Anyone else running Victron MPPT + Cerbo GX without a BMV — is the SOC actually reliable?

Just finished the wiring in my Transit and went with a Victron SmartSolar 100/30 feeding a Fogstar Drift 100Ah LiFePO4. I've got a Cerbo GX for monitoring but held off buying a BMV-712 because I thought the Cerbo pulling data straight from the MPPT might be enough to keep tabs on state of charge.

Two weeks in and the SOC readings feel... off. After a full night with no load it shows 97% rather than 100%, and after running my Webasto for a couple of hours it jumps around rather than dropping steadily. I know the MPPT can only really see what's coming in from the panels, not what's going out to the loads, so I'm wondering if that's the root cause here.

Is a BMV (or fitting a SmartShunt) genuinely non-negotiable for accurate SOC, or is there a way to get better figures from what I've already got? I've seen mentions of tweaking the battery settings in VRM — charge voltage, tail current, Peukert — but not sure where to start without risking overcharging a lithium.

Has anyone dialled this in without a dedicated shunt, or did you eventually give up and buy one?

@LDVNomad ran my shepherd's hut the same way for about eight months — the Cerbo's SOC is basically educated guesswork without a shunt, it just extrapolates from the MPPT charge data and has no idea what you're actually pulling out.

Works well enough if your loads are only through a Victron inverter or MPPT (it can see those), but anything wired direct — 12V fridge, USB sockets, random lighting circuits — is completely invisible to it.

The BMV-712 felt like an extravagance until I fitted one and realised I'd been routinely hitting 40% SOC thinking I was at 70%.

Fogstar Drift's own BMS will protect the cells, but your Cerbo will be confidently lying to your face the entire time.

CoastalBoater | 📍 South Coast | ⚡ 400Ah LiFePO4 aboard

@LDVNomad the core issue is that without a BMV (or similar shunt), your Cerbo has no actual current measurement to work from — it's essentially inferring SOC from voltage alone, which on LiFePO4 is notoriously flat across most of the discharge curve. You'll get wildly optimistic readings right up until it drops off a cliff.

I ran my boat this way briefly and found the displayed SOC could be 30% out in either direction during active loads. Added a SmartShunt 500A rather than the full BMV-712 — cheaper, does the same job electrically, and the Cerbo picks it up over VE.Direct immediately. Made an enormous difference to the accuracy.

For a van where you're relying on that data daily, I'd genuinely consider it essential kit rather than optional. 👍

On the narrowboat I've been wrestling with the same question. The thing that pushed me over the edge to finally getting a BMV-712 was realising the MPPT only "sees" what's coming in — it has no idea about your loads. So the Cerbo is essentially doing maths with half the equation missing.

Worth asking yourself: how many amps are you pulling on a typical evening? If you're just running a few LEDs and charging a phone, maybe the guesswork is tolerable. But if you've got an inverter or a compressor fridge like I do, the SOC drift gets embarrassing pretty quickly.

The BMV-712 isn't cheap but it's a one-time fix and integrates cleanly with the Cerbo over VE.Direct. Honestly should've bought it alongside the SmartSolar from the start.

Has anyone actually tested how far off the Cerbo's SOC estimate drifts over a typical day with heavy loads? I'm asking because I'm planning a similar setup in my van build — SmartSolar into a Fogstar Drift — and I was hoping to avoid the extra spend on a BMV-712 at least initially.

What I can't work out is whether the inaccuracy compounds over time or resets properly when the battery hits full charge. If it self-corrects at 100% SOC each sunny morning, maybe it's just about usable for a van that sees regular full charges?

Would the SmartShunt be a reasonable cheaper alternative to the BMV-712 here, or does losing the display defeat the point when you've already got the Cerbo screen? Seems like it could be the middle-ground option.

@StuKnight — actually tested this exact scenario on my static caravan setup. Running a SmartSolar 75/15 into a 200Ah Fogstar Drift bank, Cerbo GX without a BMV for the first few months.

What I found was the drift crept up gradually rather than going cliff-edge wrong. After a cloudy week with the electric blanket running evenings, the Cerbo had me at 67% when I finally cabled in a BMV-712 and did a proper sync — actual state was closer to 48%.

The problem is the Cerbo's working from voltage and charge current assumptions, not counting coulombs in and out. Fine in summer with predictable solar. Gets genuinely misleading once you throw in heavy irregular loads.

For a caravan I'd probably tolerate the inaccuracy. Transit full-timing? Get the BMV — it's not expensive insurance against a flat bank miles from nowhere.

@StuKnight ran that exact experiment last summer on my cabin setup — left the Cerbo running without a BMV for a week, heavy kettle usage, angle grinder, the usual chaos. By day 4 it reckoned I had 73% SOC. My Fogstar's BMS disagreed… quite loudly 😅

The Cer