Anyone else struggled to keep a 100Ah lithium topped up on cloudy days with just roof panels?

Been there, done that. Had a 100Ah Fogstar Drift on my Hymer and two 175W panels on the roof — sounds plenty on paper, doesn't it. Then you get three overcast days in the Lake District in October and suddenly you're rationing the kettle.

The maths looks great when you're sat at home planning it. Real-world UK conditions are a different story. Those panels are lucky to push out 20-30% of their rated output on a proper grey day, and if you're running a compressor fridge, laptop, lighting — it goes faster than you think.

A few things that actually helped me:

• MPPT controller — if you're still on PWM, swap it. Made a noticeable difference on low-light days.
• Tilt the panels — even a cheap £30 bracket to angle them better in winter makes a meaningful difference at lower sun angles.
• Reduce parasitic loads — my Victron Cerbo was pulling a surprising amount doing nowt useful. Tightened up the monitoring side.

I ended up adding a third panel where I could squeeze it and honestly that was the biggest single win.

What I haven't done yet is add shore power or a proper DC-DC charger from the alternator, which is probably the sensible belt-and-braces answer for anyone touring in autumn/winter here. UK cloud cover just doesn't forgive an undersized array.

Anyone running B2B chargers alongside roof panels finding it keeps things topped up reliably? Curious what sizes people are running on similar battery capacity.

@ExChippie static caravan here rather than motorhome but same headache — what's your actual panel output reading on those grey days? Because I found my "175W" panels were barely pushing 20-30W each in proper overcast.

Is your Fogstar getting full charge cycles or just sitting at 70-80% for days? Wondering if that's part of the problem or if you're genuinely pulling more than the panels can replace?

Also — did you have a Victron MPPT or one of the cheaper controllers? I've read the algorithm makes a real difference in marginal light conditions but haven't tested it myself properly.

@ExChippie mate, I lived this nightmare. Did a week in Scotland last November — two 200W panels on the roof of my Transit, Fogstar 100Ah, Victron MPPT controller. Looked bulletproof on the spec sheet.

Reality? Four grey days in a row and I was nursing 40% SoC, rationing the induction hob like it was wartime.

What actually saved me was adding a B2B charger wired to the alternator. Twenty minutes of driving fills what a whole overcast day on the panels won't. Now I treat solar as the primary, alternator as the safety net rather than the other way round.

The fundamental problem is that a 100Ah bank just isn't big enough to absorb three bad solar days without feeling the pinch. Either expand the battery capacity or have a reliable secondary input. There's no clever workaround that gets you out of basic physics.

@ExChippie October in the Lakes with roof panels is basically decorative at that point — mine were generating less than my kettle uses just thinking about boiling.

Ended up running a Victron SmartSolar MPPT and still watching the Fogstar creep down to 40% by day three in my van conversion. The real fix was swallowing my pride and adding a B2B charger so the alternator could do the heavy lifting when the sky turned into a grey blanket — which in the UK is, statistically, always.

Worth looking at:

• Renogy 40A DC-DC charger — solid bit of kit
• Parking on a slope to angle panels (desperate times)
• Accepting that British weather has a personal vendetta against off-gridders

@SussexVanLifer Scotland in November, absolute masochism, respect. 🏴󠁧󠁢󠁳󠁣󠁴󠁿

@ExChippie oh mate, I feel this in my soul 😂

Had exactly this with my van conversion. The roof panels are basically just keeping the Victron MPPT busy pretending to work on grey days.

What actually saved me was adding a DC-DC charger so I top up properly off the alternator when driving. Even a 40-minute run to the chippy puts a decent chunk back in.

Also worth looking at a folding portable panel you can angle properly — I picked up a Renogy 200W foldable and just prop it outside facing whatever patch of sky looks least miserable. Makes a surprising difference vs fixed flat panels losing half their output to poor angle.

@BitsAndBobs "decorative" 💀 brutal but fair

The Lake District in October deserves a bigger battery tbh, not just more panels

@ExChippie this is exactly why I ended up adding a shore power hookup and a decent mains charger to my static caravan setup — sometimes you just can't fight the British weather!

Curious though, did you ever try a DC-DC charger off the vehicle alternator as a backup? I've been looking at the Victron Orion-Tr Smart for my garden office setup and wondering if something similar would've saved you in the Lakes. Even just topping up while driving between spots could make a difference, no?

Also — is there a reason you went with roof-only rather than a portable panel you could angle properly? I've seen people get dramatically better output just by tilting a panel at the right angle on an overcast day rather than lying flat on a roof.