Cerbo GX Generator Automation Using Auxiliary Relay on Digital Input for Grid Sensing

DIY
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1

Hi everyone,

I am designing a Victron-based hybrid ESS system for a commercial market project. The system will consist of a Victron MultiPlus, a Cerbo GX, and a LiFePO4 battery bank.

The site currently has a Grid-Generator ATS (Automatic Transfer Switch) panel. In the new setup, when the grid fails, I do not want the generator to start immediately. Instead, the MultiPlus will invert from the battery bank to power the critical loads (POS terminals, network devices, 126W linear LED lights). If the battery SOC drops below a certain threshold, the Cerbo GX (via Relay 1) will send a “Start” signal to the generator.

Since the MultiPlus has a single AC-In, the combined output from the existing ATS panel will feed the MultiPlus AC-In. Therefore, the MultiPlus cannot natively distinguish whether the incoming AC power is from the generator or the grid.

To overcome this, I am planning to install a 230V auxiliary relay on the grid input line (before the ATS panel) and wire its dry contact output to one of the Digital Inputs on the Cerbo GX.

The Proposed Logic:

  1. While the grid is available, the auxiliary relay is energized, telling the Cerbo GX that “Grid is present”.
  2. When the grid fails, the relay drops out. Cerbo GX senses this and triggers the generator based on battery SOC conditions.
  3. While the generator is running and feeding the MultiPlus, if the grid returns, the auxiliary relay on the grid line will energize again. The Cerbo GX will immediately detect this via the Digital Input, release the generator “Start” signal (allowing the generator to cool down/stop), and the ATS will transfer the load back to the grid.

Has anyone implemented this exact setup (Auxiliary Relay + Cerbo GX Digital Input for grid sensing) on a single AC-In MultiPlus installation? How reliable is the Venus OS “Generator Start/Stop” logic when mapped to a Digital Input condition for stopping the generator?

Are there any potential issues or timing/synchronization conflicts I should be aware of (e.g., during the ATS transfer back to the grid while the generator is shutting down)?

Any insights or recommendations would be highly appreciated. Thanks in advance!

2

As I understand it, there’s a fundamental flaw in your reasoning:
In an ESS system, the AC IN terminal is always connected to the utility grid.
Connecting a generator to the AC IN terminal in an ESS system is technically not possible.
There are may postings about generator and ESS system in this forum

  1. Use the Quattro instead of a Multiplus in that configuration.

Or use

AC Input Control

After a quick read-through of your text, this comes closest to the point.

4

There is a mains-generator transfer panel upstream of the AC-IN terminal. Consequently, the AC-IN input perceives the supply as mains power. However, the mains might actually be down and the generator running. How can I distinguish between the two?

5

So, if I understand it correctly, I would set it up like this:

NO ESS!

Mains or generator connected to AC in (relay switch)

If the mains power fails, the whole system runs on battery power.

Using the Cerbo, I program the generator to start, for example, at 20% SOC and stop at 100%. Then the cycle starts over until mains power is restored.

If the battery doesn’t have a CAN bus, this should also work with the Multiplus (old style) assistants without a Cerbo GX incl generator start ;

As I understand it, with an ATS, the generator starts immediately upon a power outage.
To my understanding, this function is not desired here.

All of this is possible with the smallest Multiplus!

6

Normally, an ATS engages the generator when grid power is lost. However, I have a Victron ESS system that includes a Cerbo unit.

  1. When the grid fails, I want the Victron ESS to take over, but I do not want the Cerbo to start the generator (whereas a standard ATS logic would try to start the generator immediately, I want the Cerbo to prevent this).
    1.a. When grid power returns, the Victron ESS should disengage, and the grid should supply the system.
  2. If the Victron ESS is active and the battery level drops below a certain threshold—and grid power is still unavailable—then the generator should start.
    I am not sure if I have explained this clearly, but if this is possible, I need a way to distinguish whether the power entering the Victron AC-IN terminal is coming from the grid or the generator.
    Can I achieve this using the Cerbo or auxiliary relays? Thanks
7

English isn’t my native language, so communication might be a little rough around the edges ;-).

You need to prevent the ATS Generator from starting automatically and let Cerbo handle this (manually) instead when the battery comes low.

It’s actually quite simple.

Perhaps this video helps

8

Simple idea:

You simply route the ATS’s alternator start signal through the Cerbo’s alternator start/stop relay.

The alternator won’t start until the Cerbo closes the contact.

What brand of alternator do you have?