question

kirby avatar image
kirby asked

Setting DVCC volts for LFP 16 cells battery

I have LFP 100Ah 16 cells from "XD battery" and MultiPlus 48/5000. Both connected via Cerbo GX, via VE.Can configured as BMS Can. Related configuration already done in VE Configure.

XD Battery datasheet (got it from the vendor).

I always getting over-voltage error, as on Cerbo GX, as on battery BMS.

I started from 56V and went to 54.9V charging voltage. The last one is not enough to have 100%.

Whole process diagram:

lfp-xd-battery-diff-cell-volts.gif


At the point (1:12) where lowest voltage goes down, the inverter stopped charging.

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Is there something wrong with BMS or/and VEConfig settings, or anything else?

Why highest voltage raises and lowest goes down after stop charging?

battery chargingDVCCbattery system voltageovervoltage
lfp-xd-battery-diff-cell-volts.gif (518.1 KiB)
1674749088543.png (33.8 KiB)
1674749876002.png (26.6 KiB)
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ingo21 avatar image ingo21 commented ·
your cell voltage between highest and lowest is quite a difference 3.71 and 3.36.

i guess your over voltage error comes from the one cell which goes up to 3.71 .

do you active balance your cells while charging ?



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kirby avatar image kirby ingo21 commented ·
I don't balance it. It's on its own. I mean, I have no control on it.

Yes, obviously 3.71 is too high, I just don't understand how to prevent it...

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Alexandra avatar image Alexandra ♦ commented ·

@kirby

I agree with @Ingo21

Charge your battery with less amps, the cell balancing system in there cannot keep up.

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kirby avatar image kirby Alexandra ♦ commented ·

@Alexandra that's a good point. Tested with 15A max – the same.

Is there an option to have low amps on top battery levels? Let's say, at 95% it would set it to 5A and at 98% at 2A.


I have just hard situation with electricity, I can have 40% battery and only 2 h for charing. Next blackout would be 4-6 h. =( So, constant low amps is not an option.

Now I have 30A max for charging.

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Alexandra avatar image Alexandra ♦ kirby commented ·
@kirby

Usually the battery requests a lower amperage when connected or has supoorted comms.

You can also set your percentage when bulk is finished lower on the inverter but that mostly affects grid charging from what I understand.

Are they new batteries. Sometimes they need to be cycled a few times before they calm down and balance nicely.

What would normally happen is once the target voltage has been reached the amps back down, then it would be the absorption (or balancing) phase of charge for the time set if you have one. Then it should drop to float.

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kirby avatar image kirby Alexandra ♦ commented ·

@Alexandra

Are they new batteries.

Yes, the battery is kind of new, but in active usage 2-3 months. I have 2-4 blackouts every day.

Usually the battery requests a lower amperage when connected or has supoorted comms.

But how low apms should be? 0.1C, 0.05C?

You can also set your percentage when bulk is finished lower on the inverter but that mostly affects grid charging from what I understand.

Yes, I charge it from the grid only.
Are talking about changing Absorption Voltage for that?

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Alexandra avatar image Alexandra ♦ kirby commented ·
@kirby

Thr C rating is set by the manufacturer. So what do they recommend?

I have not seen much in the waybof information on the batteries you are using.

You can change the absorption on the inverter or cap it in DVCC on the GX.

But on the inverter, the percentage when bulk is done is set on the internal battery monitor setting.

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kirby avatar image kirby Alexandra ♦ commented ·

Hi @Alexandra

Thr C rating is set by the manufacturer. So what do they recommend?

Here is XD Battery datasheet.

In the datasheet it says up to 60A, but it fact the battery limits this value up to 30A when it's connected to Cerbo.

You can change the absorption on the inverter or cap it in DVCC on the GX.
But on the inverter, the percentage when bulk is done is set on the internal battery monitor setting.

I did set Float/Absorption Voltage as snoobler recomeded – getting High Voltage yet.

I have disabled checkbox "Enable battery monitor" in VE Configure as the battery connected to Cerbo directly.
Do you think it must be enabled?


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Alexandra avatar image Alexandra ♦ kirby commented ·
Yeah a lot of manufacturers give a max - but then recommend half that, or at least that is the current trend, especially if they know their balancing is slow. the sheet you linked recommends 100A max.

the high voltage on the cells is from a cell running higher, usually a warning is ok and alarm not so much. do you see a CCL as well? does the BMS request no charge when it is high like that?

I usually enable the internal monitor but then we use ESS so it must be programmed there. The VE bus usually uses the battery SOC anyway.

I had a random name battery do this once it took a few really deep discharges and it calmed down, might be that is all this will take.

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1 Answer
snoobler avatar image
snoobler answered ·

Is there something wrong with BMS or/and VEConfig settings, or anything else?

Imbalanced battery.

Reduce absorption to 54.4V. Float to 54.3V. Increase them 0.1V incrementally until the high voltage cuts off again. Lower it to the maximum voltage it can sustain without triggering protection. It needs to be maintained at this voltage for as long as possible to allow the BMS to balance.

Once you attain 55.2V, you can get to 98%+ charge if given enough time. For faster charging, you need to get to 56.8V.

I understand you have charge limitations; however, certain fixes require certain capabilities. if you can't provide long lower voltage charging, you need to:

1) have the battery replaced with one that works properly.

2) Do as I recommended as best you can.

3) Live with it. It should very slowly improve over time.


Why highest voltage raises and lowest goes down after stop charging?

The BMS is likely signaling for reduced current due to 3.65V being hit. Reduced current causes low voltage cell to decrease as this is normal with LFP. The decreased current still pushes the high cell into protection mode, so current is cut.

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kirby avatar image kirby commented ·

@snoobler, thanks for sharing this.

Questions

1) Do values for Absorption/Float voltage have influence in case an inverter in "External Control" mode? I don't see "Float" indicator is lighted on once I connected battery to Cerbo.

That's a report for a week, Float/Absorption in the red box.

1674803209975.png


2) And what about charge curve? Shall it be "Fixed"-only for LFP batteries?

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kirby avatar image kirby commented ·

Hi @snoobler,

Reduce absorption to 54.4V. Float to 54.3V. Increase them 0.1V incrementally until the high voltage cuts off again.

Does it mean, when I have "Absorption=54.4V, Float=54.3V" I should not have "High Voltage" error? If I understood correctly, yes.

So, I set these settings, and still getting "High Voltage" error. =(

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seb71 avatar image seb71 kirby commented ·

So it looks like the cells are not top balanced. Or you have some defective cell(s).


Reduce the Absorption voltage even further. Also decrease the charging current to 10A (C/10).

If that BMS does cell balancing, hopefully over time it will bring all cells closer at the top (when full).


Avoid discharging the battery close to empty. The issue with out of balance cells might be caused by non-matching cells (or even defective cells). Discharging the battery close to empty will make the cells even more unbalanced in cases as yours. Check the lowest cell voltage when discharging (so under your normal loads). No cell should go lower than 3.0V ever.


When/if the high voltage alerts disappear, you can gradually increase the charging current and the Absorption voltage.


Set Float voltage to 53.6V (3.35V x 16cells). Leave it at this value even if/after you get the cells top balanced.

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kirby avatar image kirby seb71 commented ·

@Seb71

Thanks for your comment. I never used battery lower 20% lower and usually it's down to 50% max.

So, I've set float 53.6V, absorption 54.4V, and these 83% set 10A charge current.

1675085134047.png

But how system should be behave with these settings?.. It just constantly continue charging the battery. I still have concerns about "Ext. control".

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seb71 avatar image seb71 kirby commented ·

Set absorption lower than 54.4V if you still get cells with high voltage warnings.


SOC is not that important here. What matters is the voltage. So look at cell voltages. When discharging, no cell should go below 3.0V, as I wrote.


But how system should be behave with these settings?

You should monitor the cell voltages especially when the battery is in the Absorption stage. If the BMS does its job, you should see that over time (after a few charge/discharge cycles) all cells are being brought to same voltage near the top (top means fully charged or close to fully charged).

Once you see that, you can gradually increase the Absorption voltage (maybe also the charging current), in small steps.

You might notice again that some cells get to higher voltage. In that case, wait for the BMS to equalize the cell state of charge again.

If/when all is OK (no high voltage alerts), increase the Absorption voltage again.

And so on.


Absolute maximum is 3.6V cell voltage (so for 16 cells, that would be 57.6V battery voltage with perfect cells), but in practice you (should) never get that high.

3.55V is a reasonable maximum, so that means the goal is to be able to set the Absorption voltage at 56.8V and not have any high voltage warnings.

If that's not possible, aim for 3.50V cell voltage, which - with equal cells - would mean 56.0V.


So a recap:

No cell should go under 3.0V or above 3.6V. Never.


Regarding the charging current - gradually increase this too, but stop when/if you reach 50A (C/2). Or you can keep it lower than 50A - say 30-35A. This is the end goal, not the charging current used at the start of this procedure.

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kirby avatar image kirby seb71 commented ·

@Seb71 Thanks for these details. So far not good (absorption=53.90/float=53.60/current=10A)... will test it.

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And quick question: Does configuration for Float/Absorption Voltage have influence when a battery in "Ext. control" mode?

Nobody answered me yet directly. I don't know, maybe that's obvious...

And another thing, when "Ext. control" mode, there is no way to check whether the battery on float/absorption or only bulk. That's why I have doubts all those modes are in use.


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snoobler avatar image snoobler kirby commented ·
Then per @Seb71 's recommendations, lower absorption further.


You may wish to reach out to XD Battery for replacement. Extreme imbalance can take weeks/months to resolve with a typical passive balance BMS.

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Related Resources

VictronConnect VE.Bus charging manual  

DVCC chapter in GX manual  

Additional resources still need to be added for this topic