AC input current limit not fully used when PowerAssist activates

[zedamoca]

System description

• Cerbo GX
• 2 × MultiPlus 48/5000-70 (parallel system)
• MPPT 450/200
• Victron Lynx Smart BMS NG 1000
• Battery bank: 4 × 5 kWh (≈20 kWh)
• Carlo Gavazzi EM24 to measure AC input
• AC input source is a generator with a 20A limit
• AC input current limit most of the time, configured to 12 A
• System operated as off-grid with generator support (not using ESS)

I am running a 2 paralel MultiPlus system in off-grid mode with generator support.

The generator is automatically started when SoC reaches 35% and stopped again at 60%.

While the load remains below the configured AC input current limit, whatever is set there, (I like to set 12A since is the minimum accepted), the system behaves as expected and draws power from AC input under External Control.

However, when the load exceeds the AC input limit, the system switches into PowerAssist. At that point, instead of continuing to draw close to the full limit configured from AC input and using the battery only to cover the excess load, the system actually reduces the power taken from AC input, to a value lower than the output of the inverters.
As a result, the battery starts to discharge slowly over time, even though AC input is available.

This creates a risky situation where SoC can continue to fall while the generator is running, potentially leading to low-SoC shutdown of loads.

Look at this nice example of today.

The inverters start pulling less power once the AC raises above the input.
example around 7:30 and then it became permanent around 9:10
In this case the sun panels brought the difference and actually filled the batteries above 60%, leading the gen to stop.

Some screenshots of my VEConfigure3:

Setting the AC input current limit to 18A, solves the problem 99% of the time, as long as I charge the car at 6A, but this is not a clean solution.

I’m so confused and I have read so much manuals that I have to ask: What am I missing?
Is there a way (setting, configuration, or control method) to force the system to always draw as close as possible to the configured AC input current limit, even when PowerAssist is active?

Thank you in advance for any helpful input!

[M_Lange (Matthias Lange - DE)]

(not 100% sure but)I think this is due to the minimum PowerAssist current.
Each MultiPlus has a minimum PowerAssist current, that means if it starts with PowerAssist it does that at least with that current.

For the MultiPlus-II 48/5000 that current is 1,6A.

The full list is available in the VE.Bus firmware section of Victron Professional.

[zedamoca]

indeed.

so, let me see if I understood this you say;

When the inverters go into PowerAssist mode, they need to feed 1.6A minimum (probable 3.2 in my case because I have 2 in paralel), being the difference given by the grid. this can lead to this difference being less than the maximum configured.

is it this simple?

[M_Lange (Matthias Lange - DE)]

That’s how I understand it too.

[zedamoca]

3.2A is in the 700 - 750W range. it matches my observations…

EDIT: its actually more

my observations are around 4A gap

this is dangerous… and it becomes even more dangerous the more inverters in paralel you have because the gap grows…