Dual alternator DC to DC charging

Interesting scenario came up in another discussion that got me thinking about my own setup and I reckon it's worth a proper thread here.

The barge use case is a cracking example — dual alternators feeding different voltages to different battery banks. But actually, how many of us have inadvertently ended up in a similar situation with vans or motorhomes? I've got a 12V starter battery and a 24V LiFePO4 leisure bank in the van conversion, and managing the charging relationship between them is genuinely non-trivial.

The obvious answer is a B2B charger (DC-DC), but here's where it gets interesting with dual alternators:

• Which alternator do you tap? The dedicated leisure alternator if you have one, or the main unit?
• Does the Victron Orion-Tr Smart play nicely when two alternators are in the mix? I've only ever run mine from a single source.
• Alternator protection — LiFePO4 can absolutely hammer an older alternator without proper current limiting in place.

On my van I'm running a Victron Orion-Tr Smart 12/24-15A, isolated version, and it's been solid. But I deliberately stayed well below the alternator's rated output because the previous owner had no idea how old the unit was.

Anyone here actually running dual-alternator setups feeding a LiFePO4 bank — whether on a narrowboat, motorhome, or otherwise? Curious whether you've got both alternators working simultaneously through separate B2Bs, or whether you're only drawing from one and leaving the other for the starter side exclusively.

Feels like there's a lot of guesswork in most advice I've seen on this.

Had this exact headache on my narrowboat — ran twin alternators feeding a Victron Orion-Tr Smart and the key thing nobody mentions is priority sequencing: if both DC-DC chargers hammer the leisure bank simultaneously you'll confuse the BMS something rotten.

Really glad this thread came up — dual alternator setups are genuinely underexplored territory here and there's a lot of real-world nuance that doesn't get documented anywhere properly.

My own van conversion only runs a single alternator feeding an Orion-Tr Smart, so I can't speak from direct dual-alternator experience — but I've been researching this heavily ahead of a potential motorhome build where I'd want redundancy.

One thing worth flagging that I don't think has been touched on yet: isolation between the two alternator circuits matters enormously. If both are feeding the same DC-DC input simultaneously without proper separation, you can create some unhappy feedback loops depending on your battery chemistry.

@SmartSolar_Geek — brilliant thread starter. And if @Stu1991's narrowboat experience is anything to go by, there's clearly a wealth of practical knowledge sitting in this community that deserves to be pulled together properly. Keep the replies coming, everyone.

Good point from @PaddyDavies about this being underexplored. One thing worth adding — if you're running two alternators into separate Orion-Tr Smarts, make sure you've got proper current sensing on each input circuit. I've seen setups on boats where one alternator ends up doing all the heavy lifting because the other's voltage droops slightly under load and the charger backs off.

Also worth considering: alternator protection relay on each unit. Alternators really don't like being disconnected under load, and some DC-DC chargers can create odd disconnect scenarios if not configured carefully.

On my own setup (12V cabin, single alternator) I use a Victron Orion-Tr Smart 12/12-30 and even that needed careful tail current tuning to stop premature absorption cutoff. Dual alternator scenarios multiply that complexity considerably.

Not done dual alternators myself but ran into something adjacent when I was wiring a Victron Orion-Tr Smart into my static van — the isolation side of things caught me out more than I expected.

One thing nobody seems to mention with dual setups: are both alternators on the same ignition feed? If not, your DC-DC charger might see a source voltage that drops in and out depending on which engine is actually running, which could confuse the charge logic badly.

Might be worth logging the input voltage over a run with a cheap Bluetooth shunt just to see what's actually happening before committing to a wiring config. Saved me a headache doing that on mine.

@MickWebb82 that static caravan wiring point is relevant — isolation between sources matters whether it's one alternator or two.

The bit nobody mentions with dual alternators feeding separate Orion-Tr Smarts: input voltage thresholds become critical. If both alternators aren't spinning at identical speeds (common on barges with belt-driven secondaries), one Orion might see sufficient input voltage while the other doesn't, and you end up with uneven charging that's hard to diagnose without logging.

Had something similar in my garden office setup — not dual alternators obviously, but two separate DC sources feeding one bank. Victron's VictronConnect logging saved me hours of head-scratching.

Worth setting your input lockout voltage conservatively so both units behave consistently, rather than leaving them on defaults. Marginal voltage differences cause more grief than people expect.