Anyone know if a 200Ah lithium battery is enough to run a small chest freezer overnight?

Got a Fogstar Drift 200Ah sitting in my van and I'm trying to work out if she'll cope with keeping a small chest freezer ticking over through the night without me waking up to a puddle of melted Magnums. 🍦

Freezer in question is a Subcold 35L — pulls about 45W when the compressor kicks in, but obviously it's not running constantly. I've seen figures suggesting maybe 20-30Ah overnight depending on ambient temp, which sounds manageable, but my setup's in the Midlands and it's currently getting proper Baltic outside, so I'm wondering if that actually works in my favour (less work for the compressor) or whether cold nights affect the battery capacity enough to cancel that out.

Running a Victron SmartShunt so I can actually monitor what's happening properly rather than just vibes-based guesswork. Charging comes from a 200W panel on the roof — which in a UK December is basically decorative.

Few specific questions:

• Anyone actually measured overnight draw on a similar-sized compressor freezer?
• Should I be worried about the battery dropping below 20% SoC if there's a few overcast days in a row?
• Is there a rule of thumb for LiFePO4 capacity loss in cold temps I should know about?

I could just plug it in and watch the SmartShunt data obsessively for a week, which let's be honest is exactly what I'm going to do anyway — but would be nice to know I'm not about to accidentally create a biohazard situation in the back of a Transit.

@Brummie84 depends massively on the freezer tbh. A decent chest freezer in a cool environment might only pull 100-150Wh overnight once it's at temp. 200Ah at 12v is 2.4kWh usable (well, ~1.9kWh if you're keeping it above 20% like you should with lithium).

So on paper yes, easily. But —

• How warm is the van interior?
• Is the freezer already cold or starting from ambient?
• What else is running?

Hot van + warm freezer starting from scratch = compressor barely stops = could chew through 400-500Wh no bother.

Had a similar setup in my static last summer, Fogstar held fine but I wasn't asking it to do much else. Chuck a Victron BMV on it so you can actually see what's being drawn rather than guessing.

Hey @Brummie84, @ZFS_OffGrid is spot on about environment being key. Worth actually measuring your specific freezer with a plug-in energy monitor (a Owl or similar) before committing - they're only a few quid and take the guesswork out completely. One thing people often overlook is how full the freezer is; a well-stocked freezer holds temperature far more efficiently than a half-empty one, so chuck some water bottles in there to fill the gaps. Also bear in mind the Fogstar Drift shouldn't really be discharged below 20% if you want to preserve cycle life, so you're realistically working with around 160Ah usable. That's roughly 2kWh at 12V, which should be plenty for most small chest freezers overnight, but measuring first is genuinely the sensible move. 👍

Good point from @ZFS_OffGrid and @DustyHiker on environment — one thing nobody's mentioned yet is the usable capacity caveat on that Fogstar Drift. Lithium nominally gives you ~95% usable, so realistically ~190Ah at 12V = roughly 2,280Wh available. Even a thirsty chest freezer pulling 200Wh overnight is only ~9% of that.

The bigger risk is actually compressor start-up surge — some cheap chest freezers spike 3-5× their running draw momentarily. Worth checking your BMS can handle it before assuming the Ah figure is the limiting factor.

In my shepherd's hut setup I measured my 60L chest freezer with a Victron SmartShunt over 24hrs — actual consumption was 280Wh total in mild weather. You should be comfortably fine overnight.

@BordersExplorer makes a cracking point about usable capacity — and with the Fogstar Drift you're actually in luck because lithium gives you a much flatter discharge curve than AGM, so you're not watching voltage sag like a disappointed parent at a school play.

One thing nobody's flagged yet: ambient temperature inside the van at night also affects compressor runtime, not just the freezer's insulation — park somewhere cool and that duty cycle drops nicely.

My chest freezer in the cabin pulls around 120Wh per night in winter but nearly double that baking in a summer field, so same unit, wildly different story depending on where you've parked.

Slap a Victron SmartShunt on it and watch the actual consumption for a night before worrying — you'll have your answer in real numbers rather than forum speculation. 🧊