How Does a BMS REALLY Work? As it seems that many here apparently do not know exactly!

[Andy-Mat]

If you are an electrics/electronics total DIY beginner, this might possibly be less than easy to understand, but if you are an e-bike battery owner/user, it is important to have a "GOTO" place on Pedelec, where you can hear and hopefully fully understand, the true facts about BMS and battery charger operation, as not fully understanding, could seriously mislead you when faultfinding, and not to erroneously rely on "imagined facts", that are possibly well meant, but are simply wrong.
Up to now on Pedelec, I have not read a single post from anyone here, that has shown that he/she knows 100% accurately, all the true facts about BMS internal operation and Battery charging, there has always been error(s) in the way the member "thinks" it works, so here is the chance to massively improve on the current situation for all of us here on Pedelec.
The most prevalent error in understanding is that the BMS tells the charger what to do and when!
It simply does not do this. The BMS does its job with or without the charger being connected.
The Charger.
The charger is mainly controlled by a small (technically seen) computer, usually in a single chip. A chip that has been programmed, to do a particular job, which is to provide a charging current, at up to 42.0 Volts (in the case of a 36 volt nominal Li-ion Battery).
Other voltage batteries have the same cell/bank voltages, but more or less banks, resulting in a lower or higher battery voltage, but working on the same exact principles. I am describing for the 36 volt battery, as I believe it is the most used voltage, generally speaking!
When the battery reaches 42.0 volts, and the charging current has dropped to 3% or less of the nominal charging current (charging current is often 2 amps for standard chargers, and 4 amps for fast chargers, though other currents also may be available), the charger will simply stop supplying a charging current/voltage, and the green (generally) full charge LED will then be lit.
Simply put, once that LED goes green, the charger has done its job and can and should be removed, to prevent long term damage to the battery
The BMS.
What the BMS is doing, is to monitor each bank of cells (10 banks in a nominal 36 volt battery), by monitoring each individual bank's voltage, and any bank that is of a higher voltage than any other, will have power drained away from it (wasted you might say), till each and every bank has the same voltage as the lowest bank.
This why all the cells in a battery, when the battery is first built, must be very closely matched in all respects, to reduce this loss of capacity to a minimum, by minute differences in voltages between banks.
Good cell manufacturers, do this matching, and sell matched quantities to battery builders. This will allow most quality batteries, to hardly need a BMS initially, till age starts to become a factor.
But they are always included for many safety reasons.
That is how each bank of cells is balanced against all the other banks, in a typical Li-ion battery.
Technically seen, the charger does not know (only the BMS will "know"), that any cell banks are lower or higher than 4.2 volts, once the overall battery voltage has reached 42.0 volts, in comparison to any other cell bank.
But the charger ssimply does not need that information to allow it to do its job anyway!
A Charger's only job, is to maintain the battery at 42.0 volts, till it is disconnected from said battery, or the mains is removed.
While wandering around on the internet, looking for an accurate and simple explanation of a the way a BMS works, I recently found this Guy, who is in my humble opinion, a true electronics Guru, and he has put together many interesting and informative Videos on YouTube, for any one here who takes the time to watch and learn.
He goes by the name of:-

mjlorton
- on YouTube. This video is a very good explanation of exactly how a BMS works. Differently in some respects to all the posts I have read here up to now!
While watching, possibly a sandwich and a coffee is recommended, to help pass the time!:-

For anyone interested in almost any other aspect of Electrics and electronics, he has placed dozens of high quality videos on YouTube, for all to see and pick up truly excellent knowledge. I myself have not watched them all, but I intend to watch any and all that interest me over the following months.
I would like each and everyone to post their thoughts after they have fully watched the video, from beginning to end.
But ONLY if they wish to!
If anyone wants to gain the knowledge secretly, that is your business, and your business only.
Remember, although clearly and simply put, some of you may need to watch it again, and that can be considered "par for the course" and no negative reflection upon yourselves.
Enjoy!
regards to all
Andy

 

[vfr400]

If you are an electrics/electronics total DIY beginner, this might possibly be less than easy to understand, but if you are an e-bike battery owner/user, it is important to have a "GOTO" place on Pedelec, where you can hear and hopefully fully understand, the true facts about BMS and battery charger operation, as not fully understanding, could seriously mislead you when faultfinding, and not to erroneously rely on "imagined facts", that are possibly well meant, but are simply wrong.

While wandering around on the internet, looking for an accurate and simple explanation of a the way a BMS works, I recently found this Guy, who is in my humble opinion, a true electronics Guru, and he has put together many interesting and informative Videos on YouTube, for any one here who takes the time to watch and learn.
He goes by the name of:-

mjlorton
- on YouTube. This video is a very good explanation of exactly how a BMS works. Differently in some respects to all the posts I have read here up to now!
While watching, possibly a sandwich and a coffee is recommended, to help pass the time!:-

For anyone interested in almost any other aspect of Electrics and electronics, he has placed dozens of high quality videos on YouTube, for all to see and pick up truly excellent knowledge. I myself have not watched them all, but I intend to watch any and all that interest me over the following months.
I would like each and everyone to post their thoughts after they have fully watched the video, from beginning to end.
But ONLY if they wish to!

Andy, you never learn, do you? Instead of hunting for random stuff on the internet and then trying to explain what you think you learned as if you're some expert, get some real parts and look at them, then figure out how they work. Ask questions about what you don't understand. Through that method, you will gain some real knowledge.

You always pick sources that are nothing to do with ebikes, like Battery University and the guy above. If you want to do some research, at least look at ebike stuff.

The guy above knows his stuff. He did make a couple of minor mistakes, but that was probably because he wasn't thinking straight while he was figuring out what to say rather than his lack of knowledge, Anyway, he's talking about lipos for model aeroplanes and he only goes over the principles of how lithium batteries work. In his next video, he shows a specific solar charging system BMS that's nothing like an ebike one and works differently.

After all that, you have the audacity to try to slander people that actually know what they're talking about.

 

[Andy-Mat]

Andy, you never learn, do you? Instead of hunting for random stuff on the internet and then trying to explain what you think you learned as if you're some expert, get some real parts and look at them, then figure out how they work. Ask questions about what you don't understand. Through that method, you will gain some real knowledge.

You always pick sources that are nothing to do with ebikes, like Battery University and the guy above. If you want to do some research, at least look at ebike stuff.

The guy above knows his stuff. He did make a couple of minor mistakes, but that was probably because he wasn't thinking straight while he was figuring out what to say rather than his lack of knowledge, Anyway, he's talking about lipos for model aeroplanes and he only goes over the principles of how lithium batteries work. In his next video, he shows a specific solar charging system BMS that's nothing like an ebike one and works differently.

After all that, you have the audacity to try to slander people that actually know what they're talking about.

Fair enough, find a good verifiable video explanation on the web somewhere, that supports all your theories.
But do not forget that Li-ion batteries are the same the world over, some bigger, some smaller, that does not change either the chemistry or the science involved.
I will happily watch it to see the differences.
I noticed that you did not lean to far "out of the window" when telling us what was wrong, as in fact, you said he made some mistakes, but neglected to inform us exactly what they were!!!.
Now why was that????
Are you yourself unsure?
Oh, by the way, he simplified it for his audience rather well I find. You might have missed that!
So how long do you estimate you will need to find something that is fully accurate? ANYTHING!
Waiting on your "new" and "accurate" information!
Thanks in advance.
Andy

 

[cyclebuddy]

The most prevalent error in understanding is that the BMS tells the charger what to do and when! It simply does not do this.

That's a very misleading statement. Not all BMS's use comms, but very, very many do - notably those batteries fitted with multi-pin charger plugs. For these e-bike systems, the BMS exerts some control over how the charger functions.

Here's how Halfords Carrera (Phylion) bike battery BMS's talk to their chargers (extracted from the battery makers own technical manual):

PIN 1: Charge “+”
PIN 2: Charge “-”
PIN 3: There is a temperature sensor in this PIN, the sensor can allow the charger to detect the battery temperature; if the temperature is too high or too low, the charging will be stopped
PIN 4: Communication with BMS: if the overcharge protection of BMS is activated, this charger can identify a signal sent from the BMS, and will cut off charging circuit
PIN 5: To identify Phylion charger, if the charger is not matched charger from Phylion, the charging will fail

 

[danielrlee]

 

[vfr400]

I noticed that you did not lean to far "out of the window" when telling us what was wrong, as in fact, you said he made some mistakes, but neglected to inform us exactly what they were!!!.
Now why was that????
Are you yourself unsure?
Oh, by the way, he simplified it for his audience rather well I find. You might have missed that!
So how long do you estimate you will need to find something that is fully accurate? ANYTHING!
Waiting on your "new" and "accurate" information!

He said the cells in series could discharge at different rates. They can't because they're in series, so the same current goes through all of them. He also said that they can discharge at different rates because of different internal resistance, but the same as before, the same current goes through each cell. He was probably thinking of a matrix of cells with cells in parallel, where both could be true. Happy?

Look up Kirchoff's First Law if you don't understand that.

 

[Andy-Mat]

He said the cells in series could discharge at different rates. they can't because they're in series, so the same current goes through all of them. he also said that they can discharge at different rates because of different internal resistance, but the same as before, the same current goes through each cell. He was probably thinking of a matrix of cells with cells in parallel, where both could be true. Happy?

He actually explained that fully and correctly, you should watch the video again.

 

[vfr400]

He actually explained that fully and correctly, you should watch the video again.

I did. He didn't.

 

[Andy-Mat]

He said the cells in series could discharge at different rates. They can't because they're in series, so the same current goes through all of them. He also said that they can discharge at different rates because of different internal resistance, but the same as before, the same current goes through each cell. He was probably thinking of a matrix of cells with cells in parallel, where both could be true. Happy?

Look up Kirchoff's First Law if you don't understand that.

Here are the three levels of Kirchoff's Law, which part to you apply to your misconception?

Isaac Physics

Isaac Physics is a project designed to offer support and activities in physics problem solving to teachers and students from GCSE level through to university.

isaacphysics.org

Because you are apparently forgetting that the law applies to the BMS itself, and each cell bank, being connected singly to the BMS.
The BMS either applies a fixed or variable resistance electronically, across each cell, to draw current, reducing the capacity and thereby to drop voltage, till it is the same as all the other cell banks.
His explanation showed that clearly.
I would guess that I was learning Kirchoff's laws, a long time before you were born....and I can say, unequivicaly, that you are misunderstanding what happens when you have cells connected together in series, and one of them has a resistor across it, bleeding off power......as happens in a BMS.
Remember, to keep things simple, both he and I did not mention "Smart" battery Packs.
You might want to read this, which also says it ball very clearly indeed:-

Battery management system - Wikipedia

en.wikipedia.org

Quoting from that article:-
A battery management system (BMS) is any electronic system that manages a rechargeable battery (cell or battery pack), such as by protecting the battery from operating outside its safe operating area[clarification needed], monitoring its state, calculating secondary data, reporting that data, controlling its environment, authenticating it and / or balancing it.[1]
A battery pack built together with a battery management system with an external communication data bus is a smart battery pack. A smart battery pack must be charged by a smart battery charger.[citation needed]
-------------------------------------------
Optimization
Distributed Battery Management system
In order to maximize the battery's capacity, and to prevent localized under-charging or over-charging, the BMS may actively ensure that all the cells that compose the battery are kept at the same voltage or State of Charge, through balancing. The BMS can balance the cells by:

• Wasting energy from the most charged cells by connecting them to a load (such as through passive regulators)
• Shuffling energy from the most charged cells to the least charged cells (balancers)
• Reducing the charging current to a sufficiently low level that will not damage fully charged cells, while less charged cells may continue to charge (does not apply to Lithium chemistry cells)
• Modular charging [6]

If you need further information to understand this, there is a massive amount on the web, very easily found, but if you still need help, just say the word, I can find it for you!
Andy

 

[Andy-Mat]

Here is a further simple explanation about cell balancing that might help you further, The cells in the middle show where power is reshared by the BMS, the best way as it wastes far less energy.
The cells on the right, are the conventional way, where each cell above the lowest voltage cell, the BMS simply "wastes" that energy away, usually with some sort of resistance applied across the individual cell Banks.
I find the illustrations also very useful too:-

How Does Cell Balancing Improve Battery Life

Cell balancing is a technique that improves battery life by maximizing the capacity of a battery pack. Know more here.

www.ionenergy.co

Here are a few of the diagrams, but without the text, which is available on the website:-

 

[Andy-Mat]

This explains why battery balancing is needed, with some simple and effective pictures:-

Li-Ion BMS - White Paper - Low voltage cut-off: a false sense of security

Text:-
White Paper - Low voltage cut-off: a false sense of security
Without a BMS, even with a low voltage cutoff, cells in an high voltage battery pack will be over-discharged

Whether a cell can stand being fully discharged depends on its chemistry:

• some cells like to be discharged once in a while (e.g.: NiCd)
• some don't mind (e.g.: NiMH)
• some do mind but survive it, though not in as good a shape (e.g.: Lead Acid)
• some are irreversibly damaged (e.g.: Li-Ion, which cannot be at below 2.0 V)

No cell can stand being reverse charged, regardless of its chemistry.
Once a cell is discharged, continuing to run discharge current through it will reverse it (the polarity of its voltage will be reversed). This is not likely to happen with a small pack, but becomes increasingly likely in batteries with more and more cells in series.
In a high voltage battery, one with many cells in series, there is a much greater chance that the overall pack voltage is not evenly divided among its cells.
In theory, if the pack starts perfectly balanced when full, all the cells will discharge evenly, and all their voltages will remain in lockstep, all the way down to discharged. In reality, the capacity of the cells will be mismatched, and the least capacity cells will reach the low voltage first. (Unless cells were pre-selected and matched to have exactly the same capacity.)
A battery with 10 LiFePO4 cells in series (whose minimum safe voltage is 2.0 V) may very well have a total voltage of 20 V. But there's no telling if some cells may be at 3.0 V, while others at 1.5 V.
LiIon cells do not deal well with over-discharging; once discharged, they cannot produce more current as the other cells in series are still doing so. Their voltage drops rapidly once discharged, so it's very easy to bring their voltage to below 2.0 V, or even reverse them.

 

[Andy-Mat]

This is very exhaustive, but you can pick out exactly what you want to learn easily:-

BU-908: Battery Management System (BMS) – Battery University

Learn about old and new methods and how innovations may change old habits

batteryuniversity.com

This one is most informative:-

BU-803a: Cell Matching and Balancing – Battery University

Learn about battery technology, battery maintenance and battery life at Battery University

batteryuniversity.com

Andy

 

[vfr400]

Here are the three levels of Kirchoff's Law, which part to you apply to your misconception?

Isaac Physics

Isaac Physics is a project designed to offer support and activities in physics problem solving to teachers and students from GCSE level through to university.

isaacphysics.org

Because you are apparently forgetting that the law applies to the BMS itself, and each cell bank, being connected singly to the BMS.
The BMS either applies a fixed or variable resistance electronically, across each cell, to draw current, reducing the capacity and thereby to drop voltage, till it is the same as all the other cell banks.
His explanation showed that clearly.
I would guess that I was learning Kirchoff's laws, a long time before you were born....and I can say, unequivicaly, that you are misunderstanding what happens when you have cells connected together in series, and one of them has a resistor across it, bleeding off power......as happens in a BMS.
Remember, to keep things simple, both he and I did not mention "Smart" battery Packs.
You might want to read this, which also says it ball very clearly indeed:-

Battery management system - Wikipedia

en.wikipedia.org

Quoting from that article:-
A battery management system (BMS) is any electronic system that manages a rechargeable battery (cell or battery pack), such as by protecting the battery from operating outside its safe operating area[clarification needed], monitoring its state, calculating secondary data, reporting that data, controlling its environment, authenticating it and / or balancing it.[1]
A battery pack built together with a battery management system with an external communication data bus is a smart battery pack. A smart battery pack must be charged by a smart battery charger.[citation needed]
-------------------------------------------
Optimization
Distributed Battery Management system
In order to maximize the battery's capacity, and to prevent localized under-charging or over-charging, the BMS may actively ensure that all the cells that compose the battery are kept at the same voltage or State of Charge, through balancing. The BMS can balance the cells by:

• Wasting energy from the most charged cells by connecting them to a load (such as through passive regulators)
• Shuffling energy from the most charged cells to the least charged cells (balancers)
• Reducing the charging current to a sufficiently low level that will not damage fully charged cells, while less charged cells may continue to charge (does not apply to Lithium chemistry cells)
• Modular charging [6]

If you need further information to understand this, there is a massive amount on the web, very easily found, but if you still need help, just say the word, I can find it for you!
Andy

You're getting all mixed up again. You need to calm down and engage your brain. We were talking about the video you linked in the OP, where the guy showed 4 cells in series without a BMS and he's explaining why the cells go out of balance. He said that the cells could have a different discharge rate, but that's impossible. For every electron that comes into a the top of a cell, another one pops out the bottom and the same current goes down the wires and through the cells, thanks to Mr Kirchoff. I thought a genius like you would know and understand Kirchoff's laws and their implications, as they're fundamental to how electricity works.

Don't you remember, I said that the video guy made a couple of minor errors, and you got all excited and wanted me to expose him as a villain otherwise I was guilty of slander, only to then become a turncoat, when you tried to defend the indefensible, and down into a deep hole you fell. Unfortunately, all that horse shite won't help you to climb out.

 

[Andy-Mat]

I did. He didn't.

Just as I expected, so I have added some other links, proving that is how it happens.
It is the same everywhere, how a resistance is place parallel to a cell, and if its charge is too high, with respect to other cells, that cell is wasted of some power.
The other method I mentioned some time ago, of transferring from a high to a low cell, is probably not used on most Bike BMS....You didn't like that explanation either,,though it is a standard method of balancing a battery, and mentioned several times in the recent links I posted!!!
Enjoy!
Have you found the web link, book or anything that supports your theories yet?
Andy

 

[joelectric]

 

[Andy-Mat]

You're getting all mixed up again. You need to calm down and engage your brain. We were talking about the video you linked in the OP, where the guy showed 4 cells in series without a BMS and he's explaining why the cells go out of balance. He said that the cells could have a different discharge rate, but that's impossible. For every electron that comes into a the top of a cell, another one pops out the bottom and the same current goes down the wires and through the cells, thanks to Mr Kirchoff. I thought a genius like you would know and understand Kirchoff's laws and their implications, as they're fundamental to how electricity works.

Don't you remember, I said that the video guy made a couple of minor errors, and you got all excited and wanted me to expose him as a villain otherwise I was guilty of slander, only to then become a turncoat, when you tried to defend the indefensible, and down into a deep hole you fell. Unfortunately, all that horse shite won't help you to climb out.

Then where is ( your own words!) "your Horse Shite" on this subject?
No links nothing!
That does tell a tale or three!!
I have seen nothing about cell balancing from you, that is also supported by web links or books or whatever and there are several ways that it is done.
You really don't have a single clue how any electronics work, I have noticed it almost every time you post, ist all from your over active imagination, not from good fundamental facts, which are easily proven!
So you should REALLY stay away from them completely, and concentrate on mechanical bike stuff. That can be very helpful to others here.....
At the moment, all you are doing is misleading others here, and making yourself a laughing stock.....
Again I ask:-
So where are the web links, supporting your theories, you have the time to be rude posting back, but not the time to prove me wrong!
Its obvious you are simply playing for time, and searching desperately, and finding only a great big hole that you dug yourself!!
And you are now teetering on the brink!
Just deserts!

 

[flecc]

You really don't have a single clue how any electronics work, I have noticed it almost every time you post, ist all from your over active imagination, not from good fundamental facts, which are easily proven!
So you should REALLY stay away from them completely, and concentrate on mechanical bike stuff. That can be very helpful to others here.....
At the moment, all you are doing is misleading others here, and making yourself a laughing stock.....

Andy, you really are a total idiot.

vfr400's knowledge on this subject far exceeds yours by a margin you couldn't even begin to imagine, he doesn't need to look up what you have to all the time. That's why his help to members not only solves their e-bike problems but is often a master class in how to do that. And he's not the young person that seem to have assumed in one of your posts.

I've never seen you solve any problem for a member, you just bury them in endless streams of words while skirting around the subject with waffle. Hinting at how wonderful you think you are is often included in your "advice" and you are so often wrong. You even posted once to me "Don't ever tell me I am wrong"! The arrogance of someone posting like that assuming they can never be wrong is breathtaking.

You are the one who has made himself a laughing stock in here, as member's adverse ratings of your posts and their posted criticisms of them show only too clearly. No-one in here is ever likely to rate vfr400 as a laughing stock, except someone with such very poor judgement as you.

As both vfr400 and I have posted, you just never learn.
.

 

[Amoto65]

Andy, you really are a total idiot.

Now that can't be disputed.....

 

[Andy-Mat]

Andy, you really are a total idiot.

vfr400's knowledge on this subject far exceeds yours by a margin you couldn't even begin to imagine, he doesn't need to look up what you have to all the time. That's why his help to members not only solves their e-bike problems but is often a master class in how to do that. And he's not the young person that seem to have assumed in one of your posts.

I've never seen you solve any problem for a member, you just bury them in endless streams of words while skirting around the subject with waffle. Hinting at how wonderful you think you are is often included in your "advice" and you are so often wrong. You even posted once to me "Don't ever tell me I am wrong"! The arrogance of someone posting like that assuming they can never be wrong is breathtaking.

You are the one who has made himself a laughing stock in here, as member's adverse ratings of your posts and their posted criticisms of them show only too clearly. No-one in here is ever likely to rate vfr400 as a laughing stock, except someone with such very poor judgement as you.

As both vfr400 and I have posted, you just never learn.
.

He has you in his "grip", but he understands Electronics less than my dog does, if that were possible.
He chooses to not answer any of my technical questions, because he does not have an answer!
He chooses to not post any links refuting what I say, because he cannot find anything that supports his flawed theories in books or on the web.
To anyone moderately intelligent, that must tell a story, a very simple story!
Also, you are like him, very rude, unfriendly, and ignorant.
Do not forget, you went for that first, I am just replying as I see fit!
Trick question, now which one of us can possibly fix that?
Tip, I cannot fix any of your poor personal traits, you are stuck with them, or you fix them yourself. Your choice.
Remember that loyalty is a fine thing, unless you are being taken in and freely used, BIG TIME......
Got it?

 

[Gavin]

Chaps, it pains me to say this but @Andy-Mat is playing you all like a fiddle here. He has clearly got some time on his hands today, and as we all know he loves to play the pantomime villain. He has started two totally weird posts today on topics guaranteed to arrouse a response.

And unfortunately, it's worked. You've all thrown yourselves into his argumentative bear-trap one after the other like lemmings and he's loving the power he has over you!!