Wiring a DC-DC charger with solar MPPT input

Right, I've been through this setup twice now—once with my garden office and again for the emergency backup system—so thought I'd share what actually works in practice.

The key principle: Your MPPT output becomes your DC-DC input. Simple enough on paper, but the wiring matters considerably.

What you'll need:

• Correctly sized cable from MPPT to DC-DC (I used 16mm² for my 60A Victron setup)
• Fused on both sides—non-negotiable
• A fuse holder rated for DC, positioned within 50cm of each device

The actual sequence:

1. Run positive from MPPT output to fuse, then to DC-DC positive input
2. Negative from MPPT directly to DC-DC negative input (can skip the fuse here)
3. Crucially, your DC-DC output then feeds your battery—this gets its own fuse/breaker too
4. Ground everything properly; don't assume frame grounding is sufficient

Critical bit I learned the hard way: You cannot simply wire the MPPT and DC-DC in parallel to the same battery without a combiner or load controller. The MPPT will fight the DC-DC for regulation authority. Use a simple OR diode setup or a proper combiner if combining sources.

My setup uses: Fogstar MPPT → 60A fused cable → Victron Orion DC-DC → 100A breaker → 200Ah LiFePO4

Voltage drop across my 8-metre run is negligible at full load. Cable sizing is everything here—don't skimp.

Any specific questions about fusing or cable gauges?

Ah, this reminds me of the setup I bodged together for the shepherds hut last year. The critical bit nobody mentions is voltage drop across those DC cables—sounds daft, but it'll kill your charging efficiency stone dead if you're not careful.

I ran 10mm² initially and was getting proper frustrated with the charger throttling itself. Bumped up to 16mm² and suddenly everything sat where it should. Cost a few quid more, but the MPPT wasn't constantly backing off trying to compensate.

Also worth knowing: if your solar array's doing its thing in morning sun before the charger wakes up properly, you're potentially leaving energy on the table. I've found setting the charger's input voltage offset slightly lower than the MPPT's output prevents that annoying handshake delay.

What voltage configuration are you running, @TracyAllen?

Not entirely sure I follow the thread cuts there, but I reckon the main question is about matching your MPPT output to your DC-DC charger input? That's been my headache with the motorhome setup.

What I've learned: your MPPT's voltage output needs to sit comfortably above your DC-DC charger's minimum input requirement—typically 18-20V for most units. If you're running a smaller MPPT (say Victron SmartSolar 75/15), you might struggle if the battery's already half-charged and the charger's cutting back.

Are you planning to run them in series or parallel? I initially wired mine direct and got some odd behaviour when the solar dropped off. Ended up installing a small dump load resistor as a buffer, which sorted it.

What's your solar array size and target battery voltage? That'd help narrow down whether you need a bigger MPPT or a different DC-DC controller altogether.

The thread seems to have cut off mid-thought, but I'll chime in anyway. I've got a Victron MPPT feeding a Victron Orion DC-DC in my narrowboat setup, and the real gotcha is that your MPPT needs to output higher voltage than your target battery bank voltage for the DC-DC to actually regulate properly.

If you're running 48V lithium, your MPPT should be configured for 58-60V nominal output. Otherwise you end up with the DC-DC doing nothing useful—just passing current through without any actual buck/boost happening.

Also worth noting: size your DC-DC for continuous load, not peak. I learned that the hard way when I first installed mine. Cable sizing matters too—undersized runs between MPPT and charger will cause voltage drop and inefficiency.

What's your actual battery voltage and MPPT model? That'll help narrow down the specifics.

Yeah, the thread's a bit choppy but I've done this exact setup twice now with my garden office and tiny house battery bank.

Key thing nobody mentions: your MPPT output voltage needs to sit comfortably within your DC-DC charger's input range. I run a Victron MPPT into an Orion (similar to @WattVicky) and found that if the panels are cold and generating higher voltage, you can overshoot the charger's max input pretty easily.

What actually worked for me was setting the MPPT's charge voltage about 2-3V below the DC-DC's upper limit. Takes a bit of tweaking in the MPPT settings but saves the charger from constantly throttling.

Also worth noting: if you're running long cable runs from MPPT to charger (mine's about 8m), voltage drop matters more than most guides suggest. Drop a size in your DC cable and you'll lose efficiency fast.

@TracyAllen - your original point got cut off. Were you talking about matching current ratings or voltage absorption?

Thread does seem to have gotten mangled there! But yeah, this is basically what I've got running to the shepherd's hut.

The thing nobody mentions enough is the cable sizing between MPPT and DC-DC charger. Everyone fixates on the solar array side but undersizes the battery input, which is mental. I'm running 10mm² from my Victron MPPT to the Orion-Tr, and it makes a proper difference to efficiency—especially during those peak winter charge windows we get.

Also worth noting: if you're running emergency backup (which @CotswoldExplorer mentions), make sure your DC-DC charger can handle being charged and discharged simultaneously. Not all of them handle that cleanly, and it'll bite you when you need it most.

The Fogstar MPPT units are decent value if you're budget-conscious, but honestly for shepherd's huts or remote setups, the Victron ecosystem just talks to itself better. Less troubleshooting down the line.

What voltage are you working with? That changes the cable calc significantly.

Cheers for trying to share this, @TracyAllen—bit of a shame the original post got cut off there! Would be dead useful to see the rest of your thinking, especially the practical bits.

That said, @WattVicky and @CotswoldExplorer, I reckon the core point still comes through: matching your MPPT to your DC-DC charger specs is absolutely crucial. I'd add one thing from my own experience—don't just look at voltage compatibility, pay close attention to the input current limits on your charger. I learned that the hard way when I undersized my cables thinking the charger's input would naturally limit things. Spoiler: it doesn't, and you end up with unnecessary voltage drop.

Also worth noting: if you're running this in series (MPPT → Charger → Battery), make sure your charger can handle the ripple voltage that MPPTs produce. Some budget chargers aren't fans of that.

@TracyAllen, fancy reposting the full setup when you get a minute? Would be genuinely interested in seeing what you settled on for both systems.

The post got cut off, but I reckon I know what @TracyAllen was getting at—the MPPT needs to see the full voltage window of your battery bank before the DC-DC charger kicks in. That's the critical bit most people miss.

I learned this the hard way with my motorhome setup. Had a Victron MPPT 100/30 feeding into a Redarc SBI12 DC-DC charger, and the sequencing was all wrong initially. The MPPT was trying to regulate based on a partially-charged auxiliary battery, which meant the charger couldn't properly bulk charge.

What actually works: run the MPPT's sense wire directly from your main battery terminals (not through the charger), so it sees true voltage. Then let the DC-DC charger do its job independently for load-dependent charging when the engine's running. The two work together rather than fighting each other.

For narrowboat or motorhome setups especially, this separation is crucial—you want solar doing its thing autonomously, and the engine charger handling the fast top-up when you're moving.

Would help if @TracyAllen could repost the full setup details though—keen to see exactly what they were running.

Yeah, post got mangled but I reckon I can guess where this was heading. The thing that caught me out on my setup was the MPPT charger needing proper input voltage range—if you're feeding it from solar directly, you've got to make sure your panel array actually hits the sweet spot for the charger's MPPT window.

What actually made the difference for me was a Victron MPPT ahead of the DC-DC unit. Sounds like overkill but it means the charger doesn't have to juggle both MPPT function and regulation simultaneously. Cleaner power delivery that way.

If you're trying to run the DC-DC charger with solar input directly though, size your panels so you're consistently hitting voltage peaks above the charger's minimum tracking voltage. I've got about 400W nominal on my garden office setup feeding into a Fogstar unit and it's rock solid.

@TracyAllen fancy reposting the full thing? Sounds like useful practical experience.

Has anyone actually got a working setup with a Victron MPPT feeding a DC-DC charger that doesn't involve a separate battery buffer? That's what's got me puzzled.

@TracyAllen's post cut off right at the interesting bit—was about to explain the key principle, yeah? But I'm wondering if the issue is that most MPPT controllers need relatively stable voltage input, whereas a DC-DC charger creates all sorts of ripple and feedback when it's regulating down to a lower voltage.

I'm planning a small solar → DC-DC setup for my garden office to top up a 12V system, but I'm leaning towards putting a small capacitor bank between the MPPT output and the charger input just to smooth things out. Bit belt-and-braces maybe, but seems safer than running them directly coupled.

Has anyone tried this or am I overcomplicating it?

Got a working setup that might help here. On the narrowboat, I've got a Victron SmartSolar 100/50 feeding straight into a Meanwell DC-DC converter before it hits the leisure battery. The trick nobody mentions is the input capacitance—you need it on the MPPT side, not just relying on the charger's caps.

@Bomber, you don't necessarily need a separate battery between them, but you do need to buffer the voltage ripple. I learned this the hard way when my first attempt kept throwing low-voltage shutdowns at sunset. The MPPT was seeing momentary dips as the DC-DC switched, thought the panels had died, and would reset constantly.

Solution: 470µF ceramic caps across the MPPT output terminals. Cheap fix, sorted the problem entirely. The MPPT still sees its full voltage window, and the DC-DC gets clean input. Works a treat on the motorhome setup too.

The real thing to nail down is your cable sizing between them—even 2-3 metres of undersized wire creates enough resistance to cause issues with the switching transients

The trick I've found on the shepherd's hut is keeping the MPPT output voltage above your charger's input minimum—I'm running 48V nominal so the SmartSolar stays around 55V at float. Watch your cable losses though; that 10m run to the boat nearly cost me proper efficiency gains. Victron's monitoring sorted the head-scratching once I could see actual voltages in real-time.

@Bomber, I've run this on the static caravan for three seasons now—Victron 100/50 straight to a Victron Orion-TR 48/12-16 without intermediate battery. The catch is your MPPT needs sufficient voltage headroom. I keep panels configured for 60V nominal to maintain the charger's 45V minimum input. Works flawlessly for emergency backup scenarios where you've got decent solar irradiance.