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

Had exactly this on my boat last season — BMV-712 was reading 74% whilst the BMS (a Daly 100A) was sitting at 61%. Drove me mad for weeks.

After a lot of head-scratching, I came to the conclusion that neither is perfectly accurate in isolation, but for different reasons:

• The BMV does coulomb counting, so it drifts over time if your charge/discharge cycles aren't syncing it properly. Key thing is making sure your tail current and charged voltage thresholds are set correctly so it resets at 100% reliably.
• The BMS SOC is often calculated from cell voltages alone, which is notoriously unreliable in the middle of the LiFePO4 discharge curve — it's basically flat for ages then drops off a cliff.

On my setup I trust the BMV-712 more day-to-day, but I use the BMS cell voltage readings as a sanity check, particularly at the extremes (below 20% or approaching full).

The other thing worth checking — are your Peukert settings right on the BMV? Wrong values there will skew everything. Fogstar have some decent guidance on this for their cells if you're running their batteries.

What BMS and battery chemistry are you running? Makes quite a difference to which one you'd lean on. And has your BMV ever properly hit a full synchronisation cycle since you installed it? That's often the root cause when the two readings diverge badly.

Others on here probably have better setups than me — would be interested to know how people with a Victron SmartShunt and a proper smart BMS handle this.

@TracyMitchell the Daly BMS is almost certainly reading cell voltage and doing a very rough OCV-to-SOC lookup — which is notoriously inaccurate under load or charge because of IR drop distorting the voltage reading.

The BMV-712 uses coulomb counting from a shunt, so it's tracking actual current in and out. Far more reliable mid-cycle, though it does drift over time if your charge efficiency factor and Peukert exponent aren't dialled in correctly.

On my static caravan setup I trust the BMV-712 day-to-day, but I cross-reference it against the Fogstar Drift cells' genuine resting voltage after a proper absorption cycle — that's your ground truth for resetting the BMV's SOC.

Key settings to check on the BMV:

• Charged voltage threshold
• Tail current percentage
• Charge efficiency factor (LiFePO4 wants ~99%)

Get those right and the BMV will track accurately for weeks between resets.

@EdHamilton is right about the Daly doing a rough voltage lookup — it's a known weakness of cheaper BMS units.

The BMV-712 is coulomb counting from a known starting point, so it's generally more accurate mid-cycle. Problem is, it drifts over time if it never sees a proper full charge to reset itself.

Had this exact situation in my shepherd's hut setup. BMV was optimistic by about 12% because the solar wasn't quite getting the batteries to absorption properly — so it never got a clean sync point.

Worth checking:

• Is your BMV actually seeing a full charge cycle to reset SOC to 100%?
• Have you set the charged voltage and tail current correctly in the BMV settings?

Those two things sorted mine completely. The Daly was actually closer to reality in my case, embarrassingly enough.

Genuine question — does the BMV-712 ever resync itself automatically, or does it only reset to 100% when it detects a full charge condition?

Asking because on my tiny house build I noticed the BMV drift further and further from reality over several weeks. Eventually traced it to the charged voltage threshold being set slightly too high, so it never thought the batteries were properly full and never got a chance to resync.

Once I dropped the charged voltage setting by about 0.2V it started hitting 100% regularly and the figures became much more reliable.

Worth checking your charge detection settings in VictronConnect if you haven't already @TracyMitchell — particularly:

• Charged voltage
• Tail current
• Charged detection time

Curious whether anyone's found a reliable way to cross-reference which reading is actually closer to truth without doing a full discharge test?

@PaulMurray the BMV-712 does resync automatically — it hits 100% when three conditions are all met simultaneously: voltage rises above your set "charged voltage" threshold, current drops below your "tail current" setting, and it holds that state for your "charged detection time" duration. All three are configurable in the settings.

Worth asking though — have you actually dialled those parameters in for your specific battery chemistry? I made the mistake of leaving mine on defaults when I first installed it on my shepherd's hut build, and it was resyncing way too early with my LiFePO4 because the voltage threshold was set for AGM. Once I corrected it the drift between my BMV and BMS got much more manageable.

What battery chemistry are you running? That'd probably help narrow down whether your thresholds are the culprit.