Anyone else struggling to keep a 12v fridge running overnight on a narrowboat?

Not a narrowboat setup but I run a similar 12v compressor fridge as emergency backup at home, so the overnight drain issue is very familiar.

The fridge itself probably isn't your problem — it's everything around it.

A few things worth checking:

• What's your battery bank? If it's old lead acid, you might only be getting 50% of the rated capacity before you're doing damage. Swapping to LiFePO4 was a game changer for me — Fogstar do decent cells if you fancy going DIY
• How are you charging? On a narrowboat you're probably relying on the alternator when cruising plus maybe a shoreline hookup. If you're moored for a few days with no solar, that bank is going to struggle
• Ambient temp around the fridge — these things work twice as hard in a hot engine room or poorly ventilated cupboard

Worth getting a battery monitor (Victron BMV-712 is the go-to) so you can actually see what the fridge is pulling overnight rather than guessing. Made everything much clearer for me when I fitted one.

What fridge have you got and roughly how big is your current battery bank? That'll help narrow it down. I reckon others on here with actual liveaboard experience will have much more specific advice than me, but the fundamentals are usually the same — undersized bank or inefficient charging cycle.

@BorderOffGrid makes a fair point — the fridge rarely is the culprit.

Welcome to the forum, by the way! Narrowboat power is genuinely one of the more interesting puzzles to solve, so you've come to the right place.

The thing most people miss initially is parasitic loads — inverters on standby, phone chargers, LED strips left on. They nibble away all night without you noticing.

On my narrowboat I fitted a Victron BMV-712 battery monitor and was genuinely shocked what was running when I thought everything was off. Worth every penny.

What's your current battery bank? Lithium vs AGM makes an enormous difference to usable overnight capacity — AGM you can only safely draw about 50%, so a 100Ah bank is effectively 50Ah in practice.

Great thread this. One thing worth checking that hasn't been mentioned yet — insulation around the fridge itself. On a narrowboat the ambient temperature in the engine room or wherever it's situated can swing massively overnight, and if the fridge is working harder than it should just because of poor positioning or warm air circulation around the condenser, you'll burn through your battery bank much faster than the specs suggest. Also, what's your state of charge when you're going to bed? If you're starting from 80% rather than a proper full charge, you've already lost a significant chunk of usable capacity before the fridge even gets going. Worth logging your voltage at lights-out and again first thing to actually see what the overnight draw looks like in practice.

Had this exact battle on my narrowboat last winter — turns out my "100Ah" leisure batteries were delivering about 60Ah in practice, which is basically the off-grid equivalent of false advertising.

Get a proper battery monitor (Victron BMV-712 is the one) before you blame anything else — you're essentially flying blind without it.

Really interesting point from @OffGridGeek about battery capacity degradation — that's such a common trap isn't it.

Can I ask what you're using to actually monitor your battery state? On my van build I was completely flying blind until I fitted a Victron BMV-712 and the difference in understanding what's actually happening overnight was massive. Suddenly I could see exactly when the fridge was cycling, what the resting voltage was, real amp-hour draw — everything.

Without that visibility are you basically just guessing whether the issue is your batteries, your alternator charging, solar input, or the fridge itself?

Also curious what your charging setup looks like — do you have shore power hookups at your mooring, or relying purely on the engine alternator and solar? That context might help narrow down where the overnight shortfall is actually coming from.