Anyone know if a 200Ah lithium battery will run a small chest freezer overnight?

Running a small chest freezer in my van conversion and trying to work out if my battery bank will actually keep it going overnight without solar input.

Got a 200Ah lithium (Fogstar Drift 12V) paired with a Victron SmartShunt so I can track consumption properly. The freezer is a Alpicool C15 — tiny thing, pulls around 45W when the compressor kicks in but obviously it cycles on and off.

From what I've read the duty cycle on these compressor fridges/freezers is roughly 30-50% depending on ambient temp. So in a cool UK night maybe looking at something like:

• 45W × 40% duty cycle = ~18W average draw
• Over 10 hours = ~180Wh

On a 12V system that's only 15Ah give or take, which seems almost too good? But then I'm never sure if I'm doing the maths right or missing something obvious.

Few questions for the more experienced folks:

1. Does that duty cycle estimate sound realistic for a UK overnight (say 10-15°C ambient)?
2. Should I be worried about the freezer struggling more if the van gets warmer inside?
3. Is there anything else drawing power overnight I should factor in — the Victron BMV maybe?

Ideally want to keep the battery above 20% SOC so not draining below 160Ah usable. Feels like there's loads of headroom but don't want to wake up to defrosted food on my first proper trip out 😅

Anyone actually run something similar and got real-world numbers?

@Brummie86 depends heavily on the freezer's actual draw. Most small chest freezers pull 30-60W but they're duty-cycling — typically 30-50% on-time, so real average consumption might be 15-25W continuously.

Over 10 hours that's roughly 150-250Wh, or 12-20Ah at 12V.

Your Fogstar Drift is a 12V 200Ah, so usable capacity is around 180Ah (assuming 90% DoD). You've got substantial headroom overnight even accounting for inverter losses if you're running 230V.

Key question — is your freezer 12V compressor (e.g. Alpicool, Brass Monkey) or 230V via inverter? 12V direct is noticeably more efficient, saves maybe 10-15% versus running through an inverter.

Monitor actual consumption with a shunt-based battery monitor if you haven't already — Victron BMV-712 will show you real Ah consumed and settle the question definitively rather than guessing from

@Brummie86 mine runs a 12V compressor fridge in the garden office overnight no bother on a similar setup — the freezer's duty cycle is your mate here, it's not hammering the battery constantly like your kettle would.

@Brummie86 worth grabbing a cheap energy monitor (Shelly plug or similar) and logging actual consumption over 24hrs before committing — chest freezers vary wildly depending on ambient temp and how full they are.

My garden office setup taught me that real-world draw is often 30–40% higher than manufacturer specs once you factor in compressor startup and ambient heat.

The Fogstar Drift is solid but remember you've got roughly 160–170Ah usable if you're keeping it above 20% SoC. Run the numbers assuming worst-case duty cycle rather than average.

Also — what's your inverter situation? If you're running a 230V freezer through an inverter you're adding another efficiency loss on top. A native 12V compressor unit would be noticeably more efficient for van use.

What nobody's mentioned yet — ambient temperature matters massively. A chest freezer sat in a hot van in summer is going to cycle far more than one in a cool space. I've had similar arguments with myself over my narrowboat setup.

Also, what's your inverter efficiency? If you're running a 240V freezer through an inverter rather than a 12V compressor unit, you're losing 10-15% straight away before the freezer even does anything. That eats into your 200Ah quicker than people realise.

And what's your actual usable capacity? Fogstar Drift is decent but are you running it to 80% DoD or hammering it to 100% every night? That'll kill it faster than the freezer will.

Good shout from @BurnWalker on ambient temps — I'd add that how full the freezer is makes a real difference too. A well-packed freezer holds its temperature far more efficiently than a half-empty one. If you're light on food, filling the gaps with frozen water bottles helps massively.

Also worth checking whether your Fogstar has a low-temperature cutoff — some lithium cells don't like discharging in very cold conditions, which could catch you out if you're parked somewhere exposed in winter.

Rough ballpark: a small chest freezer might draw 30-60Ah overnight depending on everything mentioned. Your 200Ah Fogstar (usable ~180Ah) should cope, but I wouldn't rely on estimates alone. @OffGridGuru's suggestion of an energy monitor is the only way to know for certain with your specific setup.