How to get audible alarm for low SoC on Cerbo / Ekrano GX?

Been wrestling with this one on my tiny house build and wondering if anyone else has hit the same wall.

I've got a Cerbo GX running v3.74 with a Victron SmartShunt and a Fogstar Drift 200Ah battery. I want a proper audible alarm when SoC drops below, say, 20% — nothing fancy, just something loud enough that I'd actually hear it if I'm outside chopping wood or whatever.

I know the Cerbo has relay outputs, and I've wired up a small 12V sounder to Relay 1. The relay does trigger for certain alarm conditions, but I'm struggling to get it to fire specifically on low SoC threshold rather than just general battery alarms.

A few questions I'm trying to work out:

• Can you map a specific SoC trigger to a relay directly in VRM or the GX device menus, or does it need a Generator Start/Stop condition workaround?
• Has anyone used Node-RED on the Cerbo to set this up more precisely? I've got it enabled but barely touched it
• Is there a way to do it without Node-RED for those of us who aren't particularly code-savvy?

I've had a poke around the Victron community forums but a lot of the answers seem to assume a more complex multiplus setup than what I've got. My system is fairly simple — no inverter/charger involved, just solar via a Victron MPPT.

Would be really useful to hear from anyone running a similar bare-bones setup. How have you handled low battery alerts in practice?

@PaddyGibson the Cerbo itself has no buzzer hardware, so you need to route it externally.

On my motorhome setup I use the relay output on the Cerbo (normally-open, switches at your chosen SoC threshold via Venus OS alarm rules) wired to a 12V piezo buzzer. Takes about 20 minutes to configure in Settings → Relay → Function → Generator start/stop — actually repurpose it for low battery alarm logic instead.

Alternatively if you've got a Victron MultiPlus or MPPT on the VE.Bus/VE.Direct network, those can generate audible signals through the GX Touch panel's connected devices.

The cleanest software-only route is Node-RED (available via Venus OS Large image) — trigger an MQTT alert to a networked speaker or smart plug with a siren. Slightly more involved but incredibly flexible once it's running.

What's your inverter situation? That might open up simpler options.

@BordersExplorer is right about the hardware limitation. What I do on the narrowboat is use one of the Cerbo's relay outputs (Relay 1 is normally-open by default) — set a relay rule in the GX device menu to trigger below your chosen SoC threshold, then wire a 12V piezo buzzer across it. Cost me about £3 off eBay.

Alternatively if you've got a Venus OS device on your network, you can use Node-RED (built into Large image) to push a notification to your phone via Telegram or Pushover — handy when you're away from the boat.

The relay route is more reliable for a permanent install though. Just make sure your buzzer draw is within the relay's 6A contact rating — most small buzzers are well under 1A so no issues there.

Really useful thread this — been pondering the same thing for my own setup.

Quick question for @ExSquaddie49 — when you configure the relay output on the Cerbo, are you triggering it purely on SoC percentage, or are you using the battery voltage as the condition? I'm wondering which is more reliable day-to-day, especially with LiFePO4 where the voltage curve is so flat.

Also, does anyone know if you can set hysteresis on the relay condition so the alarm isn't constantly chattering on and off around the threshold? That'd drive me mad on a narrowboat — respect for dealing with that!

I've got a Fogstar Drift 200Ah too @PaddyGibson so would love to know what SoC threshold you end up settling on once you get this sorted. 90% of the time I'm guessing people set it too conservatively?

Good shout from @ExSquaddie49 on the relay approach — that's exactly how I've got it wired on the narrowboat.

One thing worth adding: in the Cerbo's Relay settings, you can configure Relay 1 as a "Generator start/stop" or set it to manual/alarm mode. For low SoC alerts, go to Settings → Relay → Function → Alarm relay — this ties it directly to the VRM alarm conditions you've already defined.

Wire a 12V piezo buzzer (Maplins-era stuff or grab one off Amazon, £3) across the relay output with a small fused spur. Pulls essentially no current so no concern there.

The Fogstar Drift doesn't have its own BMS alarm output, so the SmartShunt SoC threshold is your trigger point — set it around 20% in Settings → System Setup → Battery monitor.

Works reliably on mine, been running 18 months without issue.