Anyone else struggling to keep the compressor fridge running overnight on a 100Ah lithium?

Ran a Fogstar 100Ah on my narrowboat for two summers trying to power a 12V compressor fridge overnight — spoiler: the fridge won. 🏳️

The maths never lies: a halfway decent compressor fridge pulls 3-5Ah average overnight, so you're burning 30-50Ah before dawn even considers showing up. Chuck in a diesel heater controller, a phone on charge, and whatever else you've "definitely turned off," and you're kissing 60-70Ah goodbye. On a 100Ah lithium you really only want to dip to 20% SoC to keep it happy long-term, so you've got 80Ah usable — which sounds fine until reality laughs in your face.

A few things that actually helped me:

• Pre-chill the fridge before disconnecting from hookup/shore power
• Set fridge temp to 4-5°C, not 2°C (compressor cycles far less)
• Insulate the fridge cabinet if it's in a hot spot — a warm ambient temp murders your runtime
• Seriously consider a Victron SmartShunt so you know what's actually happening rather than guessing

Honestly though, 100Ah lithium + compressor fridge overnight with no solar input is just... optimistic. Two panels and a decent MPPT and the whole problem mostly dissolves by 10am.

What size fridge are we talking, and have you got any solar in the mix? Might be a simpler fix than you think.

Hey @WonkyHermit — narrowboat life, I feel that! The maths is brutal isn't it. One thing worth considering beyond raw capacity is your usable depth of discharge. Even with lithium you're typically working with 80-90Ah realistically once you factor in the BMS cutoff under load. A compressor fridge cycling through a warm night can easily chew 40-50Ah, and that's before you've run any lighting or charging. I'd honestly look at improving fridge insulation first — a bit of Kingspan around the cabinet made a noticeable difference on mine — before throwing more battery at the problem. Sometimes the cheapest upgrade isn't electrical. 🙂

@WonkyHermit my tiny house fridge has a "sleep mode" — it's called unplugging it when I'm skint 🔌

Jokes aside, the real villain nobody mentions is ambient temperature — a fridge working harder in a hot narrowboat engine bay is basically a financial sinkhole. Shading the unit and insulating the cabinet properly can slash duty cycle dramatically before you even think about upgrading to a second Fogstar. Sorted mine by boxing in the condenser coils with a duct to outside air — compressor barely breaks a sweat now. Sometimes the fix isn't more batteries, it's less heat.

@WonkyHermit the compressor fridge overnight problem is one I've bashed my head against repeatedly with my motorhome setup. The dirty secret nobody mentions is duty cycle variability — that fridge isn't drawing its rated current constantly, but in a warm cab in summer it might be running 60-70% of the time rather than the 30-40% the specs imply. That completely blows your overnight capacity calculations.

What actually saved me was fitting a Victron SmartShunt so I could log actual consumption over several nights rather than guessing. Turned out my "40Ah overnight" estimate was closer to 65Ah once ambient temps crept up. 100Ah lithium with your typical 80% usable capacity leaves almost nothing for anything else.

Second battery in parallel was ultimately the only realistic fix — Fogstar do decent deals on a second 100Ah if budget allows.

@WonkyHermit narrowboat here too — been down this exact rabbit hole.

The thing nobody mentions is ambient temperature doing half your work for you. Midsummer, my compressor fridge was cycling constantly because the engine bay heat was radiating into the galley. Moved it six inches, added a 12V fan drawing almost nothing across the condenser coils, and overnight consumption dropped noticeably.

Still upgraded to a 200Ah Fogstar Drift eventually — the 100Ah was always going to be a losing battle without serious solar topping it up daily. On a boat you're often moored under trees or bridges at the worst moment.

Also worth checking your fridge's low-voltage cutoff setting if it has one. Mine was cutting the compressor in and out around 11.8V which absolutely hammers efficiency. Raised it, counterintuitively, and things settled right down.

@WonkyHermit this is exactly what I'm wrestling with for my van build right now.

Quick question for the thread — does the duty cycle change significantly based on how full the fridge is? I've read conflicting things. Instinct says a fuller fridge has more thermal mass and holds temp better, but I've also seen people claim the compressor works harder initially to cool everything down.

@ExChippie72 good shout on ambient temp — my garden office setup taught me that lesson the hard way last summer. A unit that's fine at 15°C overnight becomes a complete battery killer once it's sitting in 28°C.

Has anyone actually measured real-world draw with something like a Victron BMV-712 over a full night? Curious what numbers people are actually seeing versus the manufacturer specs, which feel optimistic at best.