Narrowboat 400Ah lithium build — is my BMS actually doing anything useful or just sat there looking pretty?

So I've finally pulled the trigger on switching the boat over from lead-acid to lithium after about three years of dithering. Went with four 100Ah Fogstar Drift cells wired in 12V, protected by a Daly 100A smart BMS. Victron SmartSolar 100/30 handling the solar side, and a Victron Multiplus 12/1600/70 for the inverter/charger. On paper it all looks lovely. In practice I'm not entirely sure the BMS is actually intervening when it should be, or whether I've just been lucky so far.

Here's what's bugging me — I pushed the battery down to about 18% SOC last Tuesday (grey week, not much solar, been running the diesel boiler fan and some LED strips more than I should), and the BMS didn't cut anything off. Cells were sitting around 3.21V each according to the Daly app, so not catastrophically low, but I expected something to happen before that point. I've got the low-voltage disconnect set to 2.8V per cell which maybe is the problem — should that be higher, like 3.0V?

Also wondering whether I should be letting the Victron DVCC handle more of the protection logic instead of relying on the Daly. From what I've read on here the two can occasionally argue with each other rather than cooperate, especially around absorption. Anyone running a similar setup on a narrowboat or liveaboard situation who's actually stress-tested this properly?

@Boycie the Daly isn't bad per se, but it's essentially a dumb protector rather than an active manager. It'll cut power at hard limits but offers zero communication with your charger or inverter.

The real weakness on a narrowboat is alternator charging — Daly has no way to signal your alternator regulator to back off gracefully. You risk nuisance disconnects under load which can spike your alternator diode.

If you're running Victron kit elsewhere, a Victron SmartShunt + Cerbo GX at minimum gives you proper state-of-charge visibility. Better still, upgrade to a JK BMS — active balancing, Bluetooth monitoring, and you can integrate it properly into a DVCC setup.

What's your charging sources? Shore power, alternator, solar? That changes the advice considerably. Fogstar Drifts are solid cells, worth protecting properly.

Great shout going with the Fogstar Drift cells, solid choice for a narrowboat build. To add to what @BarryWood said — the real test of whether your Daly is earning its keep is checking whether your charging sources actually respect its disconnect signals. On boats especially, alternators can be a nasty gotcha here; if the Daly cuts on a high-voltage event while the alternator's running, you can fry the diodes without a proper alternator protection relay in the circuit. Worth double-checking that setup. Also, have you got any active cell balancing happening, or relying purely on the Daly's passive balancing? At 400Ah you'll likely notice drift between cells over time, particularly if you're regularly hitting the top end of charge. What's your primary charging source — solar, alternator, shoreline, or a mix?

@Boycie the piece nobody mentions is that on a narrowboat you've got two extra failure modes a static install doesn't: vibration loosening cell interconnects over time, and bilge humidity getting into the BMS connectors.

Worth torquing those busbars down every season and putting a dab of dielectric grease on the BMS signal wires. Had a phantom low-cell reading on my shepherd's hut build that turned out to be nothing more than a corroded balance lead — Daly threw a fault, Victron MPPT went into protection mode, and I lost power for a day chasing a ghost.

Also — are you running the Daly's load port or using it in "common port" mode? Makes a difference to how aggressively it pulls the plug under load spikes from things like your inverter or bow thruster.

@Boycie the narrowboat environment really is its own beast for a BMS. Had a similar situation at my cabin — thought the BMS was doing its job until I started actually watching cell voltages under load. The spread between my weakest and strongest cell was shocking.

What saved me was adding a Victron SmartShunt alongside. The Daly has no idea what your SOC actually is — it just reacts to voltage thresholds. The SmartShunt gives you proper coulomb counting, trends over time, and crucially you can spot a drifting cell weeks before the BMS ever triggers.

Think of the Daly as your last line of defence, not your active management layer. On a liveaboard situation where a dead bank means a cold night, you really want to know something's wrong long before the protection fires.

@Boycie I did a similar swap in my van build and the lightbulb moment for me was realising the Daly is essentially the last line of defence, not the first.

Think of it like a circuit breaker in your house — it's there for genuine emergencies, not routine management. You want your Victron MPPT and charger talking to each other before the BMS ever needs to intervene.

What transformed my setup was adding a Cerbo GX so everything could actually communicate properly. The Daly just sat there quietly doing nothing, which is exactly what you want from it.

On a narrowboat specifically, I'd also look at setting conservative charge limits — 90% rather than full — through your charger settings rather than relying on the BMS ceiling. Extends cell longevity considerably and the Fogstar Drifts respond well to that approach.