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

Running a small chest freezer overnight on my narrowboat and trying to work out if a single 200Ah LiFePO4 is going to cut it before I commit to buying a second battery.

The freezer is a Subcold Ultra 35 — rated at about 35 litres, draws roughly 60W when the compressor kicks in. From what I can find online it cycles maybe 30-40% of the time depending on ambient temp, so real-world consumption is probably somewhere around 0.4-0.6 kWh per 24 hours.

Maths seems straightforward:

• 200Ah at 12.8V nominal = ~2.56 kWh usable (assuming I don't want to go below 20% on the Fogstar Drift)
• So roughly 2.0 kWh practical capacity
• Freezer overnight (say 8 hours) = maybe 0.2 kWh?

On paper that looks fine, but I'm second-guessing myself because:

1. The boat gets quite warm in summer — moored with the hatches shut the interior can hit 28-30°C, which I imagine hammers the duty cycle
2. I've got other loads running overnight (bilge pump, nav lights if I forget to kill them, a small fan)
3. I'm not sure how hard the compressor start surge hits the BMS

Has anyone actually run a similar setup? Particularly interested whether the Fogstar Drift 200Ah handles the compressor start without tripping. I've seen a couple of threads where people mention LFP batteries being twitchy with compressor loads but nothing definitive.

Would a 200A BMS rating be sufficient or should I be looking at

@MrBodge65 been in almost exactly this situation on my own boat.

Key thing people miss — a chest freezer doesn't run continuously. Once at temperature it'll cycle maybe 20-30% duty cycle overnight, so a 45W unit is realistically pulling closer to 10-15W average.

Over 8 hours that's roughly 80-120Wh, which on a 200Ah (≈2560Wh usable at 80% DoD) is barely a dent.

Where it gets awkward:

• Startup surge — worth checking your inverter can handle it
• Ambient temperature on the boat matters a lot in summer
• What else is running overnight (bilge pump, lighting, etc.)

Honestly a single 200Ah LiFePO4 should be fine for the freezer alone. I'd sort your total overnight load first before buying a second battery — might not need it.

My Fogstar 200Ah runs a similar-sized freezer without breaking a sweat, but I'd still check your actual resting voltage come morning — 200Ah sounds generous until your BMS disagrees at 2am.

Right so here's where I'll probably contradict myself three times —

I ran a similar setup in my cabin last winter and the ambient temperature absolutely wrecked my calculations. Freezer was working twice as hard during a cold snap because the compressor cycles differently when it's fighting itself against the surrounding air. Completely didn't see that coming.

Also worth noting: what's your discharge cutoff set to? My Victron SmartShunt absolutely saved me when I realised I'd been reading state-of-charge wrong for weeks like a complete muppet.

@RetiredEngineer's point about resting voltage is solid. But honestly — the real answer is monitor one full overnight cycle before buying anything. Stick a smart meter on it, see what you actually pull. Might be fine, might need that second Fogstar. Real data beats forum speculation every time, including mine.

The thing nobody's mentioned yet is starting surge. My shepherds hut setup taught me this the hard way — the compressor kicks in and briefly pulls two or three times the running draw. Most LiFePO4 handles this fine, but it's worth checking your BMS's peak discharge rating before you assume 200Ah is straightforwardly "200Ah worth of usable power."

On my Victron-monitored system I can actually watch those compressor cycles in the app history — genuinely eye-opening when you first see the spikes. If you've got any monitoring at all, run it for one night on shore power first and just observe the pattern. You'll know exactly what you're dealing with before you're out on the cut wondering why your BMS tripped at 2am.

Great point from @CliveKnight on the surge — definitely worth knowing your inverter's peak handling capacity if you're running AC.

One thing I'd add specifically for narrowboat use: ambient temperature matters a lot here. If your bilge or storage area runs cold overnight (which it will in winter), your LiFePO4 will derate noticeably below about 5°C, so you're not getting the full 200Ah you're paying for.

Also worth checking whether your Subcold has an ECO mode — some of the newer chest freezers cycle far less frequently once they've reached temp, which dramatically changes your overnight draw figures.

@MrBodge65 do you know your freezer's actual wattage from a kill-a-watt meter rather than the rated plate? That'll tell you far more than manufacturer specs will.