Victron SmartShunt LiFePO4 SOC drift - charged voltage, tail current, charged detection time

Been wrestling with this on my cabin setup for a few months now and finally feel like I've got it dialled in, so thought I'd share and hear what others are doing.

The three settings that seem to interact the most on the SmartShunt for LiFePO4 are:

• Charged voltage — I've settled on 13.2V (for a 12V bank). Some people go as low as 13.0V but I found that caused too many false resyncs mid-cycle.
• Tail current — Running 4% of capacity. My bank is 200Ah Fogstar Drift cells so that's 8A. Dropping it lower meant the shunt took ages to decide the battery was full.
• Charged detection time — 3 minutes seems to be the sweet spot. Too short and it resyncs before absorption is genuinely done, too long and you miss the window entirely.

The real issue with LiFePO4 is that the voltage curve is so flat that the shunt really struggles compared to lead acid. It's not like the Victron is doing anything wrong per se, it's just that the underlying chemistry makes SOC estimation genuinely harder.

What I've noticed is that if your charger isn't hitting a consistent absorption phase, the tail current condition never gets met cleanly and the SOC just drifts over weeks.

Anyone running larger banks — say 400Ah+ — finding they need to adjust the tail current percentage down to compensate? And has anyone experimented with the Peukert exponent settings on top of this to tighten things up further?

Would be good to compare notes, especially from those with solar-only setups where charge profiles vary day to day.

Great post @Inverter_Pro, looking forward to seeing where you were going with that - looks like it got cut off mid-sentence!

I've had similar battles with my 200Ah Winston cells. The setting that caught me out most was the charged detection time - I had it at the default 3 minutes which was far too short. My BMS was occasionally causing brief voltage spikes that the SmartShunt was interpreting as a full charge. Bumped it up to 5 minutes and the drift improved dramatically.

Also worth mentioning - if you're using a MPPT alongside the SmartShunt, make sure the charged voltage is set slightly below your absorption voltage, not equal to it. Took me an embarrassingly long time to figure that one out.

What cell manufacturer are you running? Makes quite a difference to where you set your tail current threshold in my experience.

@Trevor1967 ha, the post got cut off mid-sentence AND @Inverter_Pro is still wrestling with SOC drift — sounds like the SmartShunt and the OP are both suffering from synchronisation issues 😄

Right, my actual contribution: the charged detection time is the one most people ignore and it's doing a lot of heavy lifting.

@Inverter_Pro the three settings that matter are charged voltage (set ~0.1-0.2V below your absorption endpoint), tail current (typically 1-4% of battery capacity), and charged detection time (I run 3 minutes on my boat bank).

The interaction between them is the bit that trips people up. If your tail current threshold is too high, the shunt declares "full" too early and you never get a proper 100% sync. Too low and it waits forever if there's any parasitic draw.

On my Fogstar 280Ah cells I had persistent drift until I dropped charged detection time to 2 minutes and set tail current at 2%. Made a noticeable difference.

Worth checking your actual absorption voltage at the battery terminals with a multimeter too — cable voltage drop means the shunt might be seeing numbers that don't reflect reality at the cells.

@BMS_Geek nailed the basics but I'd add — the charged detection time is where most people get tripped up, myself included. Had my static caravan setup resetting SOC to 100% prematurely for weeks because the tail current was being met momentarily during bulk, not actual absorption tail-off.

I'm running 280Ah Fogstar Drift cells with a Victron SmartShunt and ended up at:

• Charged voltage: 27.0V (absorption set to 27.2V on the MPPT)
• Tail current: 4% of capacity
• Detection time: 5 minutes minimum

The longer detection time really stabilised things. Worth noting that if you're also doing EV charging off the same bank (as I occasionally do), the current draw patterns can fool the shunt during detection windows — so I now schedule charging sessions after the detection window completes.

VictronConnect logs are your friend here, watch the raw data not just the SOC gauge.