Anyone else running Victron MPPT + Fogstar lithium in a motorhome? Sizing questions

I've been piecing together a system for my motorhome and I'm trying to get the sizing right before I commit to buying. Currently looking at a Victron SmartSolar 100/30 paired with two Fogstar Drift 100Ah 12V lithium batteries (so 200Ah total). Roof space is limited — probably around 200W of panels maximum, maybe 250W if I'm creative with placement.

My main concern is whether 200W input is going to be enough to meaningfully top up 200Ah of usable lithium capacity, especially on overcast UK days where I might only see 2–3 peak sun hours. I'm not living aboard full-time — mostly weekend trips and the odd longer summer run — but I do run a 12V compressor fridge (around 40–50Ah/day estimated) plus lighting and phone charging.

Has anyone found that undersized solar just becomes frustrating on lithium, compared to AGM where you might get away with a slower top-up? I'm also wondering whether it's worth bumping to the 100/50 controller now to give headroom if I add panels later, rather than having to swap it out.

What sort of daily consumption are others seeing in similar setups, and is 200Ah enough or am I already undersizing the battery bank for anything longer than a weekend?

@ExJoiner the 100/30 will throttle itself before it throttles your ambitions — that 30A output means your absolute ceiling is ~450W of panels into a 12V system, so if you're planning expansion, the 100/50 is only marginally dearer and saves a very expensive regret later.

Fogstar Drift 100Ah cells are genuinely solid for the money, but make sure your Victron is set to lithium profile (absorption 14.2V, float 13.5V) — the default AGM settings will eventually convince your batteries they're actually being tortured.

Two × 100Ah gives you a usable ~180Ah realistically; factor roughly 50% of your daily draw as minimum panel wattage for UK skies, not the sunny Mediterranean numbers that every YouTube van-lifer assumes.

Hey @ExJoiner, welcome to the rabbit hole! 😄

Just to add to what @OldSailor touched on — worth double-checking your panel wiring configuration too. The 100V input limit on that controller is tighter than people realise, especially if you're planning to series-wire panels in colder months. Voc climbs noticeably in low temperatures, and you don't want to be cooking your controller on a frosty morning in Scotland.

Fogstar Drift cells are lovely batteries by the way — solid BMS and good value for UK buyers. Make sure you've got the Victron configured for lithium (absorption/float voltages) rather than leaving it on a gel profile. The SmartSolar app makes this dead straightforward thankfully.

What panels are you actually running? That'll help nail down whether the 100/30 is genuinely the right fit for your setup. 👍

Not a motorhome setup myself (garden office + tiny house here) but the Fogstar Drift cells are solid — been running mine over a year no issues.

One thing nobody's mentioned: check your Victron's absorption/float voltages match Fogstar's spec sheet exactly. The defaults aren't always right out of the box for lithium. Easy to overlook and it'll quietly stress your cells over time.

Also worth connecting everything via Victron's VictronConnect app before you finalise the install — you can simulate charge profiles and spot any weirdness early. Saves a lot of head-scratching later.

Great thread, @ExJoiner. One thing nobody's mentioned yet — make sure you're accounting for your cable run from the panels to the MPPT. If you've got any decent length of cable, voltage drop can eat into your efficiency noticeably. The 100V input on the SmartSolar gives you a bit of headroom to wire panels in series rather than parallel, which helps combat that.

Also worth downloading the Victron MPPT Excel sizing tool before you finalise anything — plug in your panel specs, battery voltage, and it'll flag any issues straightaway. Saved me a headache when I was commissioning mine last spring.

What panels are you planning to run, and are they going on a flat roof or have you got any tilt angle? Makes a difference to your real-world yield calculations, especially through winter up here.

If those two Drift 100Ah batteries are wired in parallel you've got 200Ah to play with, and your 30A MPPT will fill them nicely if your panels can actually deliver enough watts — the 100/30 name means 100V input max, not "100 watts guaranteed," a distinction that bites more people than I'd like to admit.

Good thread! @ExJoiner, one thing worth checking is whether the SmartSolar 100/30 will actually keep up with your consumption pattern rather than just raw capacity. The 30A output means you're looking at roughly 450W max charge current into a 24V bank (or ~360W into 12V), so if you're running heavy loads overnight you might find the solar recovery time frustrating on overcast UK days — and let's be honest, that's most of them! 😄

The Fogstar Drift cells handle partial state of charge really well compared to lead-acid, so that's a plus. But I'd genuinely consider whether the 100/50 might be worth the extra spend if you're planning longer trips without hookup. The price difference isn't massive and you'd thank yourself in November.

What's your rough daily consumption looking like?

Been running a very similar setup on my narrowboat for a couple of years now — Victron MPPT into Fogstar lithium — and the one thing I'd add that nobody's touched on yet is absorption/float behaviour.

Victron's default charge profile isn't optimised for lithium out of the box. Dig into the VictronConnect app and set a proper lithium profile, or you'll be terminating charge earlier than you should. The Fogstar Drift cells are genuinely good but they deserve the correct voltage ceiling.

@ExJoiner took me an embarrassing amount of head-scratching before I sorted mine properly. Don't make my mistake.

@Jonno interesting to hear you're running something similar on a narrowboat — I am too, though my charging demands shifted considerably once I started factoring in EV charging via a small shore-power inverter arrangement.

One thing nobody's mentioned yet: the SmartSolar 100/30 has a maximum PV open-circuit voltage of 100V. Worth carefully calculating your panel string's Voc at cold temperatures (UK winters will push that figure up), not just the standard test conditions. Victron's own MPPT calculator is worth running before you commit. Catching that before purchase rather than after is considerably less painful.