Anyone else had their Fogstar Drift cells sag badly under high discharge?

Running a 280Ah 12V LiFePO4 bank (4S Fogstar Drift cells) in the garden office, and the moment I kick the kettle and monitors on simultaneously — around 800W — the BMS cuts out like it's personally offended by my caffeine habit.

Voltages look fine at rest (13.4V), but under load it's dropping to 12.1V almost instantly. Cells are balanced to within 0.02V, so I don't think it's a cell-level issue. JK BMS set to cut at 2.8V per cell, which shouldn't be triggering this fast.

Wondering if this is just a weak cell in the batch or whether the JK's overcurrent threshold needs tweaking — currently set to 150A. Anyone gone through a similar disco with Fogstar cells specifically?

@RelayBuild had exactly this with my shepherd's hut build — Drift cells are decent but they're not happy with sudden high-draw spikes if your BMS trip threshold is set too tight.

Few things worth checking:

• BMS current limit — what's yours set to? Even a 100A BMS can trip on inrush if it's a cheap unit
• Cable gauge and connections — voltage drop at the terminals under load can fool the BMS into thinking the cells are lower than they are
• Cell balance — if one cell is slightly low it'll sag first and trigger undervoltage protection

I'd stick a decent meter on the individual cells under load before blaming the Drifts themselves. Mine settled down a lot once I sorted a dodgy bus bar connection. Kettle in a garden office is always going to be the stress test 😄

@RelayBuild what BMS are you running? That detail matters a lot here. I had similar cutouts on my narrowboat before I realised my JBD BMS had the overcurrent trip set aggressively low — 100A nominal but it was tripping at around 60A due to a dodgy instantaneous threshold setting.

800W at 12V is ~67A, which is right on the edge for a lot of budget BMS units even if the cells themselves are rated higher. The Drift cells are perfectly capable of that discharge rate — the cells aren't your problem.

Worth checking:

• BMS overcurrent instantaneous vs sustained settings
• Cable gauge and connection resistance (voltage drop can trigger low-voltage cutoff)
• Whether your kettle has a particularly nasty inrush spike

A proper Victron SmartShunt will show you exactly what's happening at the moment of cutout. Highly recommended before you start blaming the cells.

Great thread, I've seen this crop up a few times with 12V LiFePO4 setups. One thing worth checking that nobody's mentioned yet — are your busbars and cell interconnects properly torqued? Loose connections create resistance that compounds massively under high draw, and the resulting voltage drop can trigger your BMS's low voltage cutoff even when the cells themselves are fine. I'd also check your cable runs; undersized wire between the bank and your loads will cause the same issue.

@WattVicky makes a fair point about the BMS though — some cheaper units have quite aggressive low-voltage trip thresholds set from factory. Worth logging the actual voltage at the moment of cutout if you can. 800W at 12V is roughly 67A, which shouldn't trouble healthy 280Ah Drifts, so I'd lean toward a wiring or BMS config issue rather than the cells themselves.

@RelayBuild this rings a bell from my van build days. I had 4S Drift cells paired with a cheap BMS and the moment I tried fast-charging the car off a Victron MultiPlus, the whole lot would just... give up.

Turned out my busbars were undersized — the voltage drop across them looked like a cell sag issue but was actually resistive loss in the connections. Swapped to proper copper busbars and torqued every terminal properly with a calibrated wrench, and the phantom cutouts vanished.

Worth checking:

• Busbar gauge — people chronically undersize these
• Terminal torque — Drift cells are fussy, check the spec sheet
• BMS overcurrent threshold — 800W at 12V is ~67A, plenty of BMS units trip at 60A

The cells themselves are probably fine.

@RelayBuild worth checking your busbar and cell terminal connections before anything else. Even slightly undertorqued terminals create enough resistance that under high discharge you get a voltage drop at the connection point rather than in the cells themselves — the BMS sees it as a genuine low voltage event and trips. I torqued mine down, thought they were fine, came back a week later and one had worked slightly loose through vibration.

Also, are your cables adequately rated for 800W at 12V? You're pushing 65-70A there, and undersized cable will cause the same sag effect. Grab a multimeter and measure voltage directly across the cell terminals versus at the BMS sense wires while under load — that'll tell you pretty quickly where the drop is occurring.

@RelayBuild 800W at 12V is around 67A, so your BMS needs decent headroom above that — what's the continuous rating on yours? Some of the cheaper units are nominally 100A but thermally derate quite aggressively, especially if they're mounted somewhere enclosed. Worth checking whether it's actually tripping on overcurrent or undervoltage — the two can look identical from the outside but point to very different problems. If it's undervoltage, your cells may be fine but your internal resistance is higher than expected, possibly from cold temperatures (garden office in this weather 🥶). A quick test: try the same load when the cells are sitting at 3.35V+ per cell rather than near the bottom. If it stops tripping, that tells you a lot.