ESS - Not reaching lower limit SoC

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tozz asked Mar 28, '21

ESS - Not reaching lower limit SoC

Hello. I built my first ESS the past few days, but having a issue with the system not wanting to drain the batteries to my configured lower limit SoC. My setup consists of:

1x MG Master LV 600A - Connected to Cerbo using VE.Can

2x MG 5000 Wh 24V 200Ah batteries in series, thus 48V (=10 kWh)

1x Victron Cerbo GX + GX Touch

1x Victron MultiPlus-II 48/5000-70/50 - AC Coupled to grid on AC-in. Outputs are unused.

1x SolarEdge Inverter - AC Coupled to the grid

1x EM24 Ethernet grid meter

The issues are:

At basically any SoC, while discharging, the low batt warning starts to blink.
At around 60% SoC inverter stops and low batt is continuously on

I noticed a few things:

In VE.Configure I can't select lithium battery system. It only shows Gel/AGM, LiFePo and some other battery types. But not 'Li-Ion' . Should i choose LiFePo ? Or what should i choose?
The cut-off voltages in the ' Inverter' tab in VE.Configure look okay (37,2V, 43,6V and 43,6V). But from what I understand, those voltages are ignored?

My settings:

Settings -> ESS -> Mode : Optimized with BatteryLife
Settings -> ESS -> Minimum SoC : 10%
Settings -> ESS -> Active SOC Limit: 15%

Settings -> DVCC -> DVCC: On
Settings -> DVCC -> SVS: On
Settings -> DVCC -> STS: On
Settings -> DVCC -> Temperature sensor: Automatic
Settings -> DVCC -> Used Sensor: Lynx Ion BMS 600A on VE.Can
Settings -> DVCC -> SCS: On
Settings -> DVCC -> SCS Status: Disabled (External control)

Configured battery type:

LiFePo4 with other type BMS (Connected via CAN-Bus)

My questions:

Which battery type should I choose in the ESS Assistant?
What should my sustain voltage me? According tot he battery datasheet the cut-off voltage is 21V. In series that would be 42V. Should I set the sustain voltage to 42V or not?
Should I enable BatteryLife with MG HE Lithium batteries? Some Internet answers says BatteryLife is not intended to be used with Lithium batteries, yet VE.Configure enables is.

Most important question:

What should I change to allow the system to discharge to my Minimum SoC = 10%

ESS discharge level

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Answer 1 · 2021-03-28T13:12:26Z

seb71 answered · Mar 28, 2021 at 01:12 PM

When using ESS, the voltage limits set there are in effect.

What kind of Li-Ion batteries are yours? Anyway, you probably should select Lithium/LiFePO4, where you have that choice.

Edit:
Apparently your battery is NMC.

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tozz commented · Mar 28, 2021 at 01:48 PM

If the voltage limits set there are in effect ,any ideas why the system stops with a low batt error at 60% SoC?

And would be LiFePo4 be the correct battery type for NMC Li-Ion? Perhaps by lack of a better option?

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Answer 2 · 2021-03-28T13:52:59Z

seb71 answered · Mar 28, 2021 at 01:52 PM

If the voltage limits set there are in effect ,any ideas why the system stops with a low batt error at 60% SoC?

The logical assumption would be that the voltage actually drops that low.

And would be LiFePo4 be the correct battery type for NMC Li-Ion? Perhaps by lack of a better option?

Yes, but voltages for NMC are different than voltages for LiFePO4. So it should only be a starting point.

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Answer 3 · 2021-03-28T13:59:26Z

seb71 answered · Mar 28, 2021 at 01:59 PM

Should I enable BatteryLife with MG HE Lithium batteries? Some Internet answers says BatteryLife is not intended to be used with Lithium batteries, yet VE.Configure enables is.

My opinion is to not enable BatteryLife for Lithium batteries. BatteryLife is better suited for Lead-Acid batteries, which need to fully recharge often.

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Answer 4 · 2021-03-28T14:01:43Z

seb71 answered · Mar 28, 2021 at 02:01 PM

What should my sustain voltage me? According tot he battery datasheet the cut-off voltage is 21V. In series that would be 42V. Should I set the sustain voltage to 42V or not?

Sustain voltage should be higher than the voltage at which the inverter is cut off.

So first you have to decide on Dynamic cut off limits.

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Answer 5 · 2021-03-28T15:30:25Z

tozz answered · Mar 28, 2021 at 03:30 PM

@Seb71 Thanks. I've now confiured the following parameters. Please let me know if you think this is okay

From the datasheet:

Max. charge voltage: 29.4 x 2 = 58.8V
Discharge cut-voltage: 21.0 x 2 = 42.0V

I changed the settings to:

Max. charge current: 60A (from datasheet)
Sustain voltage: 43V
Dynamic cut-off:
0.005C = 45
0.25C = 44
0.7C = 43
2C = 42

(VE.Configure says that cut-off voltage in ' Inverter ' tab is ignored when using dynamic cut-off)

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seb71 commented · Mar 28, 2021 at 04:06 PM

I must start by saying that I use LiFePO4 cells. I don't have practical experience with NMC cells on solar power systems and there are differences.

When deciding about voltages, always analyze the cells.

You have 7 cells in series in each "24V" battery, so 14 cells in series in total.

29.4V (max) and 21V (min) would be 4.2V (max cell voltage) and 3V (min cell voltage). That is in the ideal case, when all cells are perfectly identical. What that means is you can have 29.4V battery voltage and some cell voltages above 4.2V (and others below 4.2V).

Do you know if your BMS disconnects the load or stops the charge also based on individual cell voltage (not only based on battery voltage)?

Do you know at which voltage the BMS starts to balance the cell voltages?

I would restrict the maximum voltage (charging voltage) lower than 4.2V, but high enough for the BMS to still be able to perform cell balancing.

With NMC cells you want to avoid overcharging at all cost. Electronics might fail and then you might have a big problem.

3V cell voltage on the low end seems reasonable.

You could test with those Dynamic cu off values you listed and adjust if necessary. Maybe lower the first two values (45V and 44V). You want these dynamic cut off limits to act as a safety. In normal use you want to stop the discharge when the Minimum SOC is reached (and that should happen before the voltage reaches the dynamic cut off values).

Increase the Sustain voltage above the highest dynamic cut off voltage (above the voltage set for the cut off voltage at 0.00C discharge rate).

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tozz seb71 commented · Mar 28, 2021 at 06:08 PM

Thanks. I'm not sure if the BMS will disconnect based on individual cell voltage. However, I do see min/max cell voltage in the Cerbo, so it atleast measures is. I assume that implies it will also do something with those values.

I've read all the documentation there is from the Lynx ION BMS, but there isn't much detail about the inner workings of the device.

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tozz commented · Mar 29, 2021 at 07:17 AM

@seb71 Thanks again! Battery was depleted to 10% last night. I might need more batteries.

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