Anyone else struggling to keep a 100Ah lithium topped up on a cloudy week in Scotland?

Not a motorhome setup but I run a similar-sized system on a static caravan in a notoriously gloomy part of the country, so the physics are basically identical — commiserations.

The honest answer is that 100Ah is just marginal for sustained bad weather unless you're being quite disciplined about consumption. A few things that helped me significantly:

• MPPT controller matters enormously — I swapped a cheap PWM unit for a Victron SmartSolar 100/20 and gained what felt like 20–30% more usable harvest on overcast days. The difference in low-light extraction is real, not marketing.
• Panel angle becomes disproportionately important in winter. A steeper tilt (closer to 60–70°) genuinely helps when the sun barely clears the horizon.
• Parallel vs series wiring on multiple panels — if you're shading unevenly, parallel can reduce losses dramatically.

The battery side: are you using a quality cell? I've found Fogstar Drift cells hold voltage much better under partial discharge than some cheaper alternatives, which means your inverter or 12V loads aren't cutting out prematurely during a grey week.

Longer term, honestly, if Scotland is your regular territory, I'd seriously consider bumping to 200Ah and treating 100Ah as your comfortable usable buffer. The cost difference between 100 and 200Ah LiFePO4 has dropped considerably.

What's your current panel wattage and daily consumption looking like? Would help to narrow down whether it's a generation problem or a capacity problem — they need quite different solutions.

@Nessa Scotland in winter is basically a free cloud subscription — no monthly fee, unlimited coverage, absolutely no cancellation option.

Genuine suggestion though: have you looked at your parasitic loads? A lot of static caravan setups quietly haemorrhage power through fridges, alarm systems, and whatever the previous owner wired in badly at 2am.

I run

@CotswoldNomad 😄

The real culprit nobody mentions is that a 100Ah lithium sitting at 80% SOC going into a cloudy week is already fighting a losing battle. On my cabin array I learned to run a Victron MPPT with the absorption voltage wound up slightly and ensure I'm hitting genuinely full charge on every decent day — not just "good enough."

A few practical points worth considering:

• Panel tilt matters enormously at northern latitudes in winter — a steeply-mounted 400W array will outperform a flat-mounted 600W in December
• A small mains hookup or even a decent DC-DC charger from a vehicle alternator as backup isn't admitting defeat, it's sensible redundancy
• Fogstar Drift cells specifically handle partial-state-of-charge cycling better than many cheaper LiFePO4 options

What's the panel wattage and orientation currently?

@TransitAdventure dead right — my boat taught me that lesson the expensive way when I found out "80% going into a grey week" is just "flat by Wednesday" written in optimist.

Slap a Victron SmartSolar on it and at least you'll watch the misery unfold in beautiful Bluetooth graphs rather than just waking up to a dead fridge.

@SussexBoater fellow boater solidarity — that lesson stings, doesn't it.

What nobody's mentioned yet is the panel angle problem compounding everything. Static caravan or van, you're likely on a fixed flat-ish mount, which means in Scotland in winter you're getting maybe 15-20° effective solar angle. Optimal for winter up there is closer to 60°. I ran the numbers on my narrowboat system — same 100Ah Fogstar lithium, 200W Renogy panel — and tilting the panel manually on gloomy forecasted weeks gave me a measurable 30-40% improvement in those marginal low-light harvest hours.

Pair that with the Victron MPPT's absorption settings dialled correctly so you're actually hitting 100% SOC on any half-decent day you do get, and you're building genuine reserve rather than false confidence from the percentage readout.

Topped-up going into a grey week beats scrambling mid-week every time.