BMV smart shunt tail current setup

Been through exactly this transition with my shepherd's hut build — swapping from a single AGM to a Fogstar Drift LiFePO4 bank changes your SmartShunt tail current settings quite dramatically, and it's worth getting your head around before you just copy across the old values.

With gel/AGM you'd typically set tail current somewhere around 4–5% of capacity because those chemistries absorb charge slowly near the top and the current genuinely tapers off. LiFePO4 is a completely different beast — the charge curve is so flat that the battery can appear nearly full long before it actually is, and tail current needs dropping considerably, often to 1–2% or even lower depending on your cells.

For a 600 Ah parallel LiFePO4 bank, I'd suggest starting around 4–6A tail current (roughly 1%) and adjusting based on how your SOC tracking behaves over a few full cycles. The charged detection voltage also needs revisiting — set it just below your BMS high-voltage cutoff rather than copying AGM figures.

One thing people overlook: charge efficiency factor. LiFePO4 sits around 99%, whereas gel is closer to 85–90%. If you leave the old figure in the BMV, your SOC will drift progressively over time and you'll wonder why the shunt seems to lose its calibration.

Anyone else running parallel LiFePO4 packs with a BMV-712 or SmartShunt and found particular settings that track well through winter when charge cycles get irregular? Curious whether folks are using Victron's default LiFePO4 preset as a starting point or tuning from scratch.

Tommo55 | 847 posts

@LiamFrost70 spot on about the dramatic difference. With AGM I was running tail current around 4% which felt reasonable, but dropped mine to 1% when I switched to LiFePO4 — the absorption phase is so much shorter and the acceptance current drops off a cliff near full charge compared to lead acid.

One thing worth mentioning that catches people out: make sure your charged voltage threshold matches your actual charge parameters. I had mine set too low initially and the shunt was declaring "full" prematurely, which throws your SOC right off from the start.

Also worth leaving the battery cycling a good few times before trusting the readings — the SmartShunt seems to calibrate itself better after it's seen a handful of proper charge/discharge cycles. Took mine about a week to really settle.

ExPostie | 312 posts

Done exactly this on my shepherd's hut. The bit people miss is that LiFePO4 has such a flat charge curve that the SmartShunt can get confused about what "full" actually means.

I'd suggest dropping tail current to around 1-2% of your battery capacity, and — more importantly — set your charged voltage accurately. Fogstar Drift cells typically want something like 27.2V for a 24V bank as the trigger threshold, otherwise the shunt never registers 100% SOC properly.

Also worth enabling the Peukert exponent tweak — LiFePO4 is much closer to 1.05 than the AGM default of 1.25. Leaving the AGM settings in place will skew your readings noticeably over time.

Victron's own docs on this are actually decent for once, worth a read alongside the Fogstar spec sheet.