Shunt und Batteriemonitor zeigen falsche Werte an

Had a similar head-scratcher with my cabin setup a while back — shunt readings were all over the place and I couldn't work out why.

Turned out the issue was that not everything was routed through the shunt. The inverter was wired directly to the battery terminal after the shunt, so the monitor had no idea what the inverter was pulling. Classic mistake, easy to miss.

A few things worth checking if you're seeing dodgy readings:

• All loads and charge sources must pass through the shunt — no exceptions. Even a small direct connection will throw off the SOC over time
• Make sure your battery negative is the only thing connected to the shunt's load side
• If you're using a Victron SmartShunt (which I'd recommend over most alternatives tbh), double-check the charged voltage and tail current settings in the VictronConnect app — defaults are often wrong for LiFePO4
• Peukert exponent — set it to 1.05 for LFP, not the default lead-acid value

I run a Fogstar 200Ah LiFePO4 with a Victron SmartShunt and it's been rock solid once I sorted the wiring. Took me an embarrassingly long time to figure out the inverter bypass issue though 😅

Anyone else had weird shunt gremlins? Curious whether it's usually a wiring thing or config thing for most people here.

GlenKelly | Member

@Chopper — classic gotcha that one! The other thing worth checking is whether the shunt's negative terminal connections are properly torqued down. I had a similar head-scratcher at my place in Wales and spent ages chasing software settings before realising one of the bolts was only finger-tight. The contact resistance was causing daft readings, especially under heavier loads when things got warm and expanded slightly.

Also worth mentioning — if you're using a Victron BMV, double-check your Peukert exponent and charge efficiency factor are set correctly for your battery chemistry. Factory defaults are often way off for LiFePO4 if that's what you're running. Makes the SOC drift noticeably over time even when the shunt itself is wired perfectly.

Good points from both of you, though I'd add one that catches people out less obviously: shunt placement relative to the battery negative busbar.

If you've got multiple batteries paralleled up — as I do with my static caravan setup running a pair of Fogstar Drift 100Ah units — every negative connection must return via the shunt, not directly to the busbar. Even one rogue earth path bypassing the shunt will give you systematically incorrect state-of-charge readings that drift worse over time.

Also worth checking: the sense wire voltage on your Victron BMV. If there's significant resistance in that connection the monitor is seeing a slightly different voltage to what the battery's actually at, which compounds the inaccuracy.

These issues tend to manifest as the SOC slowly diverging from reality rather than giving obviously wrong instantaneous readings, which is why they're so difficult to diagnose initially.

Really useful thread this — got me wondering about my own static caravan setup now.

Quick question for the group: does the order of connections on the negative busbar actually matter if everything is going through the shunt? I've got a Victron SmartShunt and the wiring looks correct on paper, but I'm occasionally seeing the State of Charge drift by a few percent overnight even with zero load showing.

Could that be a parasitic draw the shunt isn't catching, or is it more likely a calibration issue with my Fogstar Drift cells not having a proper tail current profile set up? I've seen some chat about tweaking the charged voltage and tail current thresholds in VictronConnect but never actually done it properly.

Does the shunt essentially need "re-learning" the battery capacity after you swap chemistry?

RobBennett93 | Member

@LutonAdventure for a static caravan you'll likely be fine as long as the shunt is the only negative connection point — everything else should star off the battery negative post via the shunt, not bypassing it.

One thing nobody's mentioned yet: battery temperature compensation can throw your SoC readings off badly if your monitor supports it but the temp sensor isn't connected properly (or at all). Had this exact issue on the narrowboat last winter — Victron BMV was reading 85% when the batteries were clearly struggling. Turns out the temp sensor had worked loose. Worth checking if your monitor has one.

Also double-check your battery capacity setting in the monitor itself — wrong Ah figure entered will skew everything from the start, and it's easy to fat-finger on setup.