Victron BMV-712 showing different SOC to my Daly BMS — which one do I trust?

Had exactly this issue when I first set up my garden office system. The two will almost never agree perfectly, and it caused me no end of confusion early on.

The short answer is — trust the BMV-712 for day-to-day monitoring, but use the BMS as your safety net.

Here's my thinking:

• The Daly BMS is primarily there to protect the cells (over/under voltage, overcurrent etc.). Its SOC reading is often just a rough estimate based on voltage curves, and it can drift badly if it never hits a full charge cycle
• The BMV-712 uses coulomb counting — it's actually measuring current in and out of the battery. Far more accurate for real-world SOC tracking
• That said, the BMV needs a proper synchronisation point to stay accurate. If your system never reaches 100% absorption/float, it'll drift too

The key thing I'd check is whether your BMV is configured correctly — battery capacity, Peukert exponent, charge efficiency factor. Fogstar Drift cells behave differently to older LiFePO4 chemistry, so default Victron settings might need tweaking.

Also worth making sure your BMV shunt is on the negative terminal of the battery, before anything else connects, otherwise it's not counting everything.

What's the actual discrepancy between the two readings? And are you on a Daly 8S or 16S setup? Might help narrow it down.

@GeorgeJohnson83 nailed it. Had the same head-scratching moment with my garden office setup.

One thing worth adding — make sure your BMV-712 has a proper shunt doing all the work. If anything else is on the battery negative side of the shunt, your coulomb counting goes out the window and the SOC drift gets worse over time.

Also worth doing a full charge/discharge cycle after setting your battery capacity correctly in the BMV. Mine was set to a default figure and was miles off until I corrected it to match my actual Fogstar cells.

The Daly BMS SOC is basically just voltage-based estimation — fine for protection, rubbish for accuracy. BMV wins every time once it's calibrated properly.

@GeorgeJohnson83 is right but worth being precise about why — the Daly BMS uses cell voltage to estimate SOC, which is notoriously non-linear across most of a LiFePO4 curve. The BMV-712 uses coulomb counting, which is fundamentally more accurate provided you've configured it correctly.

Key settings people get wrong:

• Charged voltage threshold
• Tail current percentage
• Peukert exponent (less critical for LiFePO4 but still matters)

If those aren't dialled in properly, the BMV will drift over time and need a full charge to resync. Mine took about a week of tweaking before it was reliable on my cabin setup.

The Daly SOC reading is really only useful as a rough sanity check or low-voltage cutoff trigger — nothing more.