ESS oscillates between charging and discharging

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felr asked Mar 4, '23

ESS oscillates between charging and discharging

In my ESS system, the maximum inverter power is externaly set to 0 W when a certain SOC is reached to prevent further battery discharge. In the range between 100% and 60% this regulation worked without problems. As now slowly more sun is available again I have lowered the limit to 50%.

Now a strange behavior shows up here:

If the limit value is reached, the inverter switches off as configured. At the same time, the Multiplus starts to charge the battery with 100-200W from the grid, so that the limit value is exceeded again after a short time and the inverter switches on again. Thus, the system oscillates between discharging and charging for hours.

Why does the MP charge from gird at all when ESS is set to Optimized (w/o Battery Life)? The battery voltage never falls below the low voltage threshold and the SOC never reaches below the ESS minimum SOC. Do you have any idea how to prevent this behavior?

Multiplus-II battery charging ESS

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Alexandra ♦ commented · Mar 04, 2023 at 12:13 PM

What do you have your dynamic cut off set to? You could be triggering sustain? Or something like that producing the weird behaviour.

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felr Alexandra ♦ commented · Mar 04, 2023 at 05:23 PM

Thank you for this tip. Just checked my configuration. I'm using Pylontech LFP batteries, which are configured according to the Victron support document to a float voltage of 46V for all discharge currents. The voltage during periods with this issue is always above that.

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Alexandra ♦ felr commented · Mar 04, 2023 at 05:27 PM

Pylons float at 51v and have a dynamic cut off of 46v or at least i assume thats what you mean. Are the cells relatively balanced then? When was the last time they were at full charge?

And the pylontec is not changing its signal for charge and discharge?. My thinking is there is conflicting signals somewhere anyway.

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felr Alexandra ♦ commented · Mar 04, 2023 at 05:41 PM

Yes, with 46 V I meant the dynamic cutoff voltage. Got somehow mixed up during translation.

Last full charge was two days ago and the cells are well balanced (no cell voltage difference below 90% SOC and above max. 30 mV). Since then the battery voltage never went below 48 V.

The pylontech batteries recognize the changing current flow direction almost instantly and switch their state from discharging to charging and back accordingly. So I don't think communication between the batteries and the Cerbo unit is an issue here.

If I externally send the Modbus command to disable the charger in the Multiplus units, charging stops. After enabling it again the oscillation continues.

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felr Alexandra ♦ commented · Mar 04, 2023 at 06:56 PM

Another idea: In my configuration "Has DC System" is enabled as I do have some DC loads (about 40W). When Max Inverter Power is set to 0, does the system try to power the DC loads from grid instead of the battery? This could explain the low-power charging.

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Alexandra ♦ felr commented · Mar 06, 2023 at 05:41 PM

@FelR

As far as i am aware it does not power DC loads from AC input. So yes the draw down and recharge makes sense with that piece of information.

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felr Alexandra ♦ commented · Mar 06, 2023 at 07:38 PM

To me it doesn't really make sense. The inverter is deactivated using a modbus signal at 50% SOC. ESS mimium SOC is still far off at 10%.So the beheavior I would expect, is the DC loads to still be powered by the battery while the Multiplus simply does nothing until the ESS minium SOC is reached. Instead it obviously tries to compensate for the discharge by the DC loads.

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