DIY 24v Battery - do I need to balance cells?

[Gic1622]

Hi everyone!

I just receive my 8 lishen 3.2V 200Ah cells from China.
Values are : 3.282, 3.278, 3.280, 3.275, 3.194, 3.195, 3.197 and 3.194.

I also have a Daly bms with separate port and a balancer.

This is my first battery project and I'm looking for advices... ?

Do I need to balance them or the voltage range is OK?

In addition, since I also have a stabilized power supply(0-30V / 0-5A), do I need to complete charge or is it useless?

The battery will join a 1200Wc off grid solar system including Victron charger and Victron battery protection.

Many thanks in advance!

 

[snoobler]

Assuming these are not "matched' cells for which you paid a noteworthy premium.

3.19-3.28 total range. Not great.

Do you have to balance them? No. Should you? Yes.

What BMS do you have?

 

[Gic1622]

Not sure about the subtility... (sorry, I'm French! ?). You mean that this is not mandatory but better to balance?
My bms is a Daly 18650 smart LiFePo4 8S 24V 100A with separate port.
I also have a balancer (https://a.aliexpress.com/_B0dx8j)

 

[snoobler]

I would. Your cells are likely at notably different states of charge.

Video and downloadable guide:

Top Balancing LiFePo4 Cells using a low cost benchtop power supply.

To get the paper, click on the orange at the top of this page. This is a tutorial with detailed steps on how I top-balance my LiFePO4 Cells using a low-cost benchtop power supply. To get the tutorial, click on the orange "Download" button...

diysolarforum.com

 

[Gic1622]

??

 

[Gic1622]

Just to keep you informed. I discharged the cells. The 8 cells were really unbalanced from 2.18 to 3.0. I did a bottom balance and every single cell reaches 2.85V.
Then, I connected my Daly smart bms and start to charge my pack thanks to my solar charger (Victron mppt smartsolar charger). Not enough sun to charge it in one time. After 1 day, I reached 75% (3100 Wh of production) After 1 night, it went down to 60 % (evening consumption, this is an off grid system).
Today, very poor sun, only 1.5 kWh, the battery reached 85%.
I closely monitor the balance of cells and BMS does a very good job. The voltage difference never exceeds 0.03V. It looks very good.

What do you think?

 

[snoobler]

Bottom balancing is generally not a good idea unless you plan to take it to empty while rarely charging it to full. If you only plan to take it to empty occasionally, yet charge it to near full most days, top balance.

The slow charges help with the balance charging.

 

[John Frum]

The 8 cells were really unbalanced from 2.18 to 3.0.

The hard limit on most cells is 2.5 volts.
Here is hoping you have an outlier that can be safely discharged to 2.18 volts.

 

[Gic1622]

The hard limit on most cells is 2.5 volts.
Here is hoping you have an outlier that can be safely discharged to 2.18 volts.

You're right. I did it one time to do a bottom balance. Now, BMS is set to protect below 2.5V.

 

[Gic1622]

Bottom balancing is generally not a good idea unless you plan to take it to empty while rarely charging it to full. If you only plan to take it to empty occasionally, yet charge it to near full most days, top balance.

The slow charges help with the balance charging.

I did both. After the bottom balance, I fully charged (solar so not so quick). It took 2 days and half. Balance has been done thanks to my bms all along the charge process. I guess I can consider it as a top balance since voltage is 27V and difference of tension is 0.004V!

 

[John Frum]

I did both. After the bottom balance, I fully charged (solar so not so quick). It took 2 days and half. Balance has been done thanks to my bms all along the charge process. I guess I can consider it as a top balance since voltage is 27V and difference of tension is 0.004V!

Unless the batteries are perfectly(which is extremely unlikely) the pack can only be balanced at one point.
For our application that is the top.

 

[Gic1622]

And what about the part of the bms plays all along the charge /discharge process?

 

[John Frum]

And what about the part of the bms plays all along the charge /discharge process?

Assuming a fet based bms, the fets switch the charge/load based on a bunch of criteria.
Such as high/low pack voltage, high/low cell voltage and high current.
Does that answer your question.
Not even sure if it was directed to me but took a shot anyway.

The other thing that a bms does is passive cell balancing.

 

[Luthj]

The BMS serves two main purposes.

1: Last line of defense to prevent cell damage from: Over/Under voltage, Over/under temp. This is done via the disconnect functionality.
2: Perform balancing to keep the cells synced through normal cycling.

 

[Gic1622]

Thanks!
After 2 days of cloudy weather, sun is back!
I now face a slight issue and I need your advice.
Balance of each single cell is perfect (less than 0.02V) as long as my Victron charger is in bulk mode. But when it switches into absorption mode, 2 cells become unbalanced, something like 0.15V.

I never did top balance. So, I unpacked my celles and now do the top balance. Do you think that is enough or do I risk to have similar trouble later?

 

[Steve_S]

Assuming a fet based bms, the fets switch the charge/load based on a bunch of criteria.
Such as high/low pack voltage, high/low cell voltage and high current.
Does that answer your question.
Not even sure if it was directed to me but took a shot anyway.

The other thing that a bms does is passive cell balancing.

CORRECTION: A BMS does not do this by design ! Balancing Passive/Active is an Extra Function / Capability that SOME BMS' have. This is NOT a default function of a BMS. It is an option, just like BlueTooth Access to the BMS, or even the ability to personally configure a BMS.

 

[John Frum]

CORRECTION: A BMS does not do this by design ! Balancing Passive/Active is an Extra Function / Capability that SOME BMS' have. This is NOT a default function of a BMS. It is an option, just like BlueTooth Access to the BMS, or even the ability to personally configure a BMS.

Daly does passive balancing.
Overkill does passive balancing.
Pretty much a commodity feature.
Correction rejected.

 

[DJSmiley]

And besides that, balance current in a BMS is generally a very low passive balancing. If your batterpack is not too much out of balance, it might be able to keep up the balance between cells, but with slightly more differences it might be insufficient (or take like forever)

For initial balancing it definitely is useless (Unless you have it balancing for like a year or so...)

I really would suggest going for a manual top balance before building. (Assuming your're using it on the higher end of SOC, hence the top-balance).
If you really want to be picky, you can use an active balancer once a year or so manually.

 

[Gic1622]

As a kind reminder, I'm French so sorry for my poor English!
Here is an update of the situation. The pack is part of an off grid system and I'm no more on site. I can now only set parameters remotely, using a 4G modem and a Bluetooth connection on a mobile with teamviewer host.

Initial parameters
BMS : OV cell protec : 3. 65 (datasheet) / OV batt protec : 28.4V
Solar CHARGER : absorption 28.4V and float voltage 27V.
When the solar switches from bulk to absorption, the cells start to unbalance. So, I decided to unpack the battery to do a balance. I don't think that is a top balance as every single cell was not fully charge. After this balance, each cell was at 3. 41V.

The day after, same problem. Above 27.3 or. 5, the system start to become unbalanced (2 or 3 cells are lower than others). So I decided to modify the parameters of charger and BMS.
Absorption : 27.9 / float 26.5
BMS : OV cell protec : 3. 6 / OV batt protec : 28V

What's your opinion?

On top of that :
1 - Would it be possible to permanently add an active balancer (5A) on top of BMS or it is not recommended to have these 2 stuffs in parallel?
2 - As I don't need a full capacity when I'm not present on site, what parameters would well protect the battery until I individually charge the cell to 3.65V or 3.6V next time I go there. Is there a risk to definitely loose capacity?

Many thanks for your kind support!

 

[Gic1622]

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