Anyone else had grief with their BMS cutting out under heavy load? Trying to sort my 200Ah LiFePO4 setup

I've been running a 200Ah LiFePO4 bank (4 x 50Ah Eve cells in parallel) in my Transit for about three months now, managed by a cheap Daly 100A BMS I picked up off AliExpress. Everything's been fine for general use — lights, phone charging, the odd bit of laptop work — but the moment I fire up my 2000W inverter to run a kettle or induction hob, the BMS trips and disconnects the whole bank. Measured the actual draw with a clamp meter and it's sitting around 170A at peak, so technically within the 200A limit I thought I had headroom for.

Had a look at the Daly's settings via the Bluetooth app and the overcurrent protection is set to 200A with a 200ms delay, so it shouldn't be triggering at 170A. Wondering if the cell-level connections are adding enough resistance to cause a voltage sag that's fooling the low-voltage cutoff instead — resting voltage is a solid 3.32V per cell but I haven't checked under that kind of load yet.

Has anyone been through something similar? Thinking of either upgrading to a JK BMS (seen a lot of love for those on here) or wiring in a separate pre-charge circuit to deal with the inverter's capacitor inrush. Not sure if I'm barking up the wrong tree though — would love to know what others have found before I start throwing money at it.

@DH_VanLife the Daly 100A is almost certainly your culprit — that "100A" rating is optimistic at best. Under real load (inverter spikes, compressor fridge startup surge etc.) you're likely hitting the actual limit pretty hard.

I ran a similar Daly on my 280Ah bank and had constant nuisance trips. Swapped it out for a JK BMS and haven't had a single cutout since. The active balancing is a bonus too.

Few things worth checking first though:

• Are your cell interconnects properly torqued? Resistance there causes voltage sag that can trick the BMS
• What's your actual peak load drawing?

If the JK feels pricey, the Daly Smart (Bluetooth version) at least lets you see what's actually triggering the protection — worth knowing before you throw money at it.

Squib19 | 47 posts

@DH_VanLife worth checking whether your four cells are properly balanced before assuming it's purely a BMS capacity issue. Even with parallel cells, if one is slightly weaker it'll drag the others down under heavy load and trigger the overcurrent protection prematurely. Stick a multimeter on each cell individually when you're seeing the cutout — ideally under load if you can manage it safely.

Also, what's your actual peak draw? Inverters especially can spike well above their rated wattage on startup. A 1000W inverter can momentarily pull 150A+ during inrush, which'll hammer that Daly every time regardless of your steady-state current being fine.

@HazelPaddy makes a fair point about the rating being optimistic, but sorting the cell balancing first costs nothing and might surprise you.

Paul1999 | 203 posts

@DH_VanLife one thing nobody's mentioned yet — have you checked the actual wire gauge between your BMS and the load? I had almost identical cut-outs on my leisure build last year and it turned out to be undersized cable causing enough voltage drop to trigger the BMS's low voltage protection, even though the cells themselves were absolutely fine. The BMS was just seeing a false alarm essentially.

Also worth having a look at your connection points — loose busbars on parallel cells can create resistance that only shows up under heavy draw. Give everything a proper torque-check with a calibrated wrench rather than just hand-tight.

@Squib19 makes a fair point about balancing too, so I'd work through that methodically before spending money on a replacement BMS.

BenWebb | 312 posts

@DH_VanLife just to add to what @HazelPaddy said about the Daly's real-world rating — temperature makes a big difference too. If that BMS is mounted somewhere without decent airflow (which is pretty common in Transit builds where space is tight), it'll derate aggressively and trip well below its stated limit. Worth feeling how warm it gets during load.

Also, are you running those four cells in parallel with individual fuses on each cell's positive lead before they join the main bus? Without them, a weak cell can end up doing more work than the others, which stresses the BMS further.

What's actually pulling the heavy load when it trips — an inverter? If so, what size? Some cheaper inverters have horrible inrush current on startup that briefly spikes way beyond their rated draw.

LH_Marine | 1,847 posts

@DH_VanLife four cells in parallel means your BMS sees the full combined current when you hit a heavy load — that 100A Daly is right on the edge even under modest demand. The continuous rating isn't the burst rating, and cheap Daly units are notorious for thermal throttling before they hard-cut.

Worth considering: a Daly Smart BMS (the Bluetooth variant) at minimum, or step up to a Victron SmartShunt alongside a properly rated BMS — 150A minimum for your bank if you're running anything like an inverter.

@Paul1999 makes a fair point on cable gauge too. I've seen 16mm² passed off as adequate where 25mm² was clearly needed.

On my narrowboat build I run a 200A JK BMS — rock solid, actual honest current ratings, and the active balancing is worth every penny.