Anyone else using a Victron Cerbo GX with mixed battery brands — getting dodgy SOC readings?

Running a 400Ah LiFePO4 bank in my static caravan setup — two 200Ah Fogstar Drift cells paired with an older 200Ah Renogy unit I had knocking about. All wired in parallel, going through a Victron SmartShunt into the Cerbo GX. On paper it should be fine, but the SOC display on VRM keeps doing odd things — jumping from around 78% down to 65% overnight with no load connected, then "recovering" by morning. Voltage across the bank sits rock solid at 13.28V the whole time, so it's not an actual discharge event.

The core issue seems to be that the Fogstar cells have a slightly different internal resistance profile to the Renogy, which I suspect is confusing the SmartShunt's current integration. I've re-run the battery setup wizard twice, set capacity to 400Ah, Peukert to 1.05, charge efficiency to 99%, and tail current at 2%. Still getting the phantom drift. Worth noting the Renogy has its own internal BMS reporting nothing unusual via its Bluetooth app.

Has anyone successfully run a mixed-brand parallel LiFePO4 bank with a SmartShunt and actually got stable SOC tracking long-term? Wondering whether the fix is a proper synchronisation cycle (full charge to absorption, hold until tail current drops), or whether I need to look at something like a battery balancer across the two brands. Open to being told I've bodged this from the start, too.

@BatteryTony had almost exactly this on the narrowboat — mixed Fogstar and a leftover Renogy unit causing the Cerbo to lose its mind with SOC drift.

The core issue is that without a shared BMS talking to the Cerbo via VE.Can or VE.Direct, it's just working from the SmartShunt's coulomb counting, and mixed cells with slightly different internal resistance throw that calibration off over time.

A few things that helped me:

• Force a full sync cycle — let the bank hit absorption voltage cleanly so the shunt resets to 100%
• Tighten your Charged voltage and Tail current settings in the shunt config
• The Renogy BMS almost certainly isn't talking to Victron at all — treat it as a dumb cell effectively

Honestly, long-term the mixed bank is your real problem. Fogstar Drift cells are decent but they'll never dance perfectly with a mismatched unit.

@BatteryTony mixing brands in parallel is asking for grief tbh. Different internal resistance means they charge and discharge unevenly — your Cerbo's SOC is just doing its best with conflicting data.

Couple of things that actually helped on my boat:

• Full discharge cycle to re-sync the SOC baseline
• Set your tail current properly in VictronConnect — most people leave it at default and wonder why it's reading 97% forever
• If your Renogy hasn't got a BMS that talks to the Cerbo, it's basically invisible to the system

The Fogstar Drifts will likely outperform that older Renogy unit anyway. Wouldn't be shocked if one day you find the Renogy barely contributing.

Long term? Match your bank. Cheap in the short run, headache forever. 🔧

@BatteryTony this is really relevant to my situation — I'm planning a mixed bank for my garden office build and hadn't even considered the SOC accuracy implications with the Cerbo GX.

Quick question: are you using a SmartShunt in addition to the Cerbo, or relying purely on the Cerbo's built-in SOC calculation? I've read that a dedicated SmartShunt with properly configured battery capacity and Peukert settings can dramatically improve accuracy even with a mixed bank — but I'm not sure if that holds when the cells have different characteristics pulling against each other.

Also, did you configure each battery's parameters separately anywhere, or is the Cerbo just treating the whole parallel bank as one unit? Wondering if that's where the readings go wrong.

@BatteryTony I had similar grief with my Cerbo until I realised the SOC drift was partly down to the shunt placement. Make sure your BMV shunt is on the negative of the entire bank rather than individual batteries — if it's measuring current from only one branch, the readings will be all over the place with mixed cells behaving differently.

Also worth going into the Cerbo settings and doing a full discharge/charge cycle to recalibrate the SOC from scratch. Mine was permanently confused until I did that.

@HollyGazer for a garden office build, honestly try to budget for matched cells from the off — saves a lot of head-scratching later. The Fogstar Drifts are good value if you're buying new.

@GrumpySparky is right about the internal resistance issue, but it's manageable with proper monitoring in place.