# BMS & Advanced balancing



## riwe (Nov 17, 2011)

Hi all,
I’m trying to get my head around the different types of BMS out there. I found information about both active and passive balancing of cells in series. 

Active balancing has the possibility to transfer energy between cells during charge and discharge in order to keep the cells on the same “level”. 

Do you have any experience of active balancing verses passive? 

Best regards

Rikard


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## smpavlik (Mar 28, 2011)

There are few threads discussing the problem. As I know all of them end with passive balancing approach. It's possible to redistribute energy among batteries but schematics become more complicated and expensive than supposed to be. The best idea is an individual charging but this solution is complicated and expensive too


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## Ziggythewiz (May 16, 2010)

Lots of threads; they all end in arguments.

Do a little searching and a lot of reading. Decide what works for you, that you feel most comfortable with. It's your money.


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## frodus (Apr 12, 2008)

Here's a whitepaper on Passive versus Active topic:
http://liionbms.com/php/wp_passive_active_balancing.php

Here's a BMS selector to help you choose the right one for your application:
http://liionbms.com/php/bms-selector.php


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## riwe (Nov 17, 2011)

Ziggythewiz said:


> Lots of threads; they all end in arguments.
> 
> Do a little searching and a lot of reading. Decide what works for you, that you feel most comfortable with. It's your money.


Hahaha! Ok sounds like a hot potato. I'll bo my home work!

Thanks.


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## riwe (Nov 17, 2011)

frodus said:


> Here's a whitepaper on Passive versus Active topic:
> http://liionbms.com/php/wp_passive_active_balancing.php
> 
> Here's a BMS selector to help you choose the right one for your application:
> http://liionbms.com/php/bms-selector.php


I'll go through that book as a starter. 

Thank you!


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## frodus (Apr 12, 2008)

If you have questions, let me know, I've got some experience with Elithion, Orion and have built my own passive and active BMS units.


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## dougingraham (Jul 26, 2011)

And then there are people like me who do not have any kind of BMS and it really doesn't look like one is needed at all.

In fact at the moment I don't even have a volt meter or ammeter display because it appears that it does nothing but distract you from driving the car.

I don't worry about range because I charge every day and I know I currently have at least three times the range I need and I will have more than 5 times the range once I get the other battery box in the car. I do clear the trip counter after every charge and I do look at the charger display when I turn it off so I know how many AH went back into the batts.

I initially tested every cell for capacity and then bottom balanced the pack. The charger is set to trip off when the weakest cell reaches about 3.6 volts. At the end of the first 500 miles I checked the balance and found that the worst difference between the cells after resting overnight was 0.0006 volts at full charge. At the end of a 75% discharge and resting for several hours the worst difference was 0.0003 volts. I could probably tune this but what would be the point? I will check this again at 1000 miles but I don't expect to see any issues. And for the record there is more voltage difference when charging than this but it isn't real so active balancing during charge doesn't do anything for you. You are chasing something that doesn't mean anything.

I have an engineering background. Would I like to know the voltage of every cell? Sure I would, for about the first two weeks of operation. Would knowing this help me in any way? Yeah, if a cell fails for some reason it would save me a few minutes tracking down which one. Would knowing this prevent a cell from failing? I don't see how. And for the cost of even a cheap bms I can buy quite a lot of spares cells.


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## frodus (Apr 12, 2008)

Doug,
How do you measure that a single cell reaches 3.6V? Are you doing this essentially by pack voltage? or are you monitoring with Cell-logs or something?

But this isn't really a BMS versus NO BMS argument.... (I hate them and stay out of them, I have my preference, but it's more for data logging/monitoring).



My experience with Passive and Active are below (I'm not including LVC/HVC in the below types, as that's a separate entity within the BMS design).

*The passive BMS* is a normal shunting BMS. When the voltage gets to a level, it starts shunting current around the cell through a resistor to bleed off extra power so the cell is charged slower than the rest. There's lots of these available. Some are fully programmable, some are dumb and the voltage is set by the design. Fairly Cheap and easy (compared to other BMS solutions). I built a small version of this using a voltage monitor chip, MOSFET and lightbulb as a load. Worked well and was pretty cheap per battery. didn't really need it for Lead Acid, it was just a project to test feasability.

*The Active BMS* is a bit like a power supply on each cell. There are a few types. One type is bidirectional, so that one cell can shuttle current to another cell that is low. Another type is just for charging, and charges each individually. Then there's the type I designed and implemented used a DC-DC on each cell and a Pack level DC-DC converter that converted down to a buss voltage of 48V. Then each DC-DC could use the buss voltage to charge a single cell in the group. This means you can actively balance during charge, discharge and while sitting. Didn't really need it for Lead Acid, it was just a project to test feasability.

What it comes down to is cost. The Active is more expensive (if you can even find an off the shelf unit) than Passive. I couldn't find anything Active out there commercially and still can't. I know RWAudio built an Active BMS as well as a board for Cell-log8's to monitor HVC/LVC.


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## Ziggythewiz (May 16, 2010)

dougingraham said:


> it really doesn't look like one is needed at all.


Agreed...



dougingraham said:


> I don't even have a volt meter or ammeter display .


... but that's just crazy talk.


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## dougingraham (Jul 26, 2011)

frodus said:


> Doug,
> How do you measure that a single cell reaches 3.6V? Are you doing this essentially by pack voltage? or are you monitoring with Cell-logs or something?


Good question. Since I characterized all my cells I have them sorted in the car from best to worst except the cell next to the best cell is the worst so I can easily side by side compare them. I set the pack voltage cutoff by watching the worst cell (a little over 100ah while the best cell is a little over 105ah). When the worst cell reached 3.60v I read the total pack voltage and then set the charger to CV at that voltage. It is interesting to note that the best cell is at a little over 3.4 volts when the worst cell reaches 3.6v (because of the bottom balance.) Since I am charging from a regular wall outlet and currently only have 33 cells the charging current only tapers from 10 amps down to 5 amps and this takes only a few minutes. I did have to fool with the CV voltage a little to keep that weakest cell from going over 3.60 volts and I think I have the overall cutoff set to 3.55v (118v is what the charger displays.) I watched it closely the first few times but everything is so consistent there is just no point.


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## dougingraham (Jul 26, 2011)

Ziggythewiz said:


> ... but that's just crazy talk.


I had hoped someone might get a kick out of that. The first week I really wanted to see it but not so much anymore. I occasionally find myself wanting to know the amps when I head up a hill at highway speed but what would knowing it change?


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## frodus (Apr 12, 2008)

dougingraham said:


> The charger is set to trip off when the weakest cell reaches about 3.6 volts.





dougingraham said:


> I set the pack voltage cutoff by watching the worst cell (a little over 100ah while the best cell is a little over 105ah). When the worst cell reached 3.60v I read the total pack voltage and then set the charger to CV at that voltage.


So, the charger is not set to trip when it detects the weakest cell reaching 3.6V. What you did is *initially* set the bottom balanced pack such that the pack level cutoff happens to coincide with a voltage of 3.6V on the lowest cell. It still has no idea what is going on on the cell level, only pack level.


Note: I just wanted to clarify (not argue) so others that read this know you're not monitoring anything except pack level voltage. There's still no cell level monitoring (or even monitoring of the worst cell). It's all done manually by you.


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## Ziggythewiz (May 16, 2010)

dougingraham said:


> what would knowing it change?


I think limited monitoring is essential (though I do go overboard), even if it's behind the scenes and giving you a head's up with a CEL.

What if you had a cell go bad? It would either A)pass 3.6 before you expect it to, then completely die and possibly lead to B)it could fail shorted allowing the rest to charge higher than expected. 

3.6V + (3.6V / total cells) would not be too bad, but what if you lose 2 cells, or 3? Unless a cell fails open (or vents/smokes) you'd never know until you lost at least a noticeable chunk of the pack.


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## dougingraham (Jul 26, 2011)

frodus said:


> So, the charger is not set to trip when it detects the weakest cell reaching 3.6V. What you did is *initially* set the bottom balanced pack such that the pack level cutoff happens to coincide with a voltage of 3.6V on the lowest cell. It still has no idea what is going on on the cell level, only pack level


Correct. The charger is watching the pack voltage, not that weak cell. The CV point was selected so that the weakest cell does not get overcharged. I will probably dial it back a little more the next time I mess with it since there is no reason I need to get that one cell fully charged and the general consensus is that they will last longer if undercharged.

I received an automated email that this thread had been updated but I don't see that message here. That message pointed out that My response was off topic. I agree that my response was off topic and for that I apologize. I will try to restrain myself in the future.


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## dougingraham (Jul 26, 2011)

Ziggythewiz said:


> I think limited monitoring is essential (though I go overboard), even if it's behind the scenes and giving you a head's up with a CEL.


For this reason I will eventually put in one of the half pack comparators. That will happen when I get the other 19 cells in the car.



Ziggythewiz said:


> What if you had a cell go bad? It would either A)pass 3.6 before you expect it to, then completely die and possibly lead to B)it could fail shorted allowing the rest to charge higher than expected.


If the cell fails open or high resistance the pack won't get charged and you will know the next time you try to drive it. Also my charger will report a funky number for the number of AH put back in the pack. If the cell fails shorted then the whole pack will get a little more charge. And for this reason I will probably drop back a little more on my CV point. This is the one time I would like to see the voltage. A shorted cell would be obvious if you know the pack is supposed to be fully charged.



Ziggythewiz said:


> + (3.6V / total cells) would not be too bad, but what if you lose 2 cells, or 3? Unless a cell fails open (or vents/smokes) you'd never know until you lost at least a noticeable chunk of the pack.


Currently I have 33 cells but it will be 52 cells soon. Currently I have it cut off at 3.55 volts. With 33 cells this would go up to 3.69 volts average and with 52 cells this would go up to 3.62v. Of course in the case of a bottom balanced pack it isn't going to be an average. It is going to overcharge the weakest cell.

The three possibilities are shorted, high resistance, and open (a special case of high resistance). The only one of these that would pose a problem is the shorted situation. That one would overcharge a cell in the case of a bottom balanced pack. Either of the others would result in an undercharged pack. So the question is how likely is a hard shorted cell failure?


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## frodus (Apr 12, 2008)

Sorry Doug, I thought my tone was too harsh, so I rewrote some of what I said. I just didn't want this to turn into another BMS war. I do appreciate your input, I wanted to point out that this topic isn't for/against BMS, it's discussing the two particular types of BMS (Active and Passive).

I agree with Ziggy about "limited monitoring". IMHO some sort of cell-level monitoring is essential to protecting your pack. I could care less about balancing, but when/if a cell gets too low, too high or too warm I want the Charger (or controller, or fan) to do something about it to stop from ruining a pack. rwaudio did a nice breakout board for the Cell-log 8 boards that provide a cheap way to monitor LVC/HVC.

Unfortunately we're not always around when a cell decides to take a crap.


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## frodus (Apr 12, 2008)

dougingraham said:


> If the cell fails open or high resistance the pack won't get charged and you will know the next time you try to drive it. Also my charger will report a funky number for the number of AH put back in the pack. If the cell fails shorted then the whole pack will get a little more charge. And for this reason I will probably drop back a little more on my CV point. This is the one time I would like to see the voltage. A shorted cell would be obvious if you know the pack is supposed to be fully charged.


If one cell goes during charging, and it fails open, charge stops. If it fails shorted, things tend to snowball.... and it could happen before you have any time to react to it. If failure of a cell happens while you're driving, you're a bit SOL.... but if you monitored it, you could have a dash light indicate you have hit LVC on a cell. Maybe look into cell-logs and wiring the alarms in series to a dash indicator. It's cheap insurance and they can be modified to draw equal power on all inputs.


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## Ziggythewiz (May 16, 2010)

dougingraham said:


> For this reason I will eventually put in one of the half pack comparators. That will happen when I get the other 19 cells in the car.
> 
> So the question is how likely is a hard shorted cell failure?


Comparator's a good idea. I almost had mine in a couple weeks ago but it died in calibration.

I think high resistance would only be a temporary condition. It'll likely slag to open or closed eventually. I don't think anyone knows how common the different failures are, I don't plan to be the one to find out. I'm happy to leave that to Jack.


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## dougingraham (Jul 26, 2011)

frodus said:


> Sorry Doug, I thought my tone was too harsh, so I rewrote some of what I said. I just didn't want this to turn into another BMS war. I do appreciate your input, I wanted to point out that this topic isn't for/against BMS, it's discussing the two particular types of BMS (Active and Passive).
> 
> I agree with Ziggy about "limited monitoring". IMHO some sort of cell-level monitoring is essential to protecting your pack. I could care less about balancing, but when/if a cell gets too low, too high or too warm I want the Charger (or controller, or fan) to do something about it to stop from ruining a pack. rwaudio did a nice breakout board for the Cell-log 8 boards that provide a cheap way to monitor LVC/HVC.
> 
> Unfortunately we're not always around when a cell decides to take a crap.


Ahhh. I don't particularly want to participate in a BMS war. I've been involved in some flame wars over the years and generally nobody wins.

I agree some kind of monitor is a good idea. A half pack comparator is probably enough for a DIY person who understands how the cells work. An OEM making hundreds of thousands of packs might benefit from cell level monitoring and the cost per cell would be under a dollar. But there is always the cost benefit trade off. In a properly working pack no monitor is needed. Are these batteries that bad that under normal operating conditions they fail so often that we really need this? I certainly hope not but I have some extra cells just in case.


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## Ziggythewiz (May 16, 2010)

Even an OEM doesn't need cell level unless it's digital and they're remotely checking in on it. Just trigger the CEL depending on the level of imbalance.


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## Elithion (Oct 6, 2009)

riwe said:


> I found information about both active and passive balancing of cells in series.
> 
> Active balancing has the possibility to transfer energy between cells during charge and discharge in order to keep the cells on the same “level”.
> 
> Do you have any experience of active balancing verses passive?


Excellent question.

As it turns out, the answer has to do with the application, not the BMS:


Non-dissipative balancing ("active") should be considered if the application is heat sensitive and balancing requires a high current, because:
Cells will be changed often in the field with an unknown SOC, or
Convenience fast charging of very short duration is used, and SOC history algorithm is not used
 
Otherwise, for Li-ion cells (which require little balancing current), dissipative balancing ("passive") is very effective (technically effective and cost effective)
The fact is that all the off-the-shelf digital BMSs use dissipative balancing.
The only BMSs with non-dissipative balancing are custom made for a given application.

Actually, there is a Chinese analog protector that uses capacitive balancing. While they call it "active balancing", in reality it is at best 50 % efficient, due to simple fundamentals of physics, so it too is, after all, a dissipative balancer.

Ultimately, people choose to develop non-dissipative balancing circuits for philosophical reasons more than sound engineering reasons.

The Li-ion BMS book talks extensively about this subject; you can read it for free on Google books.


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## riwe (Nov 17, 2011)

Wow! I was not aware that this thread had grown so much. Thanks to all for your answers. The book is ordered and I will have a happy BMS christmas. 

Thanks again

Rikard


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## ruckus (Apr 15, 2009)

dougingraham said:


> I had hoped someone might get a kick out of that. The first week I really wanted to see it but not so much anymore. I occasionally find myself wanting to know the amps when I head up a hill at highway speed but what would knowing it change?


This is how I balance my checkbook. I put money in, I take money out.

Who cares what the balance is? 

-of course there is always that moment when - uh oh! you are bankrupt!


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## Elithion (Oct 6, 2009)

wessss77 said:


> I don't want to hijack a thread...


So kindly start a new thread, as you have a new topic.


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## wessss77 (Jan 4, 2013)

Elithion said:


> So kindly start a new thread, as you have a new topic.


Done...Thanks Davide


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