Low cost / High performance universal BMS

[Alcibiades Ramos]

I going to try to illustrate with an analogy. Let's say that we a bunch of rulers all different sizes but all of them divided into 10 parts. There are tall ones and short ones and some of them average size. Let us say we grab 6 of them and we end up with a couple of short, a couple of medium and a couple of tall ones. Again each one of them is divided in 10 equal parts.
Let us say that we put them all standing up inside a room but they give us two options. We can either hang them from the ceiling or we can put them standing on the floor.
Let us add some little monkeys that climb up and down the rulers. We'll call the little monkeys "Electrons Monkeys".
When top the rulers of from the ceiling we realize that some that some Electron Monkeys fall of the ruler and are not able to climb back up.
On the contrary situation when we stand the rulers on the floor and they are all even on the floor (bottom balancing) we see that the Electron Monkeys not only are able to go up and down with no problems but also in case they jump of the top of the ruler they land back up to it.
Now let us go back to mother earth and talk about LiPoFe's. Each individual cell has their own rate of discharge. They chemical things so the physical condition for them is unique to each cell. When make sure that all the cells in a pack are discharged to the same bottom voltage (the floor standing rulers) at 2.7v we know that none are going to drop off (Electron Monkey falling of the ruler).
When we charge the whole pack each cell is going to have their own rate of charge, which is pretty much the same rate of discharge. So since we know the bottom is 2.7 volts on all the cells we know that they will all reach that voltage at about the same time ergo preventing any cell from going below before the others.
...and they lived happily ever after for around 8 or 10 years until a new technology replaced those cells with new ones called Laser Monkeys!

[nlc]

I read your message multiple times, but sorry but I still have not understood the explanation

The bottom balancing you talk about implies that you discharge all cells at bottom floor voltage before charging. It seems it's the opposite of the good utilization of lithium cell, IE charge them as soon as you can, even if they are not empty. Discharge them to the bottom floor voltage before charging becomes a waste of energy.

And I don't understand why this bottom balancing will change something on the end of charge. If cells don't have exactly the same capacity, cells with the lowest capacity will reach the max voltage threshold before cells with the highest capacity. It will be mandatory to still cut the charging to not overcharge them. And the result is a pack with a total voltage lower than it should if cells with highest capacity have not been discharged to the bottom balancing voltage before charging !??

[nlc]

Some news, the software inside the slave is completely finished and everything work well. I made a small video to see the BMS in action :

http://www.youtube.com/watch?v=k3qjPXCfs6M

Please note that the music comes from a toy of my childrens, not from the BMS

The BMS worked all the night, the PC (master) simulating a pack of 255 cells (the 9 physical cells from cell number 246 to 254), read every 1.5s. In this configuration each read causes more than 1KB data to flow through all cells to reach the master, and this morning, zero communication error after almost 30.000 reads thus approx 30MB data

[Alcibiades Ramos]

Originally Posted by nlc

Some news, the software inside the slave is completely finished and everything work well. I made a small video to see the BMS in action :

Please note that the music comes from a toy of my childrens, not from the BMS

The BMS worked all the night, the PC (master) simulating a pack of 255 cells (the 9 physical cells from cell number 246 to 254), read every 1.5s. In this configuration each read causes more than 1KB data to flow through all cells to reach the master, and this morning, zero communication error after almost 30.000 reads thus approx 30MB data

I wonder if your BMS can be set to bottom balance LiFePo's. On his last show Jack Rickard explains the whole matter in detail better than I do. If you care to watch the section on the subject is at around 30:00 into the program. This guy is an encyclopedia on batteries so it would be interesting your opinion on what he says. He has a lot of years of experience in these matters.

EVTV Motor Verks | Electric Car Conversion Videos

Salut!

Alcibiades

[nlc]

Ok I will try to see the video, but I have a lot of difficulties to understood what they said, I better read english instead ear it.
What is the video which talk about that ? "A123 Cell Testing and Module Construction" ?

Is there a website where I can found detailed explanations ? Because I cannot understood how it can be better to discharge all cell to a bottom level before charging. It's the direct opposite of what it's recommended to do with lithium chemistry (li-ion, li-po, lifepo4)

[nlc]

I done some search about "bottom balancing". All that can be read on the various debates confirms what I though : I really don't see any advantage of bottom balancing !! On the contrary, there are severe disadvantages :

- You need to discharge all cell at the LVC, it's a waste of energy and it's exactly the opposite of the prefered method for lithium chemistry, charge them even if they are not empty, to increase their lifetime

- You also need to cutoff the charge when the first cell reach the HVC, because they will not reach the same full voltage in same time if cell are not completely identical (capacity, internal resistance, temperature ...), and they are not.

- When discharging the pack, you also need to check the LVC on each cell, because cell cannot be identical, even if same amount of Ah have been entered in all cell, the cell with higher internal resistance will reach the LVC before the other.

- With bottom balancing, you completely discharge all cell, thus at the next charge, you cut off the charge when the lower capacity cell reach the HVC, but at this time the higher capacity cell are not completely charged and their voltage is not at full voltage, thus your pack is not at full voltage. Thus when discharging, for a given power you will need higher current...

In fact, I cannot understand why there is a debate between top or bottom balancing, because I cannot see any advantage of the bottom balancing method If anyone has a link to an explanation about the advantage of bottom balancing I am interested !!

Anyway, in my opinion, balancing is not really mandatory, the global pack capacity will alway be in any case the capacity of the lower cell. Just cut charge when the first cell reach the HVC and cut the discharge when the first cell reach the configured LVC and your pack will alway be healty, because no cell can be over discharged or overcharged.
But an intelligent top balancing (balancing during charge and not at the end) can add an advantage, have the maximum of cells at their top voltage at the end of charge, to maximize the global voltage of the pack.

[nlc]

Concerning the display/master of my bms, I think for now not a lot of people will be interested by a complete "calculator" which manage all the car.
To begin something, I took the decision to build a board, very similar to the cycle analyst, that everyone must know !

It will be based on :

- TFT screen, probably 4.3", maybe 5"
- Speedometer input
- Current sensor input to sense the battery current
- Throttle over-ride output to be able to set a current and/or power limit, and why not, lower this limit when the pack become almost empty.
- Serial interface connected to the BMS slave boards
- SD card for datalogging and configuration
- Serial port for firmware upgrade, configuration, datalogging, and so on
- CAN bus, can be useful
- Pack fully charged output, to be able to cutoff charge if needed
- Pack fully discharged output, to be able to cutoff discharge if needed

What do you think about that ? An ev-analyser, similar to the cycle analyst for e-bike, but with big and beautiful screen, and integrated battery management ?

[nlc]

If you don't bottom balance your batteries you risk loosing the whole pack.

Don't where you read this ? I never bottom balancing any pack I had, I never loose a whole pack

When one battery goes dead at the bottom of it's capacity it takes all the others with it eventually.

How ? Have you some link or discussion about this ?

On all defect pack I had the opportunity to see, the problem always came from the BMS, not doing correctly its low voltage or high voltage detection on one cell. Result : cell destroyed after some time because of multiple overcharging or over discharging without cutoff of the BMS.
But the BMS do its job on other cell, on which voltage cannot come out from the LVC/HVC threshold because in this case the BMS cutoff charge or discharge.

On these defect pack I seen, all other cell was alway good.

[Alcibiades Ramos]

I don't know how clear I can be about this: If you don't bottom balance your batteries you risk loosing the whole pack. When one battery goes dead at the bottom of it's capacity it takes all the others with it eventually.

But be my guest whoever does not understand or follow this procedure eventually will pay the price of the whole pack.

-First Bottom Balancing the cells is only done once in the life of the pack unless you add more cells to the pack in which case you want put all the cells at the same level from the bottom of their capacity.

-Second I never said completely discharge the cells. You said that which is wrong. I said and still do: 2.75 volts per cell to be safe. You bottom balance at that voltage NOT "0 volts".

-Third I am sorry if I am blunt but I have been doing this since the 80's with radio controlled airplanes. When you loose an aircraft at 80mph over the people,s heads because of batteries failure you start investigating the reasons why.

-Fourth When you say and I quote: " it's exactly the opposite of the prefered method for lithium chemistry, charge them even if they are not empty, to increase their lifetime" you are speaking about preferences not facts. People preferred SUVs, war and politics and think they are safe yet the figures show the contrary.

-Fifth on the HVC side you are right you should cut off once the first cell reaches that state. Yet you'll be surprised what you find if you measure them individually 24hrs later. Do the test and verify my words. In any case the Cubans have a saying around here that goes like this: :It's not one thing or the other but the opposite of both". That goes only to say that both systems combine will do the trick. I say Both System Combine... just in case.

-Sixth and probably this is the most important since there is a danger factor. Cells will not take more energy than what their chemistry allows on HVC but will go downhill in a hurry if one goes through their nominal low bottom threshold. When that happens it happens in a steep curve. Look at the manufacturers specifications it's there.

Last this is really a controversy and I am not here to antagonize but I am just looking at the facts take this blog for example I think you'll find it amusing how with the same facts they arrive to totally opposite conclusions:

Endless-sphere.com • View topic - Bottom balancing?

There is an endless discussion about how arrogant Jack Rickard is about this matters. Who cares? Basically people who care more about personalities than science. Yup he might arrogant but I think he is right. Furthermore I see he is right. And it is kinda convincing when you can see things don't ya?

So look at the graphs on that blog and tell me what you notice on the graph.
There's top balancing and bottom balancing. Now this is math. Numbers no bull. Tell me where the curve is flat and I'll tell you where you have the most average energy.

Rest my case.

Originally Posted by nlc

I done some search about "bottom balancing". All that can be read on the various debates confirms what I though : I really don't see any advantage of bottom balancing !! On the contrary, there are severe disadvantages :

- You need to discharge all cell at the LVC, it's a waste of energy and it's exactly the opposite of the prefered method for lithium chemistry, charge them even if they are not empty, to increase their lifetime

- You also need to cutoff the charge when the first cell reach the HVC, because they will not reach the same full voltage in same time if cell are not completely identical (capacity, internal resistance, temperature ...), and they are not.

- When discharging the pack, you also need to check the LVC on each cell, because cell cannot be identical, even if same amount of Ah have been entered in all cell, the cell with higher internal resistance will reach the LVC before the other.

- With bottom balancing, you completely discharge all cell, thus at the next charge, you cut off the charge when the lower capacity cell reach the HVC, but at this time the higher capacity cell are not completely charged and their voltage is not at full voltage, thus your pack is not at full voltage. Thus when discharging, for a given power you will need higher current...

In fact, I cannot understand why there is a debate between top or bottom balancing, because I cannot see any advantage of the bottom balancing method If anyone has a link to an explanation about the advantage of bottom balancing I am interested !!

Anyway, in my opinion, balancing is not really mandatory, the global pack capacity will alway be in any case the capacity of the lower cell. Just cut charge when the first cell reach the HVC and cut the discharge when the first cell reach the configured LVC and your pack will alway be healty, because no cell can be over discharged or overcharged.
But an intelligent top balancing (balancing during charge and not at the end) can add an advantage, have the maximum of cells at their top voltage at the end of charge, to maximize the global voltage of the pack.

[nlc]

Good, now there are some concrete thing to discuss

I don't know how clear I can be about this: If you don't bottom balance your batteries you risk loosing the whole pack. When one battery goes dead at the bottom of it's capacity it takes all the others with it eventually.

Need more explanation, as I said if BMS do it's job, cells cannot be discharged under LVC or overcharged over HVC. If the BMS is malfunctioning on just one cell, only this cell can be damaged. If you are not agree, please explain how other cell can be destroyed ?

-First Bottom Balancing the cells is only done once in the life of the pack unless you add more cells to the pack in which case you want put all the cells at the same level from the bottom of their capacity.

This precision is really important, why you don't say that before ? Why compare a procedure which is done once (bottom balancing) and the top balancing which is eventually done each charge ?

-Second I never said completely discharge the cells. You said that which is wrong. I said and still do: 2.75 volts per cell to be safe. You bottom balance at that voltage NOT "0 volts".

Never talk about 0V, always talk about LVC

-Third I am sorry if I am blunt but I have been doing this since the 80's with radio controlled airplanes. When you loose an aircraft at 80mph over the people,s heads because of batteries failure you start investigating the reasons why.

I am not blunt I try to understood, but it's clear that there a misunderstanding, I understood that the bottom balancing you talked about and is discussed on some forum was a procedure to do every charge !!

-Fourth When you say and I quote: " it's exactly the opposite of the prefered method for lithium chemistry, charge them even if they are not empty, to increase their lifetime" you are speaking about preferences not facts. People preferred SUVs, war and politics and think they are safe yet the figures show the contrary.

Not fact ? Yet, I seen somewhere curve which show that a cell will give more Ah in its life if it is charged before it is completely discharged (at LVC I mean !)

-Fifth on the HVC side you are right you should cut off once the first cell reaches that state. Yet you'll be surprised what you find if you measure them individually 24hrs later.

It's normal each cell cannot be 100% identical (internal resistance, temperature, ...). But what does it change to the discussion ?

Do the test and verify my words.

Yes but test what ? What can be exactly be compared and tested ?

-Sixth and probably this is the most important since there is a danger factor. Cells will not take more energy than what their chemistry allows on HVC but will go downhill in a hurry if one goes through their nominal low bottom threshold. When that happens it happens in a steep curve. Look at the manufacturers specifications it's there.

Never said the contrary, and there is no relation with top/bottom balancing and this fact : the BMS must cut discharge when at least one cell reach the LVC.
But if it happens on one cell because the BMS LVC cutoff doesn't work on this cell, only this cell will be destroyed.

Last this is really a controversy and I am not here to antagonize but I am just looking at the facts take this blog for example I think you'll find it amusing how with the same facts they arrive to totally opposite conclusions:

If there is a controversy it's absurd because we talk about a procedure to do once....

Endless-sphere.com • View topic - Bottom balancing?

There is an endless discussion about how arrogant Jack Rickard is about this matters. Who cares? Basically people who care more about personalities than science. Yup he might arrogant but I think he is right. Furthermore I see he is right. And it is kinda convincing when you can see things don't ya?

I don't care about personality, if anyone tell something it must not be a affirmation but need argument and logical explanations. Only after that you can conclude.

So look at the graphs on that blog and tell me what you notice on the graph.
There's top balancing and bottom balancing. Now this is math. Numbers no bull. Tell me where the curve is flat and I'll tell you where you have the most average energy.

It seems you forgot to put the blog link ?