Swapped to lithium on my narrowboat – inverter keeps cutting out under load

Been there with this exact headache on my own boat. Nine times out of ten it's the BMS doing its job — lithium doesn't sag like lead acid, so it cuts hard when it hits a current limit rather than just drooping voltage gracefully.

Few things worth checking:

• What BMS are you running? Cheap drop-in cells (looking at you, random eBay packs) often have a BMS rated lower than the battery's advertised discharge rate
• Cable sizing — people massively underestimate this on narrowboats. Short runs feel fine until you're pulling 2kW through undersized cable and voltage drop does the rest
• Inverter low-voltage cutoff setting — if it's still calibrated for lead acid it might be tripping too early even though the lithium pack is actually fine

I switched to Fogstar Drift cells on my 60-footer last year and had similar grief until I realised my Victron MultiPlus was still on the default lead acid profile. Five minutes in VE.Configure and it sorted itself out mostly.

What inverter are you on and what load's triggering it? Kettle? Motor? Makes a difference diagnosing this properly.

Would be good to know if others here have seen BMS comms helping — I've been going back and forth on whether the Victron/BMS CAN bus integration is actually worth the faff on a boat install or just another thing to go wrong.

@MarineGaz makes a fair point about the BMS, but worth being more specific — are you seeing the inverter fault out, or is it the BMS actually disconnecting?

On my static van setup I had a similar issue when I first switched to Fogstar Drift cells. Turned out my Victron MultiPlus was configured with lead-acid charge curves still, so it was sending wonky signals to the BMS and causing nuisance trips under load.

Check your inverter's low-voltage cutoff settings too. A lot of people set these for lead acid (say, 11.5V) and then wonder why lithium behaves strangely — the voltage window is completely different.

Also, what gauge cabling are you running to the inverter? Narrowboats often have undersized runs from previous owners. Voltage drop under load can trigger protection even when the battery itself is perfectly fine.

@DODGuy's diagnostic split is the right call — though in my garden office setup I found the culprit was actually undersized cable between the Fogstar battery and the Victron inverter creating enough voltage drop to spook the BMS into thinking it was hitting its limits when it wasn't.

@NorfolkVanLifer raises something I've seen bite people repeatedly — cable sizing gets done on peak voltage assumptions from lead-acid days, then lithium's flatter discharge curve means you're pulling more amps mid-cycle than you ever did before at equivalent load.

On my own boat I had a Victron MultiPlus cutting out under the washing machine motor start surge. Turned out my 70mm² cable run was just about acceptable on paper but had three dodgy compression joints adding meaningful resistance. Voltage drop across those joints was enough to tip the BMS over its threshold.

Check your connections before you buy anything new. Torque spec those terminals properly — marine vibration works lugs loose over months. A decent clamp meter across each joint under load will show you immediately where your resistance is hiding.

What BMS are you actually running? That changes everything about threshold behaviour.

@SomersetNomad nailed the cable point. I'd add — check your fuse or breaker rating too. Swapped a cabin setup from AGM to Fogstar lithium last year and the existing 100A ANL fuse was marginal for the inverter surge. Lithium delivers current so cleanly the fuse actually sees the full inrush now, whereas the old lead acid would sag and "hide" it. Ended up upsizing to 150A and the cutouts stopped immediately. Worth checking what your BMS peak discharge rating actually is as well — spec sheet vs reality can differ on cheaper cells.

@WonkyWelder good shout on the breaker — and here's the thing nobody mentions until it's too late: your BMS peak current rating and your inverter's startup surge are having a proper argument, and the BMS always wins.

My Fogstar Drift 100Ah has a 100A continuous limit that looks fine on paper, until you fire up a kettle and the inverter fan kicks in simultaneously. BMS goes "nope" and hard-cuts faster than you can say "why is the telly off."

Worth grabbing a clamp meter and actually watching your surge current during startup rather than guessing. Victron's Bluetooth shunt is brilliant for this — catches those millisecond spikes in the history graph.

Sometimes the fix is simply staggering your loads rather than rewiring the whole boat. Start the inverter first, then switch on the appliance.