DC to DC Charger Negative Cable Connection

Right, this one's been rattling around my head since I wired up my Victron Orion-TR Smart last spring — where exactly do you lot terminate the negative on your DC-DC charger?

I've seen three schools of thought doing the rounds:

1. Straight back to the starter battery negative terminal
2. Common chassis earth point (nearest decent bodywork bolt)
3. Directly to the leisure/house battery negative busbar

On my Transit-based tiny house build I went busbar-to-busbar for both positive and negative, keeping it all "contained" rather than bouncing current through half the van's bodywork. Seemed tidy. Whether it's correct is another matter entirely — I'm grumpy, not omniscient.

The argument for chassis earthing is that it's what the van manufacturer intended the return path to be anyway, so you're just playing along. Fair enough. But I've heard horror stories about corroded earth straps creating all sorts of phantom voltage nonsense that confuses the Orion into throttling charge current.

Fogstar and a few others seem to dodge the question entirely in their literature, which is incredibly helpful, cheers lads.

Curious whether anyone's actually seen measurable voltage drop differences between methods — I'd rather argue with data than opinion for once.

What's everyone running, and have you actually tested it properly or just crossed your fingers and hoped for the best like the rest of us?

Ran mine straight to the leisure battery negative busbar — because apparently I enjoy making my future self's fault-finding sessions a living nightmare, but at least it's consistent nightmare.

The real villain here is the 12v chassis earth that looks perfectly fine until you're 200 miles up the A9 in a Scottish monsoon wondering why your Orion

The pedant in me needs to clarify something often glossed over: the negative return path must have the same cross-sectional area as your positive, and the path back to the alternator/starter battery negative must be unbroken and low-resistance.

On my narrowboat I terminate the Orion-TR Smart negative directly at the main negative busbar, which itself has a single, short, heavy run back to the engine block/battery negative. This matters enormously on steel-hulled boats where stray current corrosion will eat your hull fittings if you introduce ground loops through sloppy multi-point negative connections.

@SIE_Electric — the leisure battery negative busbar is fine only if that busbar is your true system reference point with a clean path back. If there are multiple negative connection points with differing resistances, you're potentially creating exactly the conditions that cause phantom Victron BMS comms errors and erratic charge state readings.

Mine goes straight to the chassis earth point nearest the leisure battery — because in a van conversion, the chassis is the busbar, whether you like it or not 🚐

Been wondering about this myself — currently mid-build on my Transit and I've got the Orion-TR Smart sitting in a box waiting.

@DefenderAdventure makes a good point about cross-sectional area that I'd not thought about properly. I've been planning to run both negatives (input and output) back to a central negative busbar bonded to chassis at a single point, rather than chasing separate earth points around the van.

Is there any issue with that approach? My concern is whether the input-side negative (connected to the starter battery circuit) and the output-side negative sharing a busbar could cause any grief — or does the Orion's galvanic isolation make that a non-issue?

Fogstar recommended a single earth point to avoid ground loops but I want to make sure I'm not creating a problem further down the line.