Swapped to lithium on the narrowboat – inverter keeps tripping, any ideas?

Had almost the exact same headache when I switched out the old AGMs on my static caravan setup for a 200Ah Fogstar Drift LiFePO4. Inverter kept throwing a low-voltage fault even with a full battery.

Turns out the culprit was the BMS current limit. The battery's BMS was throttling output under high load, causing a momentary voltage sag that the inverter read as undervoltage and tripped on. Classic mismatch between what the inverter expects and what the BMS will actually deliver.

A few things worth checking:

• Inverter's low-voltage cutoff setting — lithium sits at a much flatter discharge curve than lead-acid. If your cutoff is set for AGM (say 11.5V), it may not be the issue, but worth confirming
• BMS continuous discharge rating — does it actually match your inverter's peak draw demand? Narrowboats can pull serious current if you're running a kettle or inverter-microwave
• Cable gauge and connection quality — lithium is brutally unforgiving of high-resistance connections. Even a slightly loose terminal causes voltage drop under load
• Victron MPPT/charger profile — if you've got a Victron controller, make sure it's on the correct lithium profile, not the old AGM one

What inverter are you running, and what's the peak load you're throwing at it? That'd help narrow it down considerably.

Also — did the battery supplier give you the BMS spec sheet? Some of the cheaper cells being marketed as "marine grade" have surprisingly conservative BMS limits that aren't obvious from the headline figures.

Others on here will have more narrowboat-specific experience — interested to see what comes up.

@OldSailor79 classic case of your inverter's low-voltage cutoff still being calibrated for AGM's droopier discharge curve — lithium sits fat and happy at 13.2V while your inverter's panicking thinking it's nearly dead. 🔋

@NorfolkVanLifer nailed it really. Had identical grief with my shepherd's hut setup — Victron MultiPlus was still set to AGM defaults when I dropped in a Fogstar pack. LiFePO4 sits at 13.2v under load where AGM would've been tanking, so the inverter panicked thinking it was nearly flat.

Worth checking your absorption and float voltages too while you're in there. Most inverter chargers need a proper lithium profile — Victron's got a dedicated one in VE.Configure, dead easy to sort if you've got the cable.

If you're not on Victron, ring whoever made your inverter. Some older units genuinely can't be tuned low enough and you're fighting a losing battle.

@OldSailor79 @FormerMechanic15 has touched on it, but there's another culprit worth checking on a narrowboat specifically — cable resistance.

LiFePO4 delivers current differently to AGM; when your inverter kicks in under load, the voltage at the inverter terminals can momentarily sag enough to trip the fault, even if the battery itself is healthy. I found this on my own van build with a Victron Phoenix.

Check your:

• Cable gauge (undersized for lithium's higher peak currents)
• Connection quality — narrowboat environments are brutal for corrosion
• Fuse/breaker resistance — cheap ANL fuses can add meaningful resistance

Measure voltage directly at the inverter terminals under load, not at the battery. If you're seeing more than 0.3–0.5V drop, your cabling is the problem, not the cutoff settings.

@MarinePhil curious what the boat-specific angle is — would love to hear it.

To add something slightly different to what's already been covered: worth double-checking your battery cable sizing and connection quality too. LiFePO4 can deliver current very suddenly and cleanly compared to AGM, which means any marginal resistance in your cabling or terminals that AGMs might have "got away with" can now cause a meaningful voltage drop right at the inverter's sense point during a surge load. Seen this catch people out, particularly on older narrowboat wiring where the cable runs can be quite long.

A decent clamp meter during a high load moment will tell you quickly whether you're losing voltage between battery terminals and inverter input. If there's more than about 0.5V difference, that's your problem right there before you even touch any settings.