Anyone running Fogstar Drift LiFePO4 cells alongside a Victron SmartShunt — getting accurate SOC?

Just picked up a 200Ah Fogstar Drift battery for the garden office build and paired it with a Victron SmartShunt 500A. First few days the SOC readings are all over the place — showing 78% then jumping to 91% within an hour of light load (just a monitor and a few LED strips, maybe 8–10W total).

I've set the charged voltage to 14.2V, tail current to 4%, and Peukert to 1.05 as a few people suggested for LiFePO4. Charged detection delay is on 3 minutes. Battery is sitting on a 40A Victron SmartSolar MPPT pulling off a 400W panel array.

Is the issue likely to be the flat discharge curve of LiFePO4 just confusing the shunt, or have I got my settings wrong? Wondering if I need to do a proper full charge/discharge cycle first to let the shunt calibrate itself. Anyone had similar teething issues with this specific combination?

CraftyWelder | 847 posts

@LazyWarden The jumping SOC is almost certainly your synchronisation point being off. The SmartShunt needs to see a proper full charge to calibrate itself — it's essentially counting coulombs from a known baseline.

With LiFePO4 the flat voltage curve absolutely fools the shunt if you haven't set the charged voltage threshold correctly. In VictronConnect, set your "charged voltage" to around 14.2V and "tail current" to about 2-4% of battery capacity. That tells the shunt exactly when to register 100% and reset.

Also worth checking your Peukert exponent — drop it right down to 1.05 for LiFePO4, the default assumes lead-acid behaviour.

Give it a handful of full charge cycles after adjusting and it should settle down nicely. The Drift paired with a SmartShunt is actually a solid combo once dialled in properly. 👍

Got mine sorted on the boat by setting the charged voltage in the SmartShunt to match exactly what the Fogstar BMS actually cuts off at — factory default in Victron is often wrong out the box and it'll never find a clean sync point.

Ray1981 | 312 posts

@LazyWarden To add to what @CraftyWelder is saying — make sure you've set the charged voltage threshold correctly in the SmartShunt settings. The Fogstar Drift sits at 3.65V per cell, so for a 4S configuration that's 14.6V. If your threshold is set too low, the shunt never properly resets to 100% and the SOC drifts progressively worse over time.

Also worth double-checking your Peukert exponent — LiFePO4 is much flatter than lead acid, so setting it to 1.05 rather than the default 1.25 makes a noticeable difference to accuracy during discharge. Took me a good week of fiddling before my setup settled down properly. Once synchronised correctly these shunts are brilliant with LiFePO4 chemistry.

GeorgeSmith97 | 203 posts

@LazyWarden One thing nobody's mentioned yet — check your tail current setting. By default the SmartShunt uses 4% of capacity as the threshold to decide a charge cycle is "complete," but LiFePO4 batteries like the Fogstar absorb very little current at the top end, so you might want to drop that down to around 1-2%. If your tail current threshold is too high, the shunt never registers a proper full charge and the SOC never properly synchronises to 100%. That's often what causes the erratic readings people see in the first week or so. Once you've nailed the charged voltage (as @DazHenderson77 and @Ray1981 have covered) and sorted the tail current, you should find it settles down nicely. Give it a couple of full charge cycles after adjusting and it should read accurately from there.

RetiredChef | 847 posts

@LazyWarden the elephant in the room nobody's poked yet — your Peukert exponent in the SmartShunt defaults to 1.25, which is a lead-acid assumption; LiFePO4 chemistry is far flatter, so dial that right back to 1.05 and watch your SOC stop doing its impression of a broken thermometer 🌡️

Also worth enabling synchronise to 100% only after a proper full absorption cycle — if your charger isn't actually finishing the job, the SmartShunt never gets a clean reference point to anchor itself to, and it'll drift worse than my narrowboat in a crosswind.

MarineKaren | 1,204 posts

My SmartShunt was doing the same interpretive dance until I realised the synchronisation point was never actually triggering — boat sits at absorption for ages before hitting the tail current threshold, so it never "saw" 100% and just drifted into fantasy numbers. Check your charged voltage is set to exactly what the Drift actually reaches at float, not Victron's LFP default (which is slightly off for Fogstar's chemistry) — once that clicked mine's been rock solid for 18 months.

HighlandExplorer | 1,847 posts

Worth checking your charged voltage threshold in the SmartShunt config. The Fogstar Drift sits at a 3.65V/cell ceiling, so your 12V pack hits absorption around 14.2–14.4V — but the SmartShunt default charged detection threshold is often set higher than that. If the shunt never sees the pack as "fully charged," it can't reset its 100% synchronisation point and drift compounds over days.

In VictronConnect, go to SmartShunt settings → Synchronisation → drop the charged voltage to around 13.8V and set charge detection current to roughly 2% of capacity (4A on your 200Ah). Done this on my shepherd's hut setup and SOC stabilised within a couple of cycles.

Boycie74 | 312 posts

@LazyWarden one thing nobody's mentioned yet — have you set your tail current correctly? Victron defaults this to 4% of battery capacity, which is often too high for LiFePO4. For a 200Ah Drift I'd drop it down to around 1-2A. If the tail current threshold is too generous, the SmartShunt declares the battery "full" prematurely and resets SOC to 100% before it's actually there, which cascades into dodgy readings for the rest of the cycle. Worth tweaking that before anything else honestly.