Do you need to balance IF...

[Alfalfameister]

For the record, I am top balancing my LiFePO4 cells (https://diysolarforum.com/threads/top-balancing-recommendations-fastest-or-best-practices.12179/)

But, just hypothetically, and out of curiosity, do you need to top-balance if all these conditions are met/placed:

(for 280Ah cells)

1. All your cells are brand new, ranging from 3.28v to 3.33v;
2. You plan to use a charge voltage (for a 51.2v pack) of only 54.4V (which comes out to 3.4v average);
3. You have a BMS that has a 2A balancer OR you have an active balancer anyway;
4. You set your HVD to 3.6, and your LVD to 2.9 in your BMS (but your inverter charge voltage is 54.4v (avg 3.4v) and inverter disconnect is at 48v (avg 3.0v))

Basically, you plan to charge to only 90%ish (54.4V), and probably use only 70% capacity, or down to about 20% SoC.

Would you need to do an initial balance since (though it may take weeks/months), it will eventually balance (up to a certain degree) with the BMS balancer or a separate active balancer anyway?

Just curious. Thanks for any answers.

 

[Steve_S]

There is Active & Passive balancing. Passive only "burns off" high volts from cells while Active transfers Hi Volts to Lo Volt cells.
LFP is a funny chemistry, it will NEVER SIT at 3.65V, you can get them there and they will be full but will always settle a bit (very normal) and this causes some folks to get loopy thanks to confusion between Lithium Based chemistries. The most "common" recommendation, even by High End Manufacturers (including BattleBorn*) of pre-builts is to not drive the cells over 80% leaving 10% from the bottom & top.

*Battleborn has an interesting trick, by using higher capacity cells and derating them internally with their own BMS they assure that a 12V/100AH battery delivers 100AH because it is actually 120AH, and they are using Big Cylindrical cells not square prismatic's.

Passive balancing helps keep cells in parity and has advantages with a Healthy & Matched up Battery Pack. Uses little to no power when everything is working as it should. Active Balancing is better for loosely matched or aging cells that have been used hard. NEITHER can fix or correct a Bad Cell within a pack.

The COMMODITY 280AH which many of us have or are buying are GRADE-A and fit the manufacturing spec for that class of cell. They meet the grading and have passed the factory production tests for voltage & internal resistances BUT they are NOT MATCHED & Batched for identical Internal Resistances through whole cycles which is a long, costly process. This means they are more likely to wander and have differentials between the cells, if this is minimal, then passive balancing may be sufficient but if the cells vary too much, active balancing can resolve several of these issues.

It is important to point out, that 1mv per AH deviation is not unusual with larger cells. So 280mv on a 280AH cell can and does happen (remember though, these are millivolts).

The RUB:
If using 280AH cells to build a 12V, 4 Cell Pack. And only using 80%, 10% from Bottom & Top.
(2.75V per cell / 11.0V LVD) to (3.28V per cell / 13.14V HVD gives you 224AH of useable (well not quite because of the knee BUT it is close enough for "rough math". YET some folks freak out when they test out their assembled packs to only 250AH while not taking into account LVD/HVD cutoff points etc.

Appreciate that there is a Significant difference using Fully Matched & Balanced cells (and we are not just talking about Top & Bottom Balanced) but fully & properly matched for Voltage Range & Internal Resistance through full cycles, such as one would find in an EV Pack (Conventional NMC Li-Ion types) as well as LFP CTP packs like those from CATL or BYD Blades (not available to use yet to Joe Public Customer). You can easily double the cost of cells if buying fully matched & batched sets, which is not usually done for small customers..

Some Reference info that may be handy.
BU-803a: Cell Matching and Balancing – Battery University
Pre-Balancing Cells | Orion Li-Ion Battery Management System
High resistance cell | Orion Li-Ion Battery Management System

Sit down and prepare before looking at this link: (reality check for a few folks)
https://www.batteryspace.com/custom-job-for-matching-cells.aspx

PS: I just received my batch of QNBBM-8S Active Balancers, one for each 24V Pack. 2X 280AH & 2X 175AH each running with a Chargery BMS8T which will have Passive Balancing disabled (causes conflict with active balancing). I will be doing that install over the next week straight into my production system. That will be written up as a review and posted in due course.

 

[Luthj]

If your safety systems are in place, and you don't mind the pack disconnecting repeatedly from the charger, you can forego a initial balance. That being said, the charger may be connecting/disconnecting several times an hour, as the BMS HVD, and then reconnects as the balancer drops that cell. At least for the first couple cells. Once a certain level balance is achieved, with a 3.4Vpc target, several cells will reach the 3.5Vpc point together, and the charge will terminate. That's still a fair bit of time spent at 3.65Vpc for the high cells initially though.

The main issue is that the pack may only deliver as little as 60% of its rated capacity during this period. Your governed by the lowest SOC cell.

Since the voltage curve is so flat, you will only get balancing when the cells are near the knee, which may only be a few hours a day depending on the system and how its used. With say 1A of max balance current, you can pretty easily work out how long that will take. 30% imbalance is 84AH. Say 1 hour a day balancing, so 84 days to balance. In reality it may be longer, as the BMS still has to handle normal cycle to cycle balancing as well.

 

[toms]

You don’t need balancing if you are operating as you suggested. I don’t have balancing activated on my pack - just check it yearly.

 

[Bob B]

My 8 190AH cells balanced out just fine. I skipped the parallel top balance and just used my Chargery BMS8T which was set to balance on charge and storage above 3.4V. They are in a 2P4S configuration.
Once I got to the knee I reduced the charge current to only around 3A and crept up on full charge to 3.65 / cell. Once I got to full charge, I had a deviation of less than 15 mv .... and they have remained in balance since then. After the initial charge, I changed to only balance on charge above 3.4V.

I am not sure this will work for everyone and for every battery type ... but it worked for me. These cells were all within .002 V of each other when I received them. If my initial charge / balance didn't work I planned to bit the bullet and do the parallel top balance. The one thing that helped is that the Chargery BMS will passive balance at 1.2A.

 

[Alfalfameister]

The one thing that helped is that the Chargery BMS will passive balance at 1.2A.

You mean as opposed to the normal passive balance of 200mA or so?

That's actually an idea:

1) connect the batteries in their final configuration (in my case 2P16S);

2) slap BMS with balancing (I plan to use an Active Balancer and might turn off the BMS balancer);

3) charge them with shore power until they get to the "knee" at (well, my charger only goes to) 60A (which is still really only 0.1C), then upon reaching the knee, lower charge rate to about 15-16A (effectively only 1A per cell), and let the active balancer (assuming at least 1A balancing) sort it out.

Does that sound logical? Any flaws in that?

 

[Luthj]

If you have the cells in series, which is typically required by any BMS, then you won't have 1A per cell, it will be 15A.

 

[Alfalfameister]

If you have the cells in series, which is typically required by any BMS, then you won't have 1A per cell, it will be 15A.

Doh! Sorry! Brain fart for me!

So, apart from that [hopefully temporary] mental block, when I get to the knee, I can switch over to 1A charging to creep up on the top balancing?

 

[Luthj]

Sure, but don't assume your BMS is actually balancing at 1A, that's best case. You will want to watch it, and adjust the charge voltage and/or current to keep the high cells from rising.

 

[Bob B]

You mean as opposed to the normal passive balance of 200mA or so?
Yes

That's actually an idea:

1) connect the batteries in their final configuration (in my case 2P16S);

2) slap BMS with balancing (I plan to use an Active Balancer and might turn off the BMS balancer);

3) charge them with shore power until they get to the "knee" at (well, my charger only goes to) 60A (which is still really only 0.1C), then upon reaching the knee, lower charge rate to about 15-16A (effectively only 1A per cell), and let the active balancer (assuming at least 1A balancing) sort it out.

Does that sound logical? Any flaws in that?

I think that approach will probably work .... to some extent it depends on how far out of balance the cells are to start. I think I got luck with mine that I got a closely balance set.

 

[Alfalfameister]

I think that approach will probably work .... to some extent it depends on how far out of balance the cells are to start. I think I got luck with mine that I got a closely balance set.

Hard to say differences in SoC from voltage readings alone (got new cells from Amy of Shenzhen Xuba/Luyuan) and lowest voltage reading was 3.29 and highest was 3.34v.

32 cells are connected in parallel now (32P), and for the last 36 hours, are being charged (setting at 3.5) at a measly 17.8A, or 0.002C for the currently 8,960Ah 3.2v cell. 36 hours ago, reading was 3.31x volts for the 32P.

Upon connecting it in parallel, no sparks flew, as voltage difference was 0.05v at most (but who knows exact SoC).

Am thinking of slapping them into their final configuration (2P16S) with BMS (Xiaoxiang aka Overkill), turning off the balancing part of the BMS, and attaching a 6A (rated) active balancer (I think it's a Heltec). Pics of BMS and Active Balancer attached.

So, thinking of making them 2P16S, charge at 54.0v at 60A (shore charger), and, upon hitting that, charge at 56.6v at 1A and watch it closely and slowly raise voltage until I hit 3.6v per cell.

Final charging parameters will only be at 54.4v (for when the pack is already operational).

Pics of BMS and balancer:

 

[Luthj]

My cells were within 1mv of each other. SOC variation was over 10% One was almost 18% I think.

 

[Ampster]

PS: I just received my batch of QNBBM-8S Active Balancers, one for each 24V Pack. 2X 280AH & 2X 175AH each running with a Chargery BMS8T which will have Passive Balancing disabled (causes conflict with active balancing).

I think that is an important point that should be emphasized.