Swapped to lithium on my narrowboat – fridge finally stops tripping the inverter

Been following this thread with interest because I had almost the exact same saga, just on dry land rather than the cut.

My shepherd's hut setup was plagued by the same fridge-nuking-the-inverter nonsense when I was running AGM. That compressor startup surge is absolutely brutal — caught me off guard every single time, usually at the worst possible moment. Swapped over to a 200Ah Fogstar Drift lithium last spring and the difference was genuinely night and day. The battery doesn't flinch at the inrush current anymore, whereas the AGMs were effectively sagging on voltage the moment anything demanding kicked in.

What I've found is that lithium's flat discharge curve does most of the heavy lifting here. AGMs rolling down to 11.8V under load gives your inverter next to nothing to work with — lithium sits solid at 13V+ through most of its usable capacity, so the inverter never sees that horrible brown-out moment.

For anyone still on the fence about the switch — the fridge stability alone probably justifies it if you're running a compressor unit full-time.

One question for you narrowboaters though — how are you finding lithium with your alternator setup? That's the bit I've read is trickier on boats, where you can't just bolt on a DC-DC charger and call it done without checking your engine hours and alternator temperature. Curious whether you've gone with a Victron Orion or something else to protect the alternator from the lithium gobbling charge too aggressively.

The EV charging side of my life has made me fairly obsessive about charge rates and battery management, so I'd love to know what your BMS is doing during bulk charge on the engine.

Sounds very familiar @DaleVicky! The shepherd's hut context is interesting because you've got the same compressor fridge startup surge problem but without the boat's additional complication of inverter cable runs flexing and adding resistance over time.

One thing worth mentioning for anyone else reading – if you're still on AGM and suffering this, check your battery-to-inverter cable crimps before splashing out on lithium. I wasted months blaming the inverter when a single dodgy crimp was the real culprit, adding just enough resistance to collapse voltage under surge loads. Sorted it with a proper hydraulic crimp tool and fresh lugs.

That said, lithium genuinely does transform the experience. The flat discharge curve means you're not fighting voltage sag every time the compressor kicks in. Wish I'd done it sooner rather than chasing ghosts in my wiring!

@DaleVicky static caravan here, same battle. Compressor fridge on old AGM batteries was an absolute nightmare — inverter would grunt and give up every single time the compressor kicked in.

Swapped to a Fogstar Drift 200Ah lithium last autumn and it was genuinely like someone flipped a switch. The internal resistance is so low that startup surge basically disappears from the inverter's point of view.

The bit nobody tells you beforehand: your Victron SmartShunt readings suddenly start making sense too. With AGM you'd see voltage sag all over the place and wonder what was real. Lithium just sits there, flat and honest.

@SimonGrant is right that the compressor context matters — it's not about sustained draw, it's that brutal 3-5 second startup spike. Lithium laughs at it. AGM absolutely did not.

Hey @DaleVicky, great to hear from the dry land contingent! I had a similar revelation on my narrowboat last summer. What really helped me alongside the lithium swap was fitting a soft-starter on the compressor — cuts that startup surge dramatically. Might be worth considering if you haven't already, especially in a shepherd's hut where space and battery capacity are at more of a premium than on a longer boat. @FellKev same applies to you on the static. The lithium does most of the heavy lifting but the soft-starter felt like the final piece of the puzzle for me. Inverter's been rock solid ever since. What capacity did you end up going with @DaleVicky? I went 200Ah and it's been plenty for the fridge plus general living aboard.

@DaleVicky @FellKev interesting to see the static caravan crew represented here. Same core problem I had — AGM internal resistance climbs just enough that the compressor startup spike tips the inverter into overcurrent protection.

What solved it for me wasn't just the lithium swap (Fogstar Drift 100Ah cells in my case) but also checking the cable cross-section between battery and inverter. Undersized cabling adds its own voltage drop on top of the battery sag, and together they're lethal for compressor loads.

Once I ran proper 35mm² cable with crimped lugs rather than the original undersized stuff, and had the lithium's low internal resistance handling the spike, the Victron MultiPlus stopped complaining entirely. Worth auditing that cabling before assuming the batteries alone are the fix — saved me a lot of head-scratching.