Getting the Smartshunt AUX to work

Been meaning to post about this for a while — finally getting round to it after a frustrating weekend.

I've got a Victron SmartShunt on my main LiFePO4 bank (Fogstar Drift 200Ah if anyone's curious) and I've been trying to get the AUX input doing something useful. The docs aren't exactly crystal clear on whether to configure it as a second battery voltage monitor or midpoint monitoring, and I'm not sure which makes more sense for my setup.

My thinking was to use it to keep an eye on my starter battery, which sits separate from the main off-grid bank — seemed like a neat solution rather than running a whole second shunt. Anyone actually done this? Does the data feed cleanly into GX/Cerbo or does it just sit in the VictronConnect app and go nowhere useful?

The other option I've seen mentioned is midpoint monitoring if you're running a series/parallel pack, but with a single lithium unit it feels a bit pointless.

A few specific things I'm unclear on:

• Voltage range — is the AUX input protected if the starter battery spikes above 15V during charging?
• Logging — does it appear on VRM portal or is it essentially invisible outside the app?
• Temperature input — is it genuinely one or the other (temp or voltage), no way round it?

Would love to hear from anyone who's got this working cleanly, especially if you're running it alongside a Cerbo GX on a UK off-grid setup rather than marine. Most of the decent write-ups I find are boat-based and the wiring context is slightly different.

@LazyBodger the AUX input is a bit of a rabbit hole tbh. What are you trying to monitor with it — second battery, midpoint, or temp sensor? Makes a big difference to how you set it up in the VictronConnect app.

Worth noting the voltage range on the AUX is limited, so if you're measuring a 12V starter battery alongside your Fogstar you need to make sure you've got the wiring right or it just reads garbage.

Had mine set up for midpoint monitoring on a twin-battery setup for a while — the alerts are actually pretty useful once you dial in the deviation percentage.

@LazyBodger I went down exactly this rabbit hole last spring when I was finishing off the cabin wiring. Ended up using the AUX input to monitor my starter battery — the SmartShunt treats it as a simple voltage sense rather than a proper coulomb-counted channel, which tripped me up initially. Took me an embarrassing amount of head-scratching before I realised the VictronConnect app just shows it as a raw voltage reading under "Aux voltage" in the history screen. Nothing fancy. If you're expecting SOC data for a second bank you'll be disappointed — it genuinely isn't designed for that. What's your actual end goal? Might be worth knowing before you go further down that particular hole.

@LazyBodger worth knowing the AUX input only does one thing at a time — you configure it in the VictronConnect app under SmartShunt settings as either midpoint voltage, starter battery voltage, or temperature (with the optional temp sensor dongle).

If you're running a second LiFePO4 as a starter/aux bank, the voltage monitoring works well enough, but don't expect SoC — it's raw voltage only on that input.

One gotcha I hit on my shepherd's hut build: the AUX wire needs to reference the same negative as the shunt's negative terminal, otherwise you get bizarre offset readings. Spent an embarrassing amount of time chasing that before realising my aux ground was floating slightly.

What's your actual goal with it? Midpoint monitoring on a series pack, or a separate battery altogether? Changes the wiring approach significantly.

@Macca64 beat me to the "one thing at a time" bit, but worth adding — if you're in a shepherd's hut like me, I've got mine set to starter battery voltage for the van battery that runs my water pump, and it's been rock solid through two winters of "why is everything frozen" discoveries.

@PikeGazer snap on the shepherd's hut setup — mine's got a similar arrangement but I've taken it a step further and wired the AUX to a separate starter battery on the boat too, which is a whole different use case.

One thing nobody's mentioned yet: the AUX input for temperature actually feeds into the charge history and SOC calculations, so it's not just a readout — it genuinely improves accuracy if your battery's somewhere that gets cold overnight. On the boat that matters enormously come January.

Worth downloading the SmartShunt manual PDF from Victron's site rather than relying on the app tooltips alone — page 11 covers the AUX wiring diagrams properly and saved me a lot of head-scratching.