS is keeping the DC negative return path as short and direct as possible back to your battery negative busbar.

What caught me out on my Vivaro build: I initially grounded the Orion chassis point to the van body rather than running a dedicated return cable back to the battery bank. Caused all sorts of subtle issues — slight voltage offset on the BMS readings and occasional spurious low-voltage cutoffs on my Fogstar Drift cells.

The fix was running a proper 16mm² return cable directly battery-negative → Orion negative → busbar. No shared chassis ground in that loop whatsoever.

Victron's own documentation is a bit vague on this but their tech support confirmed: isolated DC-DC converters still need clean, dedicated negative paths regardless of chassis bonding elsewhere in your system.

Has anyone tested what actually happens with a floating chassis setup in a motorhome specifically? My situation is slightly different to a van build — the habitation area and cab are somewhat separated electrically, and I'm never quite sure whether to tie the leisure battery negative to the chassis or keep it fully isolated and rely on the Victron system's own reference.

Running a Fogstar Drift 100Ah at the moment with a Victron BMV-712 and I notice the reported figures shift slightly depending on where exactly I clip the shunt. Makes me wonder if there's a hidden grounding loop somewhere causing interference.

Is the rule of thumb still one single chassis ground point for the entire system, or does that change when you've got a DC-DC charger like the Orion XS bridging two separate battery banks?

@Moorey44 good question on the floating chassis - I've actually measured this on my own setup. With a properly isolated negative bus that's not bonded to the chassis, you can still get noise and ground loops through capacitive coupling, especially once you add an inverter into the mix. In a motorhome specifically, you've also got the habitation relay and any OEM systems to consider - some European chassis (Fiat Ducato based ones particularly) have manufacturer ground bonds you can't easily eliminate anyway. My strong advice would be to use a quality multimeter and check for any voltage differential between your negative bus and chassis before assuming it's truly floating. Often it isn't. What chassis is yours based on?

@Moorey44 worth adding from a marine perspective — boats require isolated DC systems by convention (and often by regulation), so I've got some experience with floating setups. The key thing nobody mentions is that your isolation only works as well as your weakest link. One poorly-wired 12V accessory with a chassis return and your "isolated" system suddenly isn't.

On my boat I use a proper insulation resistance tester periodically to verify nothing's crept in. For a motorhome it's the same principle — worth doing a quick IR test before and after fitting accessories. Megger do affordable ones, or hire one.

The Orion XS handles floating negatives fine in my experience, but document your grounding decisions properly so you're not scratching your head six months later when something odd happens.