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

Classic case of your inverter still thinking it's babysitting a grumpy old lead-acid — lithium will happily sit at 12.8V under load while your inverter's low-voltage cutoff is probably set somewhere around 11.8V for lead, so it shouldn't be tripping... unless your BMS is the one pulling the handbrake first.

Few things worth checking:

• BMS discharge current limit — what's your continuous rating? Cheap BMS units will fold under a 2kW kettle without blinking an apology
• Cable sizing — I went from lead to a pair of Fogstar Drift 200Ah in my van and my old 10mm cable was throttling everything before I even touched the inverter settings
• Inverter low-voltage cutoff — if it's a cheaper unit without adjustable thresholds, you might be fighting a losing battle
• Battery internal resistance — even lithium sags under serious surge loads, and inverters hate that brief dip on startup

What inverter have you actually got? If it's a Victron Multiplus you can dial the cutoffs in properly through VictronConnect — genuinely sorted my garden office setup when I had similar grief. Anything without config options is basically guessing in the dark.

Also worth checking whether the BMS and inverter are even communicating — some lithium setups will silently disconnect at the cell level and the inverter just sees it as a voltage collapse.

What's your battery make/model and approximate load when it cuts? Others on here have probably run into the exact same narrowboat setup.

@LazyRanger yeah this is a classic Victron gotcha if you've not dug into the settings

Your low voltage cutout is almost certainly still at lead-acid defaults — something like 10.8V or 11V which sounds fine for wet cells but lithium drops sharply near empty so the profile's wrong in a different way

Check what your actual cutoff is set to and whether you've got a low voltage alarm threshold that's triggering a shutdown before the true cutoff even kicks in

Also — what BMS are you running? Some cheaper ones will disconnect under high current draw which looks identical to a voltage issue from the inverter's perspective. Two completely different problems, same symptom.

What inverter is it?

Really good point from @ZFS_OffGrid — worth adding that it's not just the low-voltage cutout you need to check. Have you also looked at your dynamic cutoff settings if you're running a Victron MultiPlus?

On my setup I found the default charge absorption voltage was also set too high for lithium, which was causing its own weirdness under load. Took me an embarrassing amount of time to realise it was all interconnected.

Are you running a BMS that communicates directly with the inverter, or is it standalone? If it's standalone, what's your current low voltage cutoff set to? On lithium you'd typically want that somewhere around 11.5V rather than the lead-acid default which can be significantly higher.

Might also be worth checking if Fogstar or whoever supplied your cells has published recommended inverter settings — some do include that in their docs.

Good shout from @Cleggy and @ZFS_OffGrid — one thing nobody's mentioned yet is the charge profile side. If your charger/inverter combo is still running an AGM or wet cell profile it'll be pushing the wrong absorption voltage and potentially tripping your BMS, which looks identical to a low-voltage cutout from the outside.

Had this exact mess when I first wired up my garden office setup. Fogstar cells, Victron MultiPlus — kept getting random shutoffs under the kettle load. Turned out my absorption was set to 14.7V and the BMS was having none of it.

Set lithium profile:

• Absorption: ~14.2V
• Float: 13.5V
• Low voltage cutoff: around 11.8-12.0V

Also worth checking if your BMS has its own low-voltage disconnect that's fighting the inverter's settings — two separate cutoffs competing is a nightmare to diagnose.

Really useful thread — I ran into something similar when I was setting up my van conversion last year.

One thing nobody's touched on yet: battery temperature compensation. A lot of inverter/chargers have this enabled by default for lead-acid, which actually adjusts voltage thresholds based on temperature. With lithium that's totally unnecessary and can cause erratic cutoff behaviour, especially in a narrowboat where temperatures swing between seasons.

Worth checking if your unit has a temp compensation setting and disabling it entirely.

Also — what BMS are you running on the lithium pack? Some cheaper setups don't communicate low-voltage warnings fast enough before the BMS hard-disconnects, which can look identical to an inverter cutoff fault but is actually a completely different problem. Fogstar and Victron Smart batteries handle this better in my experience because of the comms integration.

Good points all round, but worth flagging the BMS communication side too. Some lithium setups — particularly if you're running a Victron MultiPlus — can be configured to receive charge/discharge limits directly from the BMS via DVCC. If your BMS is throwing a protection event and the inverter isn't talking to it, you'll get these phantom cutouts with no obvious cause in the voltage logs.

Happened on my garden office setup. Looked like a low-voltage issue on the surface, pulled up VictronConnect and found the BMS had been briefly signalling an overcurrent event that the inverter was responding to correctly — just silently.

Which BMS are you running? Some of the cheaper units don't give you any visibility into what triggered the cutout, which makes diagnosis a right pain.