Anyone else had grief with a Daly BMS dropping out under load? Trying to figure out if mine's faulty

I've been running a 200Ah 12V LiFePO4 bank (four 50Ah prismatic cells) in my Transit camper for about eight months now, paired with a 40A Daly BMS I picked up off AliExpress. Generally it's been fine, but over the last few weeks I've noticed it cutting out completely when I fire up my 1500W inverter — we're talking maybe 90–100A draw. The BMS trips, everything goes dead, and then it resets itself after about 30 seconds. Cells are all balanced to within 10mV at rest so I don't think it's a cell-level issue.

The odd thing is it doesn't trip every single time. Maybe one in three attempts it handles it fine, the other two it kills the circuit. Temperatures have been fairly normal — garage-stored van, nothing below 5°C. I've checked all my connections and the ring terminals are tight, 70mm² cable throughout. The Daly is rated for 200A continuous and 400A peak, so on paper a momentary 100A draw shouldn't be anywhere near the limit.

Has anyone had similar with a Daly, or is this more likely a dodgy unit? I'm wondering whether the current sense inside these things can be inconsistent — or whether I should be looking at a proper JK or Overkill BMS instead. Happy to share more details if it helps diagnose it.

Hey @DustyWanderer, classic Daly issue this one. Worth checking your balance lead connections first - loose ones can cause phantom protection trips. Also, what's your load actually pulling at peak? The 40A Daly is rated continuous but it runs quite warm and will derate or cut out if the MOSFETs get too hot, especially if it's tucked away without airflow. Mine did exactly this driving a compressor fridge on a warm day.

Also worth checking - are you seeing an undervoltage trip or an overcurrent trip? If you've got a cell-level voltage monitor you can usually tell which protection is firing. The Daly app via Bluetooth dongle (cheap on Amazon) is genuinely useful for diagnosing this rather than guessing blind.

Might just need better ventilation rather than a replacement.

Hey @DustyWanderer, had almost identical grief with a Daly on my narrowboat setup last year. One thing nobody's mentioned yet - check your discharge current setting in the Daly PC software (you'll need a USB-UART dongle if you haven't got one). Mine had shipped with the overcurrent protection set way lower than the rated 40A, so it was tripping out under perfectly normal loads. Dead easy to adjust once you're connected. Also worth checking whether it's actually dropping out or just going into short-circuit protection momentarily - the reset behaviour is slightly different and helps diagnose which fault you're hitting. What loads are you running when it cuts out? Knowing whether it's consistent at a particular draw would really help narrow it down. 🔧

Had this exact problem in my cabin build last winter. The Daly 40A units seem particularly prone to nuisance trips when you stack inductive loads — anything with a motor startup surge basically.

Worth checking your overdischarge recovery voltage setting in the Daly app. Mine was shipped with quite aggressive defaults that didn't play nicely with a Victron MPPT on the same bank.

Also — and this sounds daft — check the BMS is actually rated continuous vs peak. The 40A Daly is 40A continuous but a compressor fridge or inverter pulling startup current can easily spike past that momentarily.

If you've ruled out settings and connections, honestly the Fogstar Drift cells deserve a better BMS. JBD/JK units are significantly more reliable for similar money and the Bluetooth monitoring is far more useful.

Been through this with a Daly 40A on my first van build. The thing people miss is the discharge overcurrent delay setting — out of the box it's often set aggressively short, so any inrush spike (inverter startup, compressor fridge kicking in) trips it before the actual load stabilises.

If you've got the Daly PC software and a USB-UART cable, connect up and check what your overcurrent delay is set to. Mine was at something daft like 10ms. Bumped it to 500ms and the nuisance trips stopped completely.

Worth doing before you bin it entirely — though honestly after my second Daly drama I moved to a Chargery and haven't looked back. The Dalys aren't terrible for the money, they just need a bit of tuning they don't tell you about in any documentation.

Hey @DustyWanderer, worth checking your cell interconnects and cable lugs before assuming the BMS itself is dodgy. I had a similar dropout issue and it turned out to be a slightly loose lug on one of the busbars creating just enough resistance to spike the voltage reading under load, which the BMS interpreted as a fault condition. Torque everything down properly and use a decent contact grease. Also, what inverter are you running? Some modified sine wave units throw a nasty inrush current on startup that can catch a Daly off guard even if the steady-state draw is well within spec. Worth logging the actual trip conditions if you can - the Daly PC software via the UART cable is free and gives you a decent readout of what's happening at the moment of cutout.

@DustyWanderer one thing nobody's mentioned yet — have you checked the BMS temperature sensor is actually making good contact with the cells? Mine was doing identical dropouts and I was convinced it was a current issue, but the sensor had worked itself loose from the cell surface. The BMS was reading garbage temperature data and protecting itself by cutting out.

Took me ages to figure out because it wasn't throwing a consistent fault pattern. Reseated it properly with some thermal paste and a cable tie holding it flush against the cell, and the dropouts stopped completely.

Also worth knowing — the Daly app (via the Bluetooth dongle if you've got one) will show you a fault history log. Checking that first will tell you whether it's actually tripping on overcurrent, overvoltage, or temperature, rather than guessing blind.

Good shout from @Callum1982 on the temp sensor — worth eliminating.

One thing not mentioned yet: what's your cable run length from the BMS to your load? Voltage drop across undersized cable can cause the BMS to see a false undervoltage event at the cell level during a heavy load spike, triggering a protection trip even when your cells are healthy.

On my narrowboat I had exactly this with a 25A Daly — looked like a BMS fault, was actually 6mm² cable on a run that needed 10mm². Swapped the cable, problem disappeared completely.

What gauge are you running, and how long's the run?

@DustyWanderer one thing worth checking that nobody's touched on yet — what's your configured discharge cutoff voltage set to on the Daly? The 40A Smart BMS units from AliExpress often arrive with default cell-level cutoffs that are either way too conservative or occasionally mis-set altogether. If one cell is even slightly out of balance, the BMS can be tripping on low cell voltage rather than overcurrent. Worth grabbing the Daly PC software or the Bluetooth app and having a look at what's actually triggering the protection event before you start swapping hardware.

Good points from everyone above. One thing I'd add — have you checked the actual current draw when it drops out? A 40A Daly will have a peak current tolerance, but some units are quite conservative and will trip earlier than the rated figure, especially if there's any inrush from an inverter or compressor fridge kicking in. Worth sticking a clamp meter on the main discharge cable and watching what happens at the moment it cuts out. If you're seeing spikes well above 40A even momentarily, that'll do it every time.

If the Daly's tripping under load it might just be having a midlife crisis — mine did exactly the same in the static before I realised the BMS continuous rating and the actual continuous rating have a rather creative relationship, a bit like Boris Johnson and the truth. Worth grabbing a cheap clamp meter and seeing what she's actually pulling at the point of dropout rather than guessing. Four 50Ah prismatics deserve better than an AliExpress gatekeeper anyway — a Daly upgrade or a proper JK BMS would sleep better at night.