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.

1674749088543.png

1674749876002.png

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)
9 comments
2 |3000
Toggle Comment visibility. Current Visibility: Viewable by all users

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

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 ?



1 Like 1 ·
Show more comments
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.

1 Like 1 ·
Show more comments
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.

8 comments
2 |3000
Toggle Comment visibility. Current Visibility: Viewable by all users

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

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?

0 Likes 0 ·
1674803209975.png (261.9 KiB)
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. =(

0 Likes 0 ·
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.

2 Likes 2 ·
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".

0 Likes 0 ·
1675085134047.png (40.6 KiB)
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.

1 Like 1 ·
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.

1675094254186.png

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.


0 Likes 0 ·
1675094254186.png (16.9 KiB)
seb71 avatar image seb71 kirby commented ·

You still have a large voltage difference between cells. Hopefully the BMS will manage to equalize them.


If no improvement after 1-2 weeks, you should consider returning the battery (if that is an option).


On second thought, you should contact the seller right now, at least for an opinion about your cell voltage issue. Such big differences between cells are not quite acceptable.


I think that "External Control" indicates that (some aspects of the) charging are managed by the battery BMS.


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

Check in VRM (graphs/widgets from Advanced).

0 Likes 0 ·
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.

1 Like 1 ·

Related Resources

VictronConnect VE.Bus charging manual  

DVCC chapter in GX manual  

Additional resources still need to be added for this topic