Narrowboat 400Ah lithium bank — is my Victron MPPT undersized for winter?

Running a 400Ah Fogstar Drift lithium bank on the boat with a Victron SmartSolar 100/30 and 400W of panels (2 x 200W Renogy in series). Works a treat in summer — hitting 25–28A easily on a good day. But now we're into November and I'm moored up in a fairly shaded spot on the Cam, I'm barely seeing 6–8A peak and the bank is slowly draining over a few days of overcast weather.

I know the obvious answer is "more panels" but I'm wondering whether the controller itself is the bottleneck. The 100/30 is rated 30A max output, so in theory I've got headroom — but at these low-irradiance levels is there an argument that a more efficient MPPT algorithm (say, stepping up to a 150/35 or 150/45) would squeeze more out of the panels in poor conditions? Or am I just expecting too much from 400W in a British November?

Secondary question: I run a 30A DC-DC charger off the alternator when I'm cruising (Victron Orion-Tr Smart), so the bank does get topped up on moving days. But we're doing maybe 2–3 hours of cruising per week right now. Is that realistically enough to compensate, or should I just accept that winter on a narrowboat means running the Honda eu22i more than I'd like?

Concrete numbers would be really helpful if anyone's been through this. What's your actual solar yield in December/January on a similar setup?

@AngliaOffGrid mate, your 100/30 is literally leaving free amps on the table — 400W ÷ ~24V = theoretically ~16A anyway on a 12V system, but if you're 24V you're hitting that 30A ceiling and weeping into your bilge all winter when every photon counts.

Upgrade to the Victron SmartSolar 150/45 and you'll actually use those panels rather than throttle them — winter sun is stingy enough without your MPPT joining the go-slow.

@AngliaOffGrid the winter angle is the real story here — low sun angle means your panels are rarely hitting anywhere near rated wattage anyway, so the 100/30 ceiling becomes less of a bottleneck than you'd think.

What'll actually bite you in December isn't the MPPT, it's the hours. Even a perfectly sized controller can't conjure amps from four hours of weak British winter sun.

My shepherd's hut setup taught me this the hard way — I upgraded the controller first, then realised the panels themselves were the real constraint once daylight dropped below six hours.

Before spending on a bigger MPPT, I'd genuinely track your actual daily harvest through the Victron app across a few grey weeks. If you're regularly hitting that 30A ceiling in winter, then you've got a case for upgrading. I'd wager you won't be.

Hey @AngliaOffGrid — worth adding that on a narrowboat you've also got the shading issue to think about. Even a bridge or overhanging trees mid-winter can knock your already-limited output right down. That said, with 400Ah of lithium you'll be accepting charge aggressively when it's available, so squeezing every watt matters more than in summer. Have you considered tilting your panels? Even a simple adjustable mount can make a meaningful difference to your harvest angle between October and February. Also check your wiring losses — undersized cable runs can quietly rob you of a few amps before you've even started. Might be worth logging a full day on the VictronConnect app to see where your actual peaks are sitting before deciding on an upgrade. 👍

@AngliaOffGrid one thing nobody's mentioned yet — lithium's lower internal resistance means it'll actually accept charge far more aggressively than lead-acid, so whatever amps your panels can produce in winter, the Fogstar bank will happily take them without complaint. That's genuinely good news. The flip side though is that on those rare crisp, clear January days with fresh snow reflecting light upward, you can get surprising spikes in panel output. Your 100/30 will cap at 30A regardless, so you're protected there. I'd also check your battery temperature sensor is properly configured in VictronConnect — lithium charge parameters in near-freezing conditions matter more than the controller size itself. The Fogstar Drift has a BMS low-temp cutoff, but you want the MPPT playing nicely with it rather than relying solely on the battery to protect itself.

Great points from everyone above. One thing I'd add specifically for narrowboat winter scenarios — have you considered your mooring orientation? If you're regularly bow-in facing north at a marina, even a perfectly-sized MPPT won't save you. Also worth checking whether your 100/30 is actually logging its daily harvest via the VictronConnect app — the historical data will tell you far more than guesswork about whether you're genuinely hitting the controller's limits or whether the panels themselves are the bottleneck. My gut says for a 400Ah lithium bank doing winter liveaboard duties, you'd benefit from a 100/50 and another 200W panel if roof space allows. The controller upgrade is fairly painless and future-proofs you nicely. @PaulJackson makes a fair point about lithium acceptance rates too — you want a controller that can keep up when conditions briefly improve.

@AngliaOffGrid with a 400Ah bank your 100/30 is basically a garden hose trying to fill a swimming pool in January — a 100/50 or even the 150/70 would stop you weeping into your Bovril by February.