Anyone else struggled to get accurate readings from a cheap 100A shunt monitor in cold weather?

Picked up a Victron BMV-712 last autumn after my old £15 eBay shunt monitor started giving me completely daft readings over winter — talking 40–50% state of charge swings within minutes of putting the kettle on. Suspected it was the temperature messing with the shunt resistance but couldn't really prove it.

Running a 200Ah lithium (Winston cells, 4S) in my van, with the shunt mounted in the back near the battery box. Overnight temps dropped to around -4°C a couple of weeks ago and even the BMV was reading oddly until things warmed up a bit — showing around 87% SoC but then jumping to 94% once the cabin warmed up. Not a massive difference but it got me thinking about shunt placement and whether mounting it somewhere slightly warmer would help.

Has anyone done any proper testing on this, or is it just one of those things you live with? Also wondering if anyone's experimented with insulating the shunt itself — seems a bit daft but I've seen a few people mention it on American forums and I'm not sure how relevant it is to our climate.

Jim1985 | 847 posts

@Tommo10 Yeah, had almost identical grief with a cheap shunt before I upgraded. The issue with those budget units is often the shunt resistor itself — the tolerance goes right out the window once temperatures drop, so the voltage drop readings become meaningless. Some of them also have absolutely terrible cold compensation on the reference voltage circuitry.

The BMV-712 is a proper step up. One thing worth doing once you've got it installed — make sure you set the Peukert exponent and charge efficiency factor correctly for your specific battery chemistry. The defaults are fine for basic use but dialling them in properly makes a noticeable difference to accuracy, especially in cold when your actual battery capacity is already derated anyway.

What battery bank are you running? Flooded lead acid behaves quite differently to AGM or lithium in the cold.

My old £8 shunt monitor once told me I was simultaneously at 20% and 87% charge — Schrödinger's battery, basically.