Worth fitting a DC-DC charger when you've already got solar going into the same battery bank?

Running a 200Ah lithium (Fogstar Drift) in the motorhome with 300W of solar on the roof through a Victron 75/15 MPPT. Works well most of the time, but obviously in winter or on cloudy weeks the panels don't keep up.

Currently the alternator charges the leisure battery through a basic split-charge relay, which I know isn't ideal for lithium — it just tops it up a bit rather than doing a proper charge. Been looking at a Victron Orion-Tr Smart 12/12-30 to replace it, but I'm wondering if there's much point when the MPPT is already wired to the same bank. Feels like they'd just be fighting each other or something.

Does the Orion play nicely alongside an MPPT on the same battery? Anyone running both on a Fogstar or similar LiFePO4? Keen to know if it actually made a noticeable difference to state of charge after a long drive on a grey day.

@HO_Marine yes, absolutely worth it. Solar and a DC-DC aren't competing — they're complementary. Your alternator's doing nothing useful for your lithium without one anyway (split charge relays are rubbish for lithium).

Fitted a Victron Orion-Tr Smart 30A in my motorhome last year. Cloudy weeks in Scotland sorted. The two sources just stack — MPPT handles solar, Orion handles engine charge, both talk nicely if you go Victron throughout.

Don't skimp and get a cheap unit either. Your Fogstar Drift deserves proper CC/CV charging, not some unregulated relay bodge.

@DownsCruiser is right, and I'll add a real-world angle from living aboard.

Last February I had a solid 10-day grey stretch moored up — panels barely producing 5–10% of rated output. The DC-DC charger (running a Victron Orion-Tr Smart 30A isolated) saved me every single time I motored anywhere, even a short 45-minute cruise to the next lock. Picked up a meaningful chunk of charge without touching the alternator health.

The beauty is the Orion talks to your existing Victron ecosystem nicely — everything visible in VictronConnect alongside the MPPT data. You're not duplicating effort, you're genuinely plugging a seasonal gap.

With a Fogstar Drift specifically, make sure your DC-DC is set to the correct lithium charge profile — Fogstar publish their recommended absorption and float voltages. Worth a quick check before you wire anything in.

The DC-DC is doing a different job entirely — it's charging from the alternator properly rather than just piggybacking off the chassis battery. Both inputs sit nicely alongside each other on a lithium bank.

I've got a similar setup in the shepherd's hut but obviously no alternator — just solar plus hook-up. What I would say is don't cheap out on the DC-DC unit. Victron Orion-Tr Smart is the obvious pick, non-isolated if your chassis and leisure batteries share a common negative (most do).

Worth double-checking your 75/15 settings are dialled in for lithium too while you're at it — Fogstar publish recommended charge profiles on their site. Small thing but makes a difference over time.

One thing worth mentioning that nobody's touched on yet — with a Fogstar Drift specifically, you'll want a proper DC-DC anyway because lithium can confuse some alternators when connected without isolation. The DC-DC acts as a buffer, protecting your alternator from the battery just hammering it with a massive initial charge demand. I've seen a few people on here burn out alternators on newer motorhomes going direct. A Victron Orion-Tr Smart would pair nicely with your existing Victron kit too — you can monitor everything through the same VictronConnect app which is handy. The Bluetooth between devices makes the whole setup feel properly integrated rather than cobbled together.

Great points all round. One thing I'd add to what @KentOffGrid mentions about the Fogstar Drift — if you're regularly doing longer drives between sites, a decent DC-DC like the Victron Orion-Tr Smart will actually coordinate nicely alongside your existing MPPT. Both can charge simultaneously without fighting each other, since the Orion handles its own current limiting independently. On a grey January week where your 300W is barely producing, even a 30A DC-DC running for a two-hour motorway leg can put 60-odd amp-hours back in — that's genuinely meaningful. For a 200Ah bank it's well worth the investment.

Jumping in to back up what @JLB_Boats and @KentOffGrid have already said — yes, absolutely worth it. The two systems complement each other really nicely rather than competing. I've got a similar setup and the DC-DC means that any driving time, even a short 45-minute run to a site, is actually doing useful work on the battery rather than just tickling it through the alternator.

One practical thing I'd add — if you go for a Victron Orion Smart, you can monitor both your MPPT and the DC-DC charger together through the VictronConnect app, which makes keeping an eye on your state of charge much simpler. Especially handy in those gloomy winter months when you're juggling every charging opportunity you can get. Makes the whole system feel properly joined up rather than a collection of bits. 👍

Agree with all the above, but one thing nobody's mentioned yet — if you go the DC-DC route, have a look at the Victron Orion-Tr Smart isolated units. Being able to monitor both your solar and the DC-DC input through the VictronConnect app alongside your existing 75/15 gives you a really clean picture of what's going into the Drift. Particularly handy in winter when you're trying to work out whether the engine run was actually worth doing. The 30A version should pair nicely with your 300W setup without either source competing awkwardly.

Been wondering the same thing on my boat setup. Quick question for the thread — does the DC-DC charger need to be sized to match your alternator output, or is it more about what your battery bank can actually accept? I've seen the Victron Orion (mentioned by @24V_Nerd) in various amperages and not sure which would make sense alongside an existing MPPT. Is there a risk of both charging sources fighting each other if the BMS starts pushing back?