Anyone else running a Victron SmartSolar with a cheap Amazon battery monitor — do the readings actually match up?

Finally got my 200W panel setup sorted on the van — Victron SmartSolar MPPT 100/20, wired up to a 100Ah lithium (one of the Fogstar Drift ones). The Victron app is telling me I'm pulling around 11–12A on a decent day, battery sitting at 27.4V (it's a 24V system). All looks reasonable to me.

Thing is, I've also got one of those generic Bayite battery monitors clipped onto the negative — the £18 job off Amazon. It's consistently reading about 0.8–1.2A less than the Victron app on the charge current, and the voltage is out by about 0.2V too. Not massive differences, but enough to make me wonder which one I should actually trust when I'm trying to work out state of charge properly.

Has anyone done a proper side-by-side with a clamp meter or a decent multimeter to settle this? I'm half-tempted to grab a Victron BMV-712 and just be done with it, but £100+ feels steep when the rest of the system is already sorted. Curious what others have found.

Hey @Ozzy97, nice setup! The Fogstar Drift is a solid choice. Worth knowing that the SmartSolar only measures the charge current coming in from the panels — it won't account for anything you're drawing simultaneously from the battery. So if your fridge or whatever is pulling 5A while you're charging, the net change at the battery will look quite different to what the Victron shows. That's likely where the discrepancy with your battery monitor creeps in. If you want everything to talk nicely together, a Victron BMV-712 or a SmartShunt will integrate directly with the VictronConnect app and give you a proper whole-system picture. The cheap Amazon shunts aren't terrible but calibration drift can be an issue over time.

Hey @Ozzy97, great van setup! Just to add to what @MidlandsOffGrid is saying — the discrepancy you'll often see is because most cheap Amazon shunts measure all current flowing through them, whereas the Victron is only seeing the solar input side. If you've got anything else charging or discharging simultaneously (say a fridge running whilst the panel's going), the numbers will look quite different from each other. Worth making sure your shunt is installed correctly on the negative bus too — I've seen loads of installs where accessories are bypassing it entirely, which throws everything off. The Victron app is generally very accurate for what it's actually measuring though. 👍

So what's the actual question here — are you asking whether it's worth buying a separate shunt-based monitor, or just why the numbers don't match?

Because if it's the former: yes, get a proper shunt. Running a Fogstar Drift without accurate SOC monitoring is asking for trouble — you'll either undercharge or take it too low without realising. I've got a Victron BMV-712 on my shepherd's hut setup and the SmartSolar readings alone were pretty much useless for knowing actual battery state.

The cheap Amazon ones — do they even use a shunt, or just voltage-based SOC? Because voltage-based on lithium is nearly pointless given how flat the discharge curve is.

What's the spec on the monitor you've actually bought? Makes a difference whether this is worth troubleshooting or just replacing.