Anyone else had their BMS trip mid-cruise on a narrowboat?

Happened to me last Tuesday somewhere between Braunston and Napton — glorious sunny morning, kettle on, inverter humming away, and then: silence. The Fogstar 200Ah 12V LiFePO4 just... gave up. BMS tripped out, inverter went dark, and I was suddenly very aware of how much I rely on 12V for, well, everything including the tiller pilot.

Turned out the cell balance had drifted badly — one cell was sitting at 2.9V while the others were happily above 3.2V. The JBD BMS had done exactly what it should, but it still caught me completely off guard. What I don't understand is how it drifted that far without me noticing on my Victron BMV-712. The state-of-charge looked fine right up until it wasn't.

So the question is: has anyone found a reliable way to catch balance drift before the BMS pulls the plug? I'm wondering if the Victron Cerbo GX with proper cell-level monitoring would've flagged this earlier, or whether I need to be doing manual top-balance top-ups more regularly. Currently running 240W of Renogy panels on the roof — not sure if irregular charging from the alternator while cruising is making this worse.

TracyGraham64 | 847 posts | ⚡ Solar & LiFePO4 Evangelist

@DucatoDream oh that's a horrible feeling isn't it — absolute silence when you're expecting the kettle to whistle!

Had almost identical happen on our boat last summer. Turned out the BMS was tripping on a voltage spike from the inverter startup, not actually low state of charge. The kettle drawing 1500W+ can cause a momentary voltage sag that the BMS interprets as an undervoltage fault, especially if your cells aren't perfectly balanced.

Few questions: what's your inverter rated at, and had the battery been sitting overnight without solar input? If the cells were slightly imbalanced going into the morning, that bottom cell could've dipped just enough to trigger the trip under load.

Try a full balance charge before your next cruise and see if it recurs. Also worth checking your BMS logs if the Fogstar app exposes them.

DougPearce | 1,203 posts | ⚓ Liveaboard | LiFePO4 Obsessive

@DucatoDream classic over-discharge trip, I'd wager. On my boat I log every BMS event and the pattern is almost always the same — kettle (1.2–1.5kW surge) on top of an already-depleted pack first thing in the morning hits the low-voltage cutoff hard.

Few things worth checking:

• Cell voltage imbalance — one weak cell drags the whole pack under the BMS threshold even when aggregate SOC looks acceptable
• Cable sizing — undersized run between battery and inverter causes voltage sag that looks like a low-cell event to the BMS
• Victron BMV-712 — if you're not already logging this data you're essentially flying blind

Fogstar packs are generally solid but the stock BMS low-voltage threshold can be quite aggressive. Worth connecting via their app post-reset and pulling the fault log before you do anything else.

OakSoul | 134 posts | 🏡 Tiny House | Off-Grid Curious

@DucatoDream did you check whether the kettle and inverter running simultaneously pushed you over the discharge current limit rather than the voltage floor? I ask because I had something similar with my Fogstar setup — not on a boat obviously, but in my tiny house — and it turned out the BMS was tripping on a current spike rather than actually being depleted. What's your inverter rated at, and did the Fogstar app (if you're using the Bluetooth version) show the state of charge just before it cut out? Might help narrow it down before assuming it's a depth-of-discharge issue like @DougPearce suggests.

LuckySocket | 412 posts | 🚤 Narrowboat | 12V LiFePO4 Convert

@DucatoDream gutting when that happens, especially on such a nice morning! One thing worth checking beyond what @DougPearce and @OakSoul have mentioned — was the battery actually fully charged the evening before, or did you set off with it already a bit depleted? Kettles on inverters are brutal; my 2kW kettle pulls close to 170A at 12V for a couple of minutes. If the BMS saw a sudden voltage sag under that load on an already tired battery, it may have tripped on undervoltage rather than true over-discharge. Worth pulling the BMS logs if your Fogstar supports that via Bluetooth. Also worth checking your cable connections — a loose terminal creates resistance, causes extra sag, and can fool the BMS into tripping unnecessarily. Had exactly that happen on my Springer last summer near Fenny Compton, coincidentally not far from you!

OffGridDoug | 847 posts | 🚐 Motorhome | Victron Devotee

Worth pulling the BMS data log if your Fogstar unit supports it — the 12V 200Ah does expose some diagnostics via the app. What you're looking for is whether it was a low-voltage cutoff or a overcurrent event. A 2kW+ kettle through a 12V inverter is pulling 180A+ at the battery terminals — that's right at the edge of what that BMS is rated for, especially if your cable runs have any meaningful resistance dropping voltage further. I had something similar in my Ducato build before I uprated my busbar connections. One dodgy crimp and your apparent voltage at the BMS sensor reads lower than actual SoC, triggering a premature trip. Check your terminal connections with a proper clamp meter under load before assuming the BMS itself is faulty.

Simon1995 | 203 posts | 🚤 Narrowboat | LiFePO4 Enthusiast

Had almost the exact same thing happen on my boat near Fenny Compton last summer. Worth checking your cell voltage balance before anything else — my BMS tripped because one cell had drifted out of balance and hit the low voltage cutoff while the others were still fine. The pack overall looked okay but that one rogue cell dragged everything down.

Did a top-balance on all cells individually using a bench power supply, and it's been solid ever since. Also worth checking your busbar connections — a loose connection can cause a voltage drop that fools the BMS into thinking a cell is lower than it actually is. @OffGridDoug is right about the data log too, that'll tell you straight away whether it was a voltage, temperature, or overcurrent trip.