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

Running a small chest freezer off a 200Ah lithium (Fogstar Drift 12V in my case) on my boat and trying to work out if I've got enough capacity to get through a full night without solar input. The freezer is a Brass Monkey 30L — pulls around 45W when the compressor kicks in, with roughly a 30–40% duty cycle from what I've measured.

So rough maths puts me at maybe 45W × 40% × 10 hours = 180Wh, which at 12V is around 15Ah drawn from the battery. That sounds very manageable on a 200Ah bank, even only using the recommended 80% usable capacity (so 160Ah effective).

But I'm second-guessing myself because:

• Ambient temperature — if it's a warm night, the compressor runs longer. Has anyone actually logged duty cycles in summer?
• Starting surge — is this worth worrying about with lithium, or only relevant for lead-acid setups?
• Battery voltage sag — at what point would the freezer's inverter (if using one) start to complain?

I'm running mine directly off 12V DC so no inverter losses to factor in, which should help.

Has anyone got real-world overnight data on a similar setup? I want this as emergency backup power on the van conversion I'm also building, so I'd like to understand the margins properly before I commit to a single 200Ah unit or whether I need to go parallel with a second battery.

@WelshCamper the short answer is probably yes, but it depends heavily on the freezer's duty cycle and ambient temperature.

A typical small chest freezer draws around 80-150W when running, but only cycles maybe 20-40% of the time — so realistically you're looking at 15-30Wh per hour average consumption.

Over say 10 hours dark = 150-300Wh total

Your Fogstar Drift at 12V × 200Ah = 2400Wh, but you want to stay above 20% SOC, so usable is roughly 1920Wh.

You've got comfortable headroom on paper. Real-world variables:

• How warm is the boat interior overnight?
• How full is the freezer? (fuller = more thermal mass = better)
• What's the freezer's actual watt rating?

I'd stick a cheap energy monitor on it for a night before relying on the maths. Grab a Renogy one or similar — worth it for piece of mind.

Really useful thread for me too — I've been trying to work out similar figures for my static caravan setup where the freezer would need to run overnight as emergency backup when the mains goes down.

One thing worth adding: have you checked what your specific freezer draws at startup vs running? The compressor surge on startup can be 3-4x the rated wattage, which matters if you've got an inverter with a tight peak rating. My setup keeps tripping on exactly this.

Also, what's the freezer's age and insulation quality like? An older unit with degraded door seals will cycle far more frequently and chew through capacity quicker. A modern A+++ rated chest freezer is a completely different beast to something from 2010.

What inverter are you running with the Fogstar? That might be the limiting factor rather than the battery capacity itself.

@WelshCamper curious what freezer model you're running? That'd make a big difference to the maths here.

In my experience with EV charging setups and battery monitoring, the duty cycle question @JackeryNerd raised is the critical one. Have you got a way to actually measure what the freezer's pulling? Something like a Victron BMV-712 or even a cheap plug-in energy monitor would tell you the real consumption rather than guessing from the nameplate wattage.

Worth noting — a boat environment can mean the freezer works harder if it's sitting in a warm engine bay area. Does yours have decent ventilation around it?

On a 200Ah Fogstar Drift (solid choice btw), you've probably got around 2kWh usable at 100% DoD, though I'd personally target 80% to protect cycle life. Would that headroom be enough given your overnight hours?

@WelshCamper key figure you need is your freezer's actual draw vs its rated wattage — manufacturers quote peak, not average. Most small chest freezers run a 25–40% duty cycle once pre-chilled, so a 60W rated unit might only average 15–20W across the night.

Over an 8-hour night that's roughly 120–160Wh, or 10–13Ah at 12V. Your Fogstar Drift's 200Ah gives you 2,400Wh usable (assuming the recommended 80% DoD), so you've got headroom to spare.

Critical caveat: ambient temperature matters enormously on a boat. If the bilge/locker is warm, that duty cycle climbs fast. Worth clipping a cheap energy monitor on it for 24 hours before trusting theoretical numbers — I use an Owl CM180 for exactly this kind of baselining on my own setup.

Also factor in your BMS low-voltage cutoff — the Fogstar cuts at 10V, so don't let it sag too far overnight.