# bms can lithium be used with out them



## Ziggythewiz (May 16, 2010)

Depends on the type of lithium. Some really need it to not go fireball, others, like LiFePo4 do not need it with proper care.

A BMS monitors every cell in the pack to ensure none go under or over safe voltages. It may do this by tripping alarms or cutting the pack to prevent damage, or may shunt cells to prevent overcharge, or remove power from higher cells to bring up lower ones.


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## dougingraham (Jul 26, 2011)

evnz said:


> bms can lithium be used with out them and whats there job
> Thanks
> Owen


LiFePo4 does not strictly speaking need a BMS. And if someone chooses to not use one then they are the BMS and really do need to understand what is going on. Of course you can argue that having a BMS without any understanding of what it is telling you is also dangerous.

The job of a BMS depends on what it was designed to do. There are several different types. Since I am in the camp that believes they are not necessary for LiFePo4 type installations I am not the person to ask about any particular devices. Basically they can do the following:

1) monitor the voltage of every cell during charge and indicate full charge or an error if one cell becomes overcharged. Optionally they can shunt away some current on the higher cells to top balance the pack.

2) monitor the voltage of every cell during discharge and indicate a low voltage condition of a cell to the driver or the controller so as to prevent that weak cell from being ruined.

3) monitor the temperature of each cell and indicate a fault if out of temperature range for charging or discharging.

I hope that answered your question.


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## danh818 (Dec 14, 2011)

Great explanation Doug.


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## evnz (Jul 24, 2010)

Thank for the replys so main thing i get out of this is as i have less knowlage i will be best to have them so i dont cook them specaily when they cost a 1/3 of my yearly pay 
Thank again
Owen

Ps what type bypass the charged cell for the low cells?


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## onegreenev (May 18, 2012)

> 1) monitor the voltage of every cell during charge and indicate full charge or an error if one cell becomes overcharged. Optionally they can shunt away some current on the higher cells to top balance the pack.


Why top balance when you never drive full. Its safer to have all your cells balanced at the bottom where the danger lies. 



> 2) monitor the voltage of every cell during discharge and indicate a low voltage condition of a cell to the driver or the controller so as to prevent that weak cell from being ruined.


This implies that the driver WILL HEED the warning signal and actually stop. What really happens is that the driver sees a strong voltage but a cell or two may be low. Being a strong overall voltage you keep driving because you KNOW you really have more to use. It will kill a cell. Ask me how I know. Top balanced is just wrong because you always drive less than full. Bottom balance and don't charge so high. That way your rear is covered. You know, cover your ass. 




> 3) monitor the temperature of each cell and indicate a fault if out of temperature range for charging or discharging.


Most BMS systems do not have this function. It is a GOOD function if you have cells that require a temp monitoring because they are not really safe to use. LiPO should use a thermal BMS. But you still need to bottom balance and you need a well matched pack to begin with.

Be safe. How do you decide which of the myriad of BMS systems you should use if any? I still have no clue. I still don't trust them. My cells were damaged by a factory BMS system that failed. I have many that were ruined because of a large failure. Failure of a BMS is a REAL issue.


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## onegreenev (May 18, 2012)

evnz said:


> Thank for the replys so main thing i get out of this is as i have less knowlage i will be best to have them so i dont cook them specaily when they cost a 1/3 of my yearly pay
> Thank again
> Owen
> 
> Ps what type bypass the charged cell for the low cells?


Before you go out and buy a system you need to do a lot more research. There are plenty of us that do not use a BMS and have done so very successfully. There are a few things you need to know for it to work well and safely but its pretty simple. Really. 

How are you going to decide if you do decide to use one? Truly how will you KNOW you have chosen the RIGHT one? Really.


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## eva-michael (Apr 13, 2010)

evnz said:


> Thank for the replys so main thing i get out of this is as i have less knowlage i will be best to have them so i dont cook them specaily when they cost a 1/3 of my yearly pay
> Thank again
> Owen
> 
> Ps what type bypass the charged cell for the low cells?


I will recommand you to use a battery system with battery management system. The most important thing is low voltage protection. Battery cell should be cut off at no less than 2.0V/cell and 2.4V/cell in everage for battery pack. Otherwise, it may cause demage to cell if cell voltage comes too low without stop.


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## dougingraham (Jul 26, 2011)

onegreenev said:


> Why top balance when you never drive full. Its safer to have all your cells balanced at the bottom where the danger lies.


I agree. But it is not practical for a BMS to bottom balance a pack and since one of the goals is to protect a cell from overcharge this can assist with that. What the BMS maker wont tell you is that you only need to do this once and it will stay that way unless the BMS is badly designed and unbalances the pack.



> This implies that the driver WILL HEED the warning signal and actually stop.


Yes. In particular the Wife/Girlfriend/Daughter is just going to ignore a warning indicator as long as the car seems to be driving ok. I think you would need to put the car into some sort of terrible limp mode. Maybe limit the top speed to 15mph. Even at that I suspect they would drive the battery to death.


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## dougingraham (Jul 26, 2011)

evnz said:


> Thank for the replys so main thing i get out of this is as i have less knowlage i will be best to have them so i dont cook them specaily when they cost a 1/3 of my yearly pay


The problem with a BMS is all the added complexity. If you dont understand what it is doing I don't see how it can help you. And if you do understand it then you dont need it.

LiFePo4 is pretty simple to deal with.

1) Bottom balance the cells when you install them. This more than anything else this will prevent you from driving a pack to death. What happens is you are driving along and the battery goes dead. Pretty much every cell at the same time. The car stops. You tow the car home or to a charger and charge it up. No damage. What kills these cells is reverse charging them. A weak cell that goes to zero volts will be reverse charged by the rest of the pack if you keep driving. Bottom balance will cause all the cells to dump at the same time. The car stops because there simply isn't enough voltage to make it go.

2) Deliberately undercharge the pack. All you need to accomplish this is a charger that can turn off when a particular voltage is reached. The first time you charge the pack after the bottom balance you find the weakest cell. You can do this when the pack is nearing full by locating the cell with the highest voltage. Once you have located the weakest cell you monitor that cell until it reaches something a little over 3.5 volts and then read the total pack voltage and set the charger to stop at that total pack voltage. Done!

3) Make certain that there are no connections to the pack except at the endpoints. If there are other connections then these connections are a potential source of pack imbalance.

4) Try not to overdischarge the pack. The pack will last longer if you don't do deep discharges very often. Generally no deeper than 80% is considered a good idea for long life and 70% is better.

5) Avoid extreme discharge currents. Most of the cells can tolerate a continuous 3C discharge and a burst discharge of 8C. If you are going to exceed these levels on a regular basis then you probably need a larger pack.


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## EVfun (Mar 14, 2010)

onegreenev said:


> Before you go out and buy a system you need to do a lot more research. There are plenty of us that do not use a BMS and have done so very successfully. There are a few things you need to know for it to work well and safely but its pretty simple. Really.


I run successfully without a BMS using a top balanced pack. It has the advantage that when the pack approaches fully charged there is a real strong signal to the charger, in the form of a sharp increase in voltage, to tell the charger to back off. I set my charger for 133 volts maximum for a 38 cell pack (3.50 vpc.) It times out 15 minutes after reaching voltage regulation so I charge to about 95%. I had the timer on my old charger fail, but no cell reached 3.6 volts in 5 or 6 hours before I shut the charger off. 

I don't push my pack capacity -- that is important because you CANNOT over-discharge with a top balanced pack without destroying the weaker cells in the pack. Most BMS systems maintain a top balanced pack so this warning applies to pack with a BMS too. LiFePO4 cell reversal is just about assured cell death (they internally short.)


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## dougingraham (Jul 26, 2011)

EVfun said:


> I run successfully without a BMS using a top balanced pack.


I have gone back and forth a couple of times between top and bottom balance. Yes, a top balance makes it incredibly easy to tell when the pack is nearly full. The rapid rise in voltage is hard to overlook. I originally argued with myself that you want the charger to always stop because you are not usually watching it at that time. And with what the bottom balance scenario is trying to stop someone is always in the car. However human nature seems to win out and there will always be someone who says I can make it and pushes the car to brick a cell or cells. It is almost impossible to brick a cell with bottom balance. The charger is still going to stop charging even if the rapid rise at the end of charge is not quite so rapid. Since I want to allow other people to drive my car I have decided on the bottom balance. I am going to undercharge quite a lot since I have a lot more range than I need and this makes the rapid rise a moot point since I don't ever want even the weakest cell to get to that point. We have plenty of range we don't need to worry about the last 5% like you would on a Lead Sled and since it is supposed to be better for the batteries to not fully charge that is another reason not to fully charge and therefore top balance isn't necessary.


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## Elithion (Oct 6, 2009)

evnz said:


> bms can lithium be used with out them


Do you mean "Can Lithium-ion batteries be used without a BMS?"

Yes, absolutely. It is done all the time, especially by hobbyist members of this group.

Or do you mean "Should Lithium-ion batteries be used without a BMS?"

The answer is: depends on how long you want your battery to last.

- Without a BMS, you'll have to replace cells as they are killed by overcharging or over-discharging; how often? depends on how much work you put into minding the pack manually, and how carefully you manage to use only a portion of the charge in the pack.

- With a BMS, you set it up correctly, and pretty much forget it: the BMS will keep you from abusing any cell.



evnz said:


> whats there job


Do you mean "what's their job?"

The first job of a BMS if to prevent you from operating any individual cell in the battery pack outside its SOA (Safe Operating Area). It does so from shutting down charging or discharging if ANY cell is about to be operate outside its SOA. [EDIT: The BMS *MUST* be wired in such way that it can *DIRECTLY *shut down the charger and the load.]

The second job is to allow you to store and use the maximum amount of energy in the battery. It does so by top balancing the battery: making sure that all the cells are completely full at the end of charge. [EDIT 1: balancing at any point will ensure maximum charge, but to also ensure maximum energy you have to balance at the top. EDIT 2: Balancing has nothing to do with safety: the first job deals with safety]

The third job is to report to you the state of the battery, for your convenience. [EDIT: Note that some people want to do the protecting on their own and won't let the BMS do its job automatically, so they will use the BMS just as a monitor.)

The first job is absolutely required. Careful: some BMSs do not do this job.
The second job is optional, but very desirable.
The third job is nice, but not really necessary.

You can select a BMS among all the offerings with this BMS selector utility.


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## Roy Von Rogers (Mar 21, 2009)

Doug has it right on, also something I have been preaching is the size of a pack. Yes I know batteries are costly, but after all, batteries is what makes it a EV, so put enough cells in a vehicle so you seldom have to worry about going empty, and only use the center portion with regular usage.

Constantly using the full pack is like always waiting for you gas tank to reach empty before refill.

The point is dont let the tank (batteries) get under a quarter empty, and dont overfill.


Roy


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## evnz (Jul 24, 2010)

Thank you all have given me a good amount to think on. I am lost to bottom balance tho is that where you use the controler to stop the car when the volts drop to low

Owen


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## Roy Von Rogers (Mar 21, 2009)

evnz said:


> Thank you all have given me a good amount to think on. I am lost to bottom balance tho is that where you use the controler to stop the car when the volts drop to low
> 
> Owen


 
The reason for bottom balance is only due to not all batteries having the exact same capacity. So if you empty all the cells equally down to about 2.9v, and then charge them in series to 3.5v times the cells, and stop the charger, they should all have about the same capacity.

So, if for some reason you get too close to the end, the voltage will be so low you will not be able to drive at speed, and all the cells should have the same voltage, and none will go to zero, reverse and destroy thermselfs.

Roy


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## ev-converter (May 24, 2012)

Elithion said:


> Do you mean "Can Lithium-ion batteries be used without a BMS?"
> 
> Yes, absolutely. It is done all the time, especially by hobbyist members of this group.
> 
> ...


I agree with you that a BMS SHOULD be used to protect against over discharge and overcharge. Working with and having the first street car with Li-ion in 2004, I didnt have a BMS and none was available. The fear that something was going to happen when charging or putting the foot to the metal on the freeway is just too much. Now with the BMS one should have NO concern, as the BMS will warn you, shutoff the charger and/or reduce speed before the cells are damaged. Been there and done it. Most dont want a BMS because they dont want to part with the coins it takes to get it. At around $12 per cell it is cheap insurance. By the way, all production cars use a BMS and they also due top balance during charging. If the big guys do it, it must be the right way.


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## Roy Von Rogers (Mar 21, 2009)

ev-converter said:


> I agree with you that a BMS SHOULD be used to protect against over discharge and overcharge. Working with and having the first street car with Li-ion in 2004, I didnt have a BMS and none was available. The fear that something was going to happen when charging or putting the foot to the metal on the freeway is just too much. Now with the BMS one should have NO concern, as the BMS will warn you, shutoff the charger and/or reduce speed before the cells are damaged. Been there and done it. Most dont want a BMS because they dont want to part with the coins it takes to get it. At around $12 per cell it is cheap insurance. By the way, all production cars use a BMS and they also due top balance during charging. If the big guys do it, it must be the right way.


Depends whether your talking about a monitoring system, or managing the individual cell charging. One should monitor anything thats important to the well being of any piece of machinery. But I'm always a bit leary of those who make money offf such devices claiming you must have one of their devices.

I would discourage anyone from putting an active electronic device across a lithium battery, period. Nothing wrong with monitoring and using such info to negating a problematic event.

Roy


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## onegreenev (May 18, 2012)

ev-converter said:


> I agree with you that a BMS SHOULD be used to protect against over discharge and overcharge. Working with and having the first street car with Li-ion in 2004, I didnt have a BMS and none was available. The fear that something was going to happen when charging or putting the foot to the metal on the freeway is just too much. Now with the BMS one should have NO concern, as the BMS will warn you, shutoff the charger and/or reduce speed before the cells are damaged. Been there and done it. Most dont want a BMS because they dont want to part with the coins it takes to get it. At around $12 per cell it is cheap insurance. By the way, all production cars use a BMS and they also due top balance during charging. If the big guys do it, it must be the right way.



As long as the BMS is working. I will never allow a BMS to take control of the charger. I lost 20 100 AH cells because of a faulty FACTORY BMS system. So that is like what 120 bucks each. 20 X 120 = $2,400 not including shipping or any of that. 

Pete 

Have a gander at what happens when a FACTORY BMS fails. Have a gander at the rats nest of wires and each one is a link to a failure. What happened is that the failed BMS failed to report that it no longer worked and failed to report any uncharged or balanced cells and allowed cells to drain. Resulting in a total of 20 lost cells. 

Notice the nice fat little piggies. They are all touching. They were snug and secure in the battery box for the past 4 years and no sign of any damage except the cells were low voltage. I pulled one out and had 5 instantly puff up like little piggies. So keeping pressure on the cells will not prevent them from being damaged. They just hold the pressure and when released they just bloat out like little piggies. 

Don't let anyone tell you that banding will prevent damage or fix damaged cells it won't. Banding ONLY HIDES problems.


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## onegreenev (May 18, 2012)

Roy Von Rogers said:


> Doug has it right on, also something I have been preaching is the size of a pack. Yes I know batteries are costly, but after all, batteries is what makes it a EV, so put enough cells in a vehicle so you seldom have to worry about going empty, and only use the center portion with regular usage.
> 
> Constantly using the full pack is like always waiting for you gas tank to reach empty before refill.
> 
> ...


Yes, sizing your pack is important but even with this you will or most will get into a situation where they WiLL go to the extreme, distance wise. Yes, it is wise to bottom balance so if and usually when it happens you don't loose your cells. You don't want your cells looking like this.


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## EVfun (Mar 14, 2010)

onegreenev said:


> Yes, sizing your pack is important but even with this you will or most will get into a situation where they WiLL go to the extreme, distance wise. Yes, it is wise to bottom balance so if and usually when it happens you don't loose your cells. You don't want your cells looking like this.


Don't do it!

A tow costs $100. A single cell costs that much or more. The factory built EVs have means to shut them DOWN if you try to go to far. Since you built it you can do what you want. Even if you installed an amp hour counter rigged to shut down the main contactor you know how to bypass it.

Don't do it!

Respect your investment. Pete shows you a great example of how a BMS can bite back. But *any* method of control can bite back in some direction. Know how your system fails and how you might "exploit" it. Choose your system and pay attention so you don't do it.

If your charger is quite controllable and you are inclined to use your full range you should bottom balance. If your charger is less adjustable and you will stay well away from full discharge use a top balance. I like top balance because I don't push the 0% SOC limit *and* I trust my self control more than any electronics device. Pete knows what works best for him, and that is good too. 

I am looking forward to some of us here putting on enough miles to wear a pack out. I have a concern that the bottom balance systems will wear out the smaller cells first because they are the only ones hitting or exceeding 3.60 vpc each charge. I'm trying to kill them from regular rapid discharge (pushing 60 amp hour cells for 6C often.) We spend our money and take our chances -- most of us are trying to add to the learning experience too.

It's all good -- so long as you don't blindly murder your cells. We are all trying to prevent that.


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## onegreenev (May 18, 2012)

Im working more on the assumption that most folks won't do any thing close to what any of us do and you need to have fail safes. Bottom balancing is a failsafe when those folks disregard the warnings. So when you put a pack together and you decide that your not going to charge more than like 3.6 volts which is actually not terrible. Many are also looking at just 3.5 volts and if you decide on like 3.5 volts and you have a cell that exceeds 3.8 on the top because you have bottom balanced cells then that cell really needs to be removed from the pack as it is way to low capacity to be well matched for your pack. I also tell folks that they need a well matched pack from the get go and if you don't your just asking for trouble. If you have 100 AH cells and you find that most are 105 AH but you find a cell that is like 90 AH you need to cull that 90 AH cell or live with that being your limit. Top balancing will be disastrous to those that take things to the extreme and if you only put in 90 AH into you pack then all your cells will be balanced and ready to perform. It would be better to have your pack better matched because not only do you want to be close in capacity you also want to hit the cure at close to the same time. So it will be important to have your cells close no matter if you decide on top or bottom and no the BMS can't fix mismatched packs. So start with a matched set. Close is fine. 

Pete 

My Leaf is not fully charged nor is it fully discharged. I can choose to ignore the warnings and some have done so but all the cells reach the bottom at the same voltage. They have to because they KNOW folks will drive the cells to the bitter end. Guaranteed. No need for top balancing because you never drive a full pack. You always drive a partly filled pack.


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