GUIDE to properly Top-Balance and Charge a LFP Battery: Part 1

[DavidKelly]

Here you admit you're prolly not grasping "my method" .

I’m being polite. Your method for determining FCV is not clearly explained, nor will you simply explain it. From my understanding it does not take temperature into account (Which makes it only work at that specific temperature and breaks at all other temperatures) and you refuse to clarify it. Your example of the LF230 is incomplete.

From the SOC OCV curve provided by the cell manufacturer your value of 3,37V only describes a FCV at 0 degrees.

I have reviewed the code. It does not explain the effect of temperature on the FCV.

Then you go on and say this.

Yes. I stand by it.

And this.

And this

I too am happy to engage in such a discussion.
But when you haven't even physically implemented things to begin with, what criticism are you offering and on what grounds ?? based on a self-admittedly incomplete grasp of what is being said ?

I feel you are deflecting and have made me less confident in your model. A simple example would suffice.

This post was written last year. In the meantime, the implementation is already seeing stable, production usage in many user's personal solar systems, everyday, at their homes.

We are all in agreement that this will not affect cell operation but only potentially protect cycle life. I have no doubt they are working without problems.
Nor is this a question of the code written by others of which you contributed to.

This relates to only using capacity to determine FCV (SOC OCV 100%) and treating all similar capacity cells equally across the board regardless of brand/ Internal resistance/ starting capacity or temperature. At the start of this thread you boldly said that everyone is wrong and your method is right. If your model was sound it would stand up to the scrutiny with easy answers to the above questions.

It simply means that BMS prevents balancing when it would 'de-balance' the cells more.
Such as, when the there's current going a high resistance cell that is actually lower in charge (but higher in terminal voltage due to Internal Resistance).
This increases the work the BMS balancer has to do and is preventable.

This adds to the above as a higher internal resistance cell will have a different FCV to the rest- not to mention dependancy on temperature.

Typically this is identified by the BMS as a weak cell fault. A pack that was well built with impedance matching should not have this issue. Better BMS such as Orion will also take internal resistance into account. But that’s a conversation for another thread.

A simple question:

At 25 degrees Celsius, using your model. What is the SOC OCV 100% Voltage (FCV) for an EVELF230 cell?

A) 3,37V
B) 3,469V
C) Something else
D) I don’t know
E) Phone EVE

 

[G00SE]

@hwy17 im curious to hear your thoughts on this thread as there seems to be some bullet points that tie into a thread you made about float/absorbtion.

 

[shvm]

Your method for determining FCV is not clearly explained

That may be a language barrier.

nor will you simply explain it.

Take a cell, and subject it to full charge (take note of the temperature & misc. Env conditions). Then let the cells rest until the terminal voltage stabilizes. Cell manufacturers usually standardize this time for new cells as 30 minutes. Plot terminal voltage vs. time. When it settles, that is the FCV.

From my understanding it does not take temperature into account (Which makes it only work at that specific temperature and breaks at all other temperatures) and you refuse to clarify it.

This adds to the above as a higher internal resistance cell will have a different FCV to the rest- not to mention dependancy on temperature.

This is where I have to call you wrong. Cell FCV (essentially a voltage) is a function of chemistry and temperature, but not Internal Resistance.
You can attach an external resistance in series with a cell and check for yourself that it still won't change its FCV.

Typically this is identified by the BMS as a weak cell fault. A pack that was well built with impedance matching should not have this issue.

A cell in a battery pack may see an increase in IR later down the road. I as an Engineer like to go beyond "should not have" and "might face this down the road during design life". A cell may have high IR but be nominal capacity.
It is our job to build systems that can tolerate simple imperfections like this. Convergent balancing is exactly that.

 

[G00SE]

At 25 degrees Celsius, using your model. What is the SOC OCV 100% Voltage (FCV) for an EVELF230 cell?

A) 3,37V
B) 3,469V
C) Something else
D) I don’t know
E) Phone EVE

I appreciate the thread and dialogue so far in it.
Much of it, I don’t have a solid grasp up. Am simply looking to treat my cells the most effective/reliable way.
I snipped this part of your other response to pose a question. (I don’t know the answer to your question btw)
Based on the data posted in this thread, if one has multiple packs of cells that are in a climate controlled room, (operating temps always between 50-80 Fahrenheit) with zero chance to exceed .5C discharge rate….
Is the jist of this that charging to 3.469v/cell both inconsequential/non detrimental, and also completely reasonable?

 

[hwy17]

@hwy17 im curious to hear your thoughts on this thread as there seems to be some bullet points that tie into a thread you made about float/absorbtion.

I'll look it over, I'm not sure I've ever read this one though.

My method is so reductive that I'm careful about getting into discussions about it directly against any of the other methods that it overturns. I need to prove it for myself a bit more before I'm ready to get more promotional about it.

But this is it: Bulk, boost, absorb, float, balance, dip, dive, duck, dodge, and dive at 3.437! Don't need active balancing, and just build then balance if your cells are fresh and from a premium vendor.

 

[G00SE]

I'll look it over, I'm not sure I've ever read this one though.

My method is so reductive that I'm careful about getting into discussions about it directly against any of the other methods that it overturns. I need to prove it for myself a bit more before I'm ready to get more promotional about it.

But this is it: Bulk, boost, absorb, float, balance, dip, dive, duck, dodge, and dive at 3.437! Don't need active balancing, and just build then balance if your cells are fresh and from a premium vendor.

The jist of what I got from the OP seems to support your other thread ideas with charge termination and single voltage set points. Hence why I thought to tag you lol

 

[shvm]

At 25 degrees Celsius, using your model. What is the SOC OCV 100% Voltage (FCV) for an EVELF230 cell?

Any LFP Cell with modern chemistry when fully rested after a full charge will have a FCV hovering just above 3.37 V.

Temperature do affect this but only slightly and that's the simple reason I am not taking that EVE data seriously determining FCV with just 5 minute rest.
At my place, summer temperatures are routinely around 35+ Degree C. And I still remember seeing cell voltages settling down in the ballpark of 3.4 V during my test after charge termination.

I guess EVE has much explaining to do.

 

[hwy17]

The jist of what I got from the OP seems to support your other thread ideas with charge termination and single voltage set points. Hence why I thought to tag you lol

Oh perfect, I will read then.

 

[shvm]

Oh perfect, I will read then.

Now that I am aware of your earlier work on charge termination
I would like to read up on them.

 

[hwy17]

Now that I am aware of your earlier work on charge termination
I would like to read up on them.

Here's where I started a mess. Also I knew more by the second half of this thread than when I started it. And I know more today than I knew at the second half as well.

Why is bulk/absorption voltage used?

I'm trying to get down to some charging basics here and interested in any resources or comments that explain the basis of 2 and 3 stage charging. Why would we want to use a higher voltage than our end goal saturation voltage? Why would we not just constant current / constant voltage all the...

diysolarforum.com

 

[shvm]

Here's where I started a mess.

I have a first hand experience in this well before even authoring this guide.

Also I knew more by the second half of this thread than when I started it. And I know more today than I knew at the second half as well.

What I have realised during the last four months or so is that Float value of LFP is even lower than my already very conservative values in this thread. I now use 53.4 V and still cycle what I like to assume 98% of my battery capacity.
The more your physically experiment, the more you know and understand.

 

[hwy17]

What I have realised during the last four months or so is that Float value of LFP is even lower than my already very conservative values in this thread. I now use 53.4 V and still cycle what I like to assume 98% of my battery capacity.
The more your physically experiment, the more you know and understand.

Yeah the true safe float voltage is probably in the lower 53's like that.

I'm working on a single voltage charging scheme theory for daily cycled solar systems, because I think a single voltage would greatly simplify things by eliminating concerns about rebulking cycles, eliminating the need for careful charge termination or any termination at all, and allowing balancing in float.

Right now, I'm starting with 3.437 because 55v is a round number, but the best single voltage could be elsewhere.

There are two criticisms my single voltage has to face:

1. Bulk/Absorbing at 3.437 won't be fast enough - This is the big one that I need to explore more in practice. After this summer I will be able to speak more confidently about it. I think that the difference at typical solar C rates is minimal.

2. Floating at 3.437 is too high - Yes, this is definitely true if it was a constant float, like a UPS application. But I think it's ok for two reasons, one that if we absorb at 3.437 in the morning of the solar cycle then we aren't really ramming the charge in, and that should leave a bit of vacancy for the float to keep trickling it in, and two that a solar cycle float is naturally limited to 1-8 hours, depending on how early you acheive the float in the day. The float will always end when the sun goes down, so the float duration is naturally limited.

 

[DavidKelly]

That may be a language barrier.

Take a cell, and subject it to full charge (take note of the temperature & misc. Env conditions). Then let the cells rest until the terminal voltage stabilizes. Cell manufacturers usually standardize this time for new cells as 30 minutes. Plot terminal voltage vs. time. When it settles, that is the FCV.

Full charge being as per datasheet to 3,65V and CV or CP. Then cell rests and lowers voltage to FCV.

I think everyone is in agreement with the above. This is the method for the SOC OCV plot supplied by the manufacturer. This test is done at a variety of temperature intervals to form the SOC OCV curves that characterise the cells.

I can assure you the Eve datasheet is correct and they hover at the higher values at those temperatures (after hours). Are you sure your cells aren’t B grade?

So to confirm:

• Does your Method calculate the FCV or do you use the SOC OCV curve from manufacturer?

• Does your method compensate for changes in temperature?

• Does your method calculate the FCV based on capacity alone?

• Do you still think everyone is doing it wrong?

This is where I have to call you wrong. Cell FCV (essentially a voltage) is a function of chemistry and temperature, but not Internal Resistance.
You can attach an external resistance in series with a cell and check for yourself that it still won't change its FCV.

How can it be a function of Chemistry and not have an effect on internal resistance. The FCV will decrease as the cell looses capacity. The increase in internal resistance will affect the FCV.

Are you implying that a resistor placed In series does not incur a voltage drop? The measured voltage will be lower. I suggest you use a resistor with a proportional resistance to that of a cell internal resistance increase.

A cell in a battery pack may see an increase in IR later down the road. I as an Engineer like to go beyond "should not have" and "might face this down the road during design life". A cell may have high IR but be nominal capacity.
It is our job to build systems that can tolerate simple imperfections like this. Convergent balancing is exactly that.

The literature is clear on this. Capacity and internal resistance are inversely proportional. A decrease in capacity will see a rise in internal resistance ( and vice Versa).

The majority of balancing algorithms will easily deal with this. The goal of an impedance matched pack is to have all cells age similarly under standard balance conditions. I appreciate your altruism as an engineer but I feel it is misplaced. I too am an engineer not that it’s important. We all want to design batteries to last.


I absolutely disagree that there is a general voltage for all LFP. The 3,37V is not standard across the board. I feel this is misleading. compare this to GFB SOC OCV that hovers at 3,4…It does seem we’ve reach agreement on the importance of temperature.

Your sample set doesn’t sound large enough to be drawing such strong conclusions. Especially if your cells are B grade, aged or not directly from Eve. I think EVE knows exactly what the are doing. I have seen the higher FCV on the regular but it will vary with age and cycle life.

 

[G00SE]

What defines a grade B cell

 

[DavidKelly]

I have a first hand experience in this well before even authoring this guide.

What I have realised during the last four months or so is that Float value of LFP is even lower than my already very conservative values in this thread. I now use 53.4 V and still cycle what I like to assume 98% of my battery capacity.
The more your physically experiment, the more you know and understand.

This is true on some of the cheap and nasty cell makes a float voltage of 51,8V- 52,5V is not uncommon for a 15S battery.

 

[DavidKelly]

What defines a grade B cell

At the factory the cells are graded. This is relative to capacity and is based on internal resistance. The can’t capacity test every cell, it would not be economical (The amount of energy!)… So they determine the cell capacity based on the relationship with internal resistance as mentioned above. If the cells internal resistance is outside of the specification on the datasheet it is rejected ( Marked B grade). If it’s real bad. It’s C grade and recycled.

These cells are then batched and sold on the secondary market through tertiary distributors ( nothing in China is wasted). Reputable manufacturers will remove the serial number and mark them B grade (will post a picture).

These cells will not perform the same as A grade or sometimes called HSEV grade ( Electric vehicle grade) cells.

B grade typically find there way into telecoms applications due to low C rates or sold as A grade to fetch the higher value.


This is why buying on Aliexpress must be done carefully. Be careful, there’s a lot of dodgy LFP out there!

 

[G00SE]

At the factory the cells are graded. This is relative to capacity and is based on internal resistance. The can’t capacity test every cell, it would not be economical (The amount of energy!)… So they determine the cell capacity based on the relationship with internal resistance as mentioned above. If the cells internal resistance is outside of the specification on the datasheet it is rejected ( Marked B grade). If it’s real bad. It’s C grade and recycled.

These cells are then batched and sold on the secondary market through tertiary distributors ( nothing in China is wasted). Reputable manufacturers will remove the serial number and mark them B grade (will post a picture).

These cells will not perform the same as A grade or sometimes called HSEV grade ( Electric vehicle grade) cells.

B grade typically find there way into telecoms applications due to low C rates or sold as A grade to fetch the higher value.


This is why buying on Aliexpress must be done carefully. Be careful, there’s a lot of dodgy LFP out there!
View attachment 210882

Do they have data to delineate the cutoff between passing and rejects?
This is the first tangible type info I’ve seen as to HOW these are classified.
Additionally, why would ANY currently available cell offered to regular consumers by 3rd party vendors (18650,imr,ezeal,sfk,docan,gobel, etc) have supplies of grade A available? Wouldn’t those be going into whatever mass scale projects they were built for?

 

[DavidKelly]

Yeah the true safe float voltage is probably in the lower 53's like that.

I'm working on a single voltage charging scheme theory for daily cycled solar systems, because I think a single voltage would greatly simplify things by eliminating concerns about rebulking cycles, eliminating the need for careful charge termination or any termination at all, and allowing balancing in float.

Right now, I'm starting with 3.437 because 55v is a round number, but the best single voltage could be elsewhere.

There are two criticisms my single voltage has to face:

1. Bulk/Absorbing at 3.437 won't be fast enough - This is the big one that I need to explore more in practice. After this summer I will be able to speak more confidently about it. I think that the difference at typical solar C rates is minimal.

2. Floating at 3.437 is too high - Yes, this is definitely true if it was a constant float, like a UPS application. But I think it's ok for two reasons, one that if we absorb at 3.437 in the morning of the solar cycle then we aren't really ramming the charge in, and that should leave a bit of vacancy for the float to keep trickling it in, and two that a solar cycle float is naturally limited to 1-8 hours, depending on how early you acheive the float in the day. The float will always end when the sun goes down, so the float duration is naturally limited.

Think your method is spot on. Might have to play with the numbers a bit depending on the cells you have but should work well. I agree with your reasoning.

I don’t think it will be too high. In many motive applications the charge target voltage is 3,458V.

Your set point will be fine on good quality cells.

Hopefully this is the outcome of this post that 3V-3,5V is good to start and then tailor to your system.

Please keep us updated on how it goes.

 

[DavidKelly]

Do they have data to delineate the cutoff between passing and rejects?

Yes. If you ask them they will share it with you on a batch that you have ordered. Certainly with Eve. They are very professional. We once used it to solve an issue we originally thought was caused by bad cells but data proved it to be a faulty BMS. I digress.

This is the first tangible type info I’ve seen as to HOW these are classified.
Additionally, why would ANY currently available cell offered to regular co

I currently am involved with a lithium ‘rejuvenation’ project for the largest Telecoms company in South Africa. Can’t say who but easy to work out. In South Africa theft and vandalism is very bad. We are then able to disassemble recovered damaged batteries and test cells. Establish the good from the bad based on impeadance testing and other characteristics. Then rebuild new batteries and return to service.

We are currently processing 13000 units.

by 3rd party vendors (18650,imr,ezeal,sfk,docan,gobel, etc) have supplies of grade A available? Wouldn’t those be going into whatever mass scale projects they were built for?

There’s a lot of us weekend warriors out there hey, Our money is as green as big projects. We also pay way more for small batches. LFP is dirt cheap in bulk - This is how you can buy a 100Ah battery for 400USD. These suppliers have formed a Business model based on buying in bulk and distributing. Their profit is the delta between high volume discounts and low volume markup.

Gobel is fantastic BTW. Ask for Ellie or Jessie.

 

[DavidKelly]

The impedance thresholds are usually on the datasheet.

 

[DavidKelly]

GFB SOC OCV for LF206 for reference. Closer to the 3,5V side at 100% on charge.

 

[G00SE]

Yes. If you ask them they will share it with you on a batch that you have ordered. Certainly with Eve. They are very professional. We once used it to solve an issue we originally thought was caused by bad cells but data proved it to be a faulty BMS. I digress.



I currently am involved with a lithium ‘rejuvenation’ project for the largest Telecoms company in South Africa. Can’t say who but easy to work out. In South Africa theft and vandalism is very bad. We are then able to disassemble recovered damaged batteries and test cells. Establish the good from the bad based on impeadance testing and other characteristics. Then rebuild new batteries and return to service.

We are currently processing 13000 units.



There’s a lot of us weekend warriors out there hey, Our money is as green as big projects. We also pay way more for small batches. LFP is dirt cheap in bulk - This is how you can buy a 100Ah battery for 400USD. These suppliers have formed a Business model based on buying in bulk and distributing. Their profit is the delta between high volume discounts and low volume markup.

Gobel is fantastic BTW. Ask for Ellie or Jessie.

Thanks for the insights

 

[rustyrotors]

GFB SOC OCV for LF206 for reference. Closer to the 3,5V side at 100% on charge.

can you share the soc-ocv data for lf304, or is it basically the same

 

[shvm]

How can it be a function of Chemistry and not have an effect on internal resistance. The FCV will decrease as the cell looses capacity. The increase in internal resistance will affect the FCV.

I can take two identical cells with same FCV. and connect them in parallel as depicted.
The capacity of the combined cell has doubled, The effective IR halved, but the measured FCV or any characteristic voltage for that matter has to be the same. Basic sanity check !!

The literature is clear on this.

Might as well cite one.

Capacity and internal resistance are inversely proportional. A decrease in capacity will see a rise in internal resistance ( and vice Versa).

You need to realise that it is a one way street.
For an aging cell, capacity ↓ will indeed cause IR ↑.
But an IR ↑ doesn't necessarily translates to capacity ↓.

I have said it before.
I am saying again, it is totally common for a cell to have higher internal resistance without affecting measured cell capacity (Ah).



@upnorthandpersonal You reading those ??
This is now reminding me of your thread

I should have known better... (sorry, ranting)

... than to go into discussion with SFK on Andy's YT comment section. For context, the issue is that the JK (new one) resets its values after x amount of time un-powered. This is a bug, obviously, and the reset values of 4.2V per cell are very bad for LFP. So SFK commented under the video...

diysolarforum.com

Are you implying that a resistor placed In series does not incur a voltage drop? The measured voltage will be lower. I suggest you use a resistor with a proportional resistance to that of a cell internal resistance increase.

Not unless current is flowing. But that is not how FCV is determined in the first place.

It does seem we’ve reach agreement on the importance of temperature.

Umm, not at all as far as I'm concerned, when it comes to the charging implementation.

In hindsight, The EVE data is still not enough for determining what voltage to float for LFP.
There is no consideration of the surface charge phenomena and how there is a supercapacitor like behaviour at charge end. There is a depletion of vast majority of mobile charge carriers (Li+ ions) too. Not recommended to stay in this region for long.

This is true on some of the cheap and nasty cell makes a float voltage of 51,8V- 52,5V is not uncommon for a 15S battery.

51.8V-52.5 V comes out to be 3.45 to 3.5 V Cell for 15S battery. That is a dangerous float voltage and is not recommend for even 'grade A' LFP cells due to aforementioned degradation mechanisms.

BTW, I have EV Grade A CALB L160F100 Cells with factory QR.

So to confirm:

• Does your Method calculate the FCV or do you use the SOC OCV curve from manufacturer?

• Does your method compensate for changes in temperature?

• Does your method calculate the FCV based on capacity alone?

• Do you still think everyone is doing it wrong?

I don't get your adamancy. There is simply much more to this that needs to be figured out on your end.

 

[DIYrich]

@DavidKelly : any thoughts on compression of cells and cycle life?