How to configure ESS for a given SOC

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damric asked Nov 11, '23

How to configure ESS for a given SOC

I would like to configure my ESS in the following way :

- to keep battery level to 80%

- to give priority to PV power to feed the loads

- feed the battery only in case of excess of PV power

To do so I put the SOC limit at 80% in ESS and the Optimized mode is activated (without BatteryLife).

The problem is the following : the battery is cycling between 80 and 83% and the priority of PV power is not given to the loads.

Indeed, even if PV power is lower to the loads, the PV are first feeding the battery in order to reach something around 83%. Then, a discharge cycle start to come back to 80%. This is not what I would like to do.

I want to keep battery to 80% for safety mode in case of failure of grid loads and I would like to feed the battery (between 80 and 100%) only in case of extra PV power.

To illustrate, the issue :

1/ Even if battery is higher to SOC limit of 80%, the system is feeding the battery with PV power :

first in bulk mode :

then in absorption mode until reaching 83%:

2/ Finally, the battery power is used is discharge mode until 80%

... and this cycle is starting again as long as PV Power is available

How I can solve this issue ? What does I need to modify in ESS configuration ?

Thanks

ESS SOC ess settings

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Answer 1 · 2023-11-11T14:26:12Z

nickdb answered · Nov 11, 2023 at 02:26 PM

ESS has a hysteresis, when it hits the minimum set it only starts discharging again 3% above the minimum. This is to prevent it bouncing against minimum SOC as power is drawn.

This is normal ESS behaviour.

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Answer 2 · 2023-11-11T15:27:55Z

damric answered · Nov 11, 2023 at 03:27 PM

Ok, thanks for your answer.

It is not very efficient to use PV power to feed the battery and then to use battery to feed loads. It seems better to have a direct use of PV power in loads .

But if it is not possible to change I will stay like this.

Thanks again.

8

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nickdb ♦♦ commented · Nov 11, 2023 at 03:42 PM

When you hit the minimum, discharge will stop, loads are serviced by AC, all PV will be used to get above minimum SOC (80% + 3%), once there, discharge is enabled and PV is preferred for loads and further charging is secondary.

You have a very high minimum, so that is not an ideal ESS, if it is struggling to cope with loads and charge then the system balance needs to be looked at again.

It is probably better to just leave it on keep charged.

This is the most efficient way of doing it, as the objective is energy storage, and maintaining minimum charge levels.

For mobile users changes have been made for PV priority, but that does not help fixed installations.

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rickc nickdb ♦♦ commented · Nov 11, 2023 at 03:58 PM

Is this 3% hysteresis documented anywhere in the ESS documentation?

I’ve only heard about it accidentally here.

This behavior bothered me enough over the summer that I wrote a script to continuously adjust the maximum inverter output to track the PV input. This avoided the 3% rapid sawtooth on the SoC level by slowing the discharge. I wanted the PV to only power the loads below a threshold SoC. I’ve stopped doing that, too many moving parts.

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damric rickc commented · Nov 12, 2023 at 09:38 AM

you wrote " I wanted the PV to only power the loads below a threshold SoC". In my situation I need the opposite : I want that PV feed loads above a given threshold SoC. Do you have an idea of how to configure ESS in this case ?

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damric nickdb ♦♦ commented · Nov 12, 2023 at 09:35 AM

The first objectives of my batteries is to prevent grid failure, this is why I need to store as much energy as possible and this is why I set SOC to 80%.

My previous ESS Setting was "keep batteries charged" but in this case when PV power is higher than loads, the energy is lost. In order to avoid to loose energy the idea is to store into the battery instead of feeding for free the grid.

If I set SOC to a lower value the system will work better (ie with more efficiency) ?

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nickdb ♦♦ damric commented · Nov 12, 2023 at 10:13 AM

Once you are 3% above your min SOC, All PV goes to loads, spare PV charges the battery up to 100%, that is default ESS behaviour. Loads above PV generation are supplemented by battery until you hit 80% again.

If you can't service loads AND charge to 100%, in reasonable weather, then your system is undersized.

It should never bounce between the 83% and 80% unless, your ratio of PV to load is off.

If you are easily hitting 100% by day, remaining there, while servicing loads, then reduce your minimum SOC as you are wasting PV.

Some, if allowed, choose to feed back to the grid, which will ensure all spare PV is used.

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damric nickdb ♦♦ commented · Nov 12, 2023 at 12:02 PM

Thanks again for your answer.

We are in winter with lot of cloudy days so it is difficult to have enough PV power. It will be easiest in summer. This is why I have this cycling between 80 and 83% during all the day (and also because I have a constant high loads demand in my system). During such days, there is never enough PV power to fill the battery up to 100%.

My PV is sized to cover with limited extra power the loads which are the same during all the year (around 2kW of constant loads vs 3.2kW of peak PV power). My battery is sized to cover a long duration of grid failure, this means an important battery capacity. Therefore 3% of battery capacity represents a lot of energy (with regards to the rest of my system), it takes time in winter to reach this threshold.

Fianlly I will follow your advice : I will go back to the "keep batteries charged" mode ; at least for the winter season.

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nickdb ♦♦ damric commented · Nov 12, 2023 at 12:07 PM

So in winter, with such a high min SOC, keep charged is probably best.

Alternatively, you can set an inverter max power limit in ESS, so that it will only use a max amount of PV/battery, the balance comes from grid.

You can even set this to 0W so everything is delivered from grid.

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damric nickdb ♦♦ commented · Nov 12, 2023 at 01:02 PM

Ok, I just make the test with this ESS max power limit you describe. This should avoid the cycles of the battery between 80 and 83%, this is a good thing. But this is not helping for my objective of 100% direct use of PV power in loads.

I have to test on a full day in order to find the best compromise.

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