Anyone else finding their 12V compressor fridge is killing their battery bank over winter?

I've got a 120Ah lithium (a Fogstar Drift) paired with a 200W panel on the van roof, and over summer it was brilliant — panel kept up no bother. But now we're into shorter days and the fridge (a Brass Monkey 40L) is cycling more because the ambient temp in the van actually drops quite a bit overnight, weirdly making it work harder to hold temp, and the solar just isn't cutting it anymore. I'm pulling maybe 3-4Ah per hour from the fridge and only getting 20-30Ah back from the panel on a dull December day.

I've been toying with the idea of adding a second 200W panel, but roof space is tight on a Transit Custom. Also wondering whether a small DC-DC charger from the alternator is worth leaning on more heavily during winter drives — currently got a Victron Orion 18A in there but I don't always do long runs.

Has anyone found a practical workaround for this seasonal gap, without just throwing a massive second battery at it? Curious what others are doing, especially those living in it full time through the colder months.

JessPhillips | 847 posts

@Jim1999 Classic winter struggle! The issue is two-fold — your panel output drops massively (realistically you might only be getting 20-40W average on a gloomy December day) while the fridge actually works harder in cold ambient temperatures trying to maintain setpoint if it's poorly insulated.

A few things worth trying: check your fridge's thermostat setting isn't lower than it needs to be, and if it's somewhere cold in the van, consider a small insulating wrap around it. Some people also put a thermal blanket over theirs overnight.

Honestly though, 120Ah against a compressor fridge through a UK winter with one 200W panel is always going to be tight. Have you got any shore power or leisure battery charging option when parked up? That'd be my first port of call before adding more panels.

KarenEvans | 1,203 posts

@Jim1999 One thing I'd add that hasn't been mentioned — have you checked your fridge's thermostat setting? I found mine was working far harder than necessary because it was still on the summer setting. Dropping it back a notch made a noticeable difference to consumption without actually affecting food safety. Also worth sticking a small thermometer inside to see what it's actually hitting rather than guessing.

The other thing I do through winter is pre-cool anything going in the fridge before it goes in — warm items make the compressor work overtime. Small habits like that can buy you a surprising amount of runtime when solar input is already marginal. How much driving are you doing? An Orion or similar DC-DC charger off your alternator can really save the day this time of year.

ValleySolar | 412 posts

@Jim1999 Something worth considering that nobody's touched on yet — where are you parking overnight? If the van's sitting somewhere cold, your fridge compressor is working much harder to maintain temperature than it would in summer. A simple layer of insulation around the fridge body (not blocking the vents obviously!) can make a surprisingly big difference to runtime. Also, what are your actual resting voltages looking like in the morning? A 120Ah Drift should have enough capacity to ride through a winter night comfortably even with minimal solar input, so if you're really struggling it might be worth checking your fridge's thermostat setting — dropping it from 4°C to 7°C can cut compressor cycles noticeably without compromising food safety.

Hamish1980 | 634 posts

@Jim1999 Worth looking at your fridge temperature setting too — a lot of people run them colder than needed, which hammers the compressor duty cycle. For food storage 4-5°C is plenty, and every degree you dial back makes a difference in winter.

Also, have you considered a small DC-DC charger so you can top up from the van's alternator on driving days? Even a 30-40 minute run can put meaningful charge back into a lithium. Combined with being a bit more disciplined about opening the fridge less frequently (letting cold air escape is surprisingly costly), you might find the situation improves without needing extra panels. The Fogstar Drift is a solid battery so at least you've got that going for you! 👍

MoorRoger | 847 posts

@Jim1999 Familiar story this time of year! One thing I'd throw in that nobody's mentioned — have you looked at your solar charge controller stats to see what you're actually harvesting daily now versus summer? I'd wager you're down to maybe 20-30% of your peak output on a grim December day in the UK. Your 200W panel might only be giving you 20-40Wh on a really overcast day, whereas your fridge could easily pull 30-50Wh depending on ambient temps. The maths just stops working. A second panel if you can squeeze one on the roof makes a massive difference over winter — even a modest 100W addition helped me enormously last January on Dartmoor.

GeorgeHarris | 218 posts

@Jim1999 Something that made a noticeable difference for me was adding thermal mass inside the fridge — I fill any empty space with frozen water bottles or even just screwed-up newspaper. The compressor runs far less frequently when it's not cooling air every time the door opens. Also worth checking the door seal with a piece of paper; they can degrade and let cold air escape without you realising. On the charging side, have you considered a B2B charger so you're topping up properly when driving? A 200W panel in December in the UK is going to struggle to do much on its own — you're probably lucky to get 3-4 usable hours even on a clear day. Combination of small tweaks rather than one big fix is usually the answer in my experience.

WillReid | 412 posts

@Jim1999 The maths are fairly brutal in winter — a 200W panel in the UK might realistically yield 1-2 usable hours of decent output on a grey December day, so you're potentially only recovering 200-400Wh whilst your fridge chews through maybe 300-500Wh over 24 hours depending on ambient temperature. Your 120Ah Fogstar is doing heavy lifting with very little recovery time.

One thing nobody's touched on: consider a secondary small panel (even a 50W) oriented differently — south-facing tilt steeply angled catches low winter sun far better than a flat roof mount. On my shepherd's hut I run a tilted ground array specifically for this reason and winter harvest improved noticeably.