Bought two second-hand 200Ah LiFePO4 cells off eBay — how do I know if the BMS is actually doing its job?

Picked up a pair of 200Ah LiFePO4 prismatic cells last week from a seller in Birmingham, came with a 24V 100A Daly BMS already wired in. Paid £180 the lot which felt like decent value, but now I'm second-guessing myself a bit. The cells arrived with no capacity test data, no cycle count history, nothing really. Seller just said they came out of a "solar storage unit."

I've got a basic Victron BMV-712 on order so I can at least monitor state of charge and current draw, but I'm wondering how much I can trust the Daly BMS to actually protect these cells properly. I did a rough charge test yesterday and the pack topped out at 28.4V before the BMS cut off, which seemed about right, but I've no idea if individual cells are being balanced or if one is running away from the others.

Is there a reliable way to check the per-cell voltages without buying a dedicated cell monitor? I've got a decent multimeter (Fluke 115) but accessing individual cell terminals on a wired-up pack feels like it could get messy. Also wondering if the Daly app via Bluetooth is actually worth trusting or if it's just giving me ballpark figures.

Hey @PeteDixon89, decent score at that price! To check your Daly BMS is actually doing its job, grab the Daly Smart BMS app via Bluetooth dongle (they're a few quid on eBay) — it'll show you individual cell voltages, temperature readings, and any protection events that have triggered.

Most importantly, deliberately test the protections: try drawing more than 100A and see if it cuts out, and check it disconnects on low voltage.

Also worth doing a capacity test on those second-hand cells — charge fully, then discharge at a known rate while timing it. If you're only getting 140Ah from a "200Ah" cell, that tells you something. LiFePO4 from random eBay sellers can be anything really. What's the cell brand printed on them?

Great buy @PeteDixon89! To add to what @TimGraham's saying about the app — once you're connected, deliberately stress-test the protections rather than just reading the idle stats. Temporarily short the balance leads or pull heavy current to see if the over-current protection actually trips. Also worth checking the BMS is cutting out at the right cell voltages: LiFePO4 should disconnect around 2.8V low and 3.65V high per cell. Second-hand Daly units sometimes have the parameters reset to defaults that don't match your actual cells, or worse, tampered values from the previous owner. A cheap USB-to-UART adapter lets you reprogram those thresholds if needed. Given it came pre-wired by someone unknown, I'd personally verify every connection with a multimeter before trusting it with a real load. £180 is a great deal but only if it's actually safe!

@PeteDixon89 one thing nobody's mentioned yet — with second-hand cells, the BMS parameters may have been configured for a different cell chemistry or capacity entirely. The Daly's charge/discharge cutoff voltages might be completely wrong for your specific cells.

Pull up the app @TimGraham mentioned and check the configured voltage thresholds. For a 24V LiFePO4 system you want:

• Overvoltage cutoff: ~29.2V (3.65V per cell)
• Undervoltage cutoff: ~22.4V (2.8V per cell)
• Balance start voltage: ~28.8V

If those are set outside those ranges — particularly the undervoltage protection — you could be silently damaging the cells on discharge without the BMS ever tripping.

Also worth doing a capacity test with a proper load before trusting this in a motorhome situation. I use a CT clamp and log the Ah discharged from full to BMS cutoff.

Good shout from @Curly on the parameter config — I'd add that with second-hand Daly units specifically, it's worth checking the cell voltage differential readout in the app once you've got it connected. If any single cell is sitting more than about 50-100mV away from its neighbours at rest, that's a red flag suggesting either a weak cell or dodgy balancing history.

Also worth doing a full cycle — charge to top, let it settle an hour, then check individual cell voltages again. A healthy LiFePO4 pack should be remarkably flat across all cells in that middle SOC range. Any cell consistently running high or low needs watching closely before you trust the pack in a real install.

For £180 with a Daly included you've done alright, just don't skip the due diligence! 👍

@Curly raises the sneaky one — also worth checking if that Daly is actually cutting off when it should, because mine looked fine in the app right up until it silently let a cell hit 2.8V and I had a very sad morning in the shepherds hut wondering why the kettle died.

What @BatteryJason is hinting at is a real rabbit hole. I ran a deliberate discharge test on my Fogstar cells when I first got them — just a resistive load, a decent clamp meter, and patience. Watch what the BMS actually does at the low voltage cutoff versus what the app claims it'll do.

The Daly app connection is notoriously flaky over Bluetooth, so don't trust the reported figures without cross-referencing a multimeter directly across the terminals. I've seen Daly units where the display happily reports 24.1V while the cells themselves are sitting unbalanced — one cell at 2.8V, rest at 3.2V.

For £180 with 200Ah capacity you've potentially got a bargain, but those cells need a proper capacity test first. Charge full, rest an hour, discharge at a known rate, log everything.