Short circuit protection on 1000A NG BMS - what current level?

Been mulling this over for a while and thought I'd get some other opinions.

Currently planning a 24v LiFePO4 build — probably around 560Ah with Fogstar Drift cells — to replace an ageing FLA bank. The main load that's giving me grief is a high-draw DC motor (angle grinder on a makeshift workshop setup, long story). Peak draw can hit somewhere in the 300-400A range for a few seconds on startup.

The question I keep coming back to: what's the actual short circuit trip threshold on the higher-rated BMS units? I'm looking at some of the 1000A-rated NG-style units but the specs sheets are... vague. "Short circuit protection" is listed but no actual current figure. Is it 2x rated? 3x? Milliseconds or microseconds?

My concern is:

• Motor inrush doesn't trip the BMS unnecessarily
• But genuine fault conditions do get caught quickly enough to protect the cells

Had a brief chat with someone running a similar setup who said their BMS tripped constantly on motor start until they added a capacitor bank across the terminals. Curious if anyone here has gone down that route or found a BMS that handles it more gracefully.

Also wondering if a Victron Smart Shunt in the mix gives any useful data here — can it log fast transient events or is it just too slow for this kind of thing?

Anyone running high-surge loads off LiFePO4 with a quality BMS that handles it well? JK BMS? Daly? Something else?

@LiFePO4Fan ooh this is one I've been wrestling with too on my narrowboat build! Quick question though — are you planning to run any big DC loads directly, or going inverter-first for everything?

The reason I ask is that on my setup I was initially torn between a 1000A NG BMS and dropping to a 500A unit, and the answer kind of depends on your worst-case load scenario. A decent inverter has its own overcurrent protection, so the BMS short-circuit threshold matters less on that side.

Also worth checking — does your specific Fogstar Drift cell spec sheet list the maximum continuous discharge rate? Because if the BMS short-circuit trip point is set too conservatively relative to genuine surge loads (motor startups, compressor fridges etc.) you'll get phantom trips. Proper nightmare on a boat at 2am, trust me! 😅

What inverter/charger are you looking at?

From bitter experience, my cabin's BMS tripped on a welder I forgot I'd plugged into the inverter — so whatever threshold you pick, your actual load spikes will make that decision for you eventually 😅

@MrBodge73 that's a classic. Done similar myself.

For the actual numbers — on a 24v system with Fogstar cells, your SCP threshold wants to be above your realistic peak load but below the cell's absolute maximum pulse rating. Most NG BMS units let you set somewhere between 1500-2000A trip point on the 1000A model, with a short delay (usually microseconds to a few ms).

The delay matters more than people realise. Too short and you'll get nuisance trips on inrush from inverter startup. Too long and you're not actually protecting anything useful.

Worth checking what your inverter's actual inrush looks like — my Victron Multiplus pulls a nasty spike at startup that caught me out initially.

Also don't forget your fuse on the positive cable still wants to be rated lower than the BMS trip, otherwise the BMS becomes your fuse. Which is not what it's designed for.

@MrBodge73 welder ghost haunting your BMS is genuinely peak off-grid experience.

My shepherd's hut setup runs Fogstar cells with a Daly BMS and I landed on 2x the max continuous discharge rate as a rough SCP threshold starting point — so if your inverter peaks at 150A, you're not wanting the BMS nuking itself at 160A.

Key thing people miss: SCP delay matters as much as the current level. Victron inverters throw a nasty startup spike — too short a delay and your BMS trips every time something large fires up. I run 200–300µs on mine, which lets the inverter stabilise without ignoring genuine faults.

The NG BMS specifically has decent configurability here — worth checking the cell manufacturer's absolute max discharge rating as your hard ceiling though.