Anyone else struggling to get accurate SOC readings with a cheap shunt on a LiFePO4 battery?

Picked up a 200A shunt monitor off Amazon a few weeks back (one of those Aliexpress rebrands, about £12) to keep an eye on my 100Ah LiFePO4 leisure battery in the van. Wired it up according to the instructions and it seemed fine at first, but I'm noticing the state of charge percentage drifts quite badly over a few days. After a full charge it'll show 100%, but by the next morning it's reading something like 74% even though the battery voltage is sitting at 13.28V which suggests it's nowhere near that low.

I'm wondering if the issue is the shunt not accounting for the flat discharge curve of LiFePO4 properly, or whether it's just a calibration problem with the monitor itself. I've set the battery capacity to 100Ah in the settings and tried adjusting the Peukert exponent down to around 1.05 which I've seen recommended for lithium, but it doesn't seem to make much difference. The thing also seems to lose its calibration completely if the van's leisure battery isolator gets switched off.

Has anyone found a reliable budget option that actually handles LiFePO4 well, or is it worth just biting the bullet and getting something like a Victron BMV-712? I don't mind spending a bit more if it's actually going to give me trustworthy readings rather than something I have to second-guess constantly.

@Ray1981 yeah had exactly this with my shepherd's hut setup. Cheap shunts tend to drift badly with LiFePO4 because the flat voltage curve means tiny errors in current measurement snowball into massive SOC inaccuracies.

Threw mine out and got a Victron SmartShunt 500A — night and day difference. Syncs properly via Bluetooth, handles the quirky LiFePO4 charge curve properly, and you can tweak the Peukert exponent and charge efficiency factor in the app.

Worth noting: make sure your shunt is the only negative connection point — any other negative paths bypassing it will completely skew readings regardless of shunt quality.

~£60 for the Victron but honestly it's the one thing I wouldn't cheap out on. SOC accuracy is pretty much useless if you can't trust it.

Hey @Ray1981, one thing worth trying is setting a proper "full charge" sync point — most of these cheap units need to be manually told when the battery is 100% so they can calibrate from there. With LiFePO4 the flat voltage curve makes it really hard for a basic shunt to self-calibrate accurately. Let the battery charge fully until your charger drops to float/absorption, then manually reset the SOC to 100%. After that it should coulomb-count reasonably well until the next sync. Also double-check your battery capacity is entered correctly in the settings — they often default to 100Ah but with slightly different assumptions. Not perfect, but it makes a noticeable difference in my experience. If accuracy really matters long-term, a Victron SmartShunt is worth the investment — night and day difference!

HarbourSam | 📍 South Coast | ⚡ Off-grid since 2019

@Ray1981 worth checking your shunt's calibration resistor value too — a lot of these budget units ship with tolerances way outside spec, which compounds the drift issue @BigAl27 mentioned.

The other thing that catches people out with LiFePO4 specifically is that the voltage curve is so flat compared to lead-acid. Even a tiny measurement error gets amplified massively when you're trying to infer SOC from it. Coulomb counting (which is what the shunt does) needs to be very accurate to stay reliable over multiple cycles.

Honestly, for around £35-40 you can get a Victron BMV-712 — night and day difference. I resisted the upgrade for ages then kicked myself when I finally made the switch. Sometimes cheap costs more in the long run! 😅

Tom2000 | 📍 Yorkshire | ⚡ Van life & shed systems

@Ray1981 Something nobody's mentioned yet — LiFePO4's flat voltage curve is actually the root cause of a lot of this grief. Unlike lead-acid, there's almost no voltage variation between say 20% and 80% SOC, so any tiny error in your shunt's current measurement compounds massively over time. A £12 unit often has a tolerance of ±1-2% which sounds fine but adds up quickly over a day of cycling.

Worth also double-checking your shunt is properly rated — some of these cheap 200A units struggle with accuracy at lower currents (say under 5A), which is exactly where a van leisure battery often sits ticking over. You might find it reads brilliantly at high loads but drifts overnight on standby draw.

A Victron SmartShunt is the obvious upgrade if the budget stretches — genuinely transformative for LiFePO4 monitoring.

Jim1980 | 📍 East Midlands | ⚡ Off-grid since 2015

@Ray1981 To add to what @Tom2000 was getting at about the flat voltage curve — this is exactly why coulomb counting matters so much with LiFePO4, and where cheap shunts really fall down. Even small measurement errors accumulate over time and your displayed SOC just drifts further and further from reality. Worth checking what current threshold your unit uses to detect "charged" state — many of these budget units default to settings designed for lead-acid and won't trigger a proper sync point correctly on LiFePO4. Try tightening that tail current threshold right down. Also make sure every single negative load and charge source runs through that shunt — if anything's bypassed it's game over for accuracy regardless of how good the unit is.

Really good thread this — I had the exact same issue on my static caravan setup with a cheap shunt. What sorted it for me was switching to a Victron BMV-712. Pricey I know, but the difference in accuracy is night and day, especially with LiFePO4's flat discharge curve (as @Tom2000 touched on).

One thing nobody's mentioned — are you accounting for self-discharge drift? If your van sits unused for a few days, these cheap units can lose their reference point entirely and the SOC reading just wanders off. The Victron handles this properly.

Is there a particular reason you're wanting to stick with the cheap shunt, @Ray1981? If it's budget, the Victron SmartShunt 500A is cheaper than the BMV and still leagues ahead of Aliexpress stuff.