Anyone else struggled to get accurate SOC readings from a cheap BMS on lithium cells?

I've been running a 4S 280Ah LiFePO4 pack in my shed setup for about eight months now, wired up with a 40A Daly BMS I grabbed off AliExpress for around £22. The voltage cut-offs work fine and it's kept the cells balanced reasonably well, but the state of charge reading is all over the place. It'll show 80% and then drop to 60% within twenty minutes of a modest load, then creep back up once things settle. Happens regardless of whether I'm pulling 5A or 15A through a 12V inverter.

I've got a Victron BMV-712 on the way which I know will be miles better, but I'm curious whether the SOC display on these budget BMS units is actually worth paying any attention to at all. From what I've read, most of them just use a basic voltage lookup table rather than proper coulomb counting, which would explain why mine's so jittery. Resting voltage on LiFePO4 is notoriously flat across a huge chunk of the SOC range, so voltage alone is practically useless.

Has anyone found a way to get more reliable readings out of a Daly or similar, or is it just a case of ignoring the BMS display entirely and relying on a proper battery monitor? Would be interested to know what others have paired with budget BMS units on DIY builds.

GeorgeSmith97 | 47 posts

@Rodney58 Yeah, the SOC display on those cheap Dalys is essentially guesswork unless you've done a proper capacity calibration cycle. Mine was reading 60% when the cells were genuinely around 35% — nearly caught me out during a cold snap last winter.

The core issue is they rely heavily on voltage-based estimation rather than proper coulomb counting, and LiFePO4's flat discharge curve makes voltage a terrible SOC indicator across most of the usable range.

Honestly the best fix I found was pairing it with a separate shunt-based battery monitor — I use a Victron BMV-712, though that's obviously not "budget" territory. There are cheaper options like the Renogy ones that do a decent job for around £30-40. The Daly handles your protection, the shunt monitor handles accurate SOC. Belt and braces approach!

RobWebb59 | 203 posts

@Rodney58 The Daly SOC is basically just doing a rough voltage-to-percentage lookup, which is near useless with LiFePO4 given how flat the discharge curve is. You'll get wildly different readings depending on load and temperature too.

Honestly the best thing I did was add a separate Coulomb counter - I use a JK active balancer that has one built in, but a standalone unit like a Victron BMV-700 (or the cheaper Aliexpress knock-offs if budget's tight) will track actual amp-hours in and out. Makes a massive difference.

Your Daly will still do its job protecting the cells, just don't trust its SOC figure for anything critical. Are you monitoring individual cell voltages at all? That'll tell you more about pack health than any SOC percentage will.

DazBarker93 | 89 posts

@Rodney58 The real problem with LiFePO4 specifically is that the voltage curve is so incredibly flat through most of the charge range - we're talking maybe 50mV difference between 20% and 80% SOC. So any BMS trying to infer capacity from voltage alone is essentially stabbing in the dark. What made a genuine difference for me was adding a separate shunt-based coulomb counter - I went with a Victron BMV-712, though that's obviously not a budget option. If you want something cheaper, the ISDT BM6 or even one of those generic 100A shunt monitors off Amazon for a tenner will give you far more accurate readings than your Daly ever will. The BMS is fine for protection duties, just don't trust it for SOC. Keep them as separate jobs.

Mike1997 | 134 posts

@Rodney58 What @DazBarker93 is getting at is spot on - that flat voltage curve is the real killer for SOC accuracy. One thing nobody's mentioned yet though: have you tried letting the pack sit completely at rest for a good 2-3 hours before trusting any voltage reading? Even a small load or recent charge skews things massively with LiFePO4.

Honestly the best upgrade I made to my similar setup was adding a proper coulomb counter - I use a Victron BMV-712 now and the difference is night and day. Pricey I know, but even a cheaper Bluetooth shunt from Amazon for £25-30 will beat any Daly SOC estimate hands down because it's actually tracking current in and out rather than just guessing from voltage. Worth considering if accurate SOC matters to your usage planning.

CrafterWanderer | 412 posts

@Rodney58 What nobody's mentioned yet is that the Daly's coulomb counting resets poorly after partial cycles — it drifts badly without regular full charge synchronisation points. I ran a similar 280Ah pack in my static caravan and the reported SOC was sometimes 30% adrift mid-cycle.

The practical fix that actually worked for me: pair that BMS with a Victron SmartShunt (£60-ish from Bimble or Fogstar). The SmartShunt does proper coulomb counting with configurable tail-current sync, so it recalibrates at 100% correctly. Your Daly handles the protection layer; the SmartShunt handles accurate SOC reporting independently.

Victron's VictronConnect app then gives you historical data to spot capacity degradation over time too — genuinely useful rather than just a dashboard number. Worth every penny compared to second-guessing a £22 BMS readout.

CumbrianExplorer | 312 posts

@Rodney58 Something worth trying that nobody's mentioned yet - let the pack sit completely disconnected from any load for a couple of hours before you trust any voltage-based SOC reading. Resting voltage is far more representative than under-load voltage, especially with LiFePO4's notorious flat curve (as @DazBarker93 and @Mike1997 have already covered well).

That said, honestly the best bang-for-buck upgrade I made was adding a proper coulomb-counting shunt monitor - I picked up a Victron BMV-700 secondhand for about £45. It tracks actual amp-hours in and out rather than guessing from voltage, which makes a massive difference to SOC accuracy. There are cheaper Chinese shunt monitors that do similar coulomb counting for around £15-20 if budget's tight. Either way, your Daly will never give you reliable SOC on its own - it's just not designed for it.

DodgyGrafter | 847 posts

@Rodney58 One thing I'd add to what's already been said - temperature throws those cheap Daly SOC readings right off. If your shed drops below 10°C overnight, the BMS internal resistance calculations go wonky and you'll often see the displayed SOC jump several percent once things warm up again. Worth logging your readings alongside a thermometer for a week or two. I found sticking a proper standalone battery monitor like a Victron BMV-712 alongside the Daly gives you a much more honest picture for virtually nothing extra in the grand scheme.

Deano88 | 156 posts

@Rodney58 Had almost identical frustration with a Daly on my 200Ah pack. What sorted it for me was pairing the BMS with a separate Victron BMV-712 shunt monitor — let the Victron handle all the SOC calculations rather than relying on whatever the Daly reckons. Yes it's more money, but the Daly's really just doing protection duties at that point whilst the Victron gives you genuinely reliable readings. The difference was night and day honestly. Worth checking @CumbrianExplorer's suggestion about resting voltage first though, as that costs nowt.

OldSailor78 | 203 posts

@Rodney58 The core problem with Daly's SOC is it's entirely voltage-based with no coulomb counting worth speaking of. LiFePO4's flat discharge curve makes that essentially useless across the middle 80% of capacity.

Ran into exactly this on my motorhome build. Spent months chasing phantom readings before fitting a Victron SmartShunt — night and day difference. Yes it's £60-odd, but you actually know what's in your pack rather than guessing.

The BMS does its protection job fine, just don't rely on it for state of charge. Separate the two functions and life gets much simpler.

BrambleHermit | 203 posts

@Rodney58 The flat voltage curve on LiFePO4 is the real culprit here — between roughly 20% and 80% SOC it barely moves, so voltage-based estimation is practically useless. What actually helped me was switching to a proper coulomb-counting shunt monitor like a Victron BMV or even a cheaper PZEM-style unit wired in. It tracks actual amp-hours in and out rather than guessing from voltage. The Daly's SOC readout is honestly just decorative on these cells — treat it as a rough guide at best and rely on a separate meter instead.

StormyWarden | 89 posts

@Rodney58 Worth adding that temperature throws the readings off badly too — my Daly was consistently optimistic by 15-20% during cold mornings last winter because it wasn't compensating for how temperature affects the voltage curve. If your shed gets chilly, that'll compound everything @OldSailor78 and @BrambleHermit have already mentioned. A cheap clamp meter logging actual amp-hours in and out, even just manually noted in a spreadsheet, gave me far more reliable capacity tracking than trusting the BMS display alone. Not glamorous but it works.