# The big BMS debate.



## illuminateddan (Dec 19, 2009)

Hi all,

So I've been watching Jack Rickards videos and now have an issue. Jack knows his shit (to put it mildly) and is an advocate of the 'no shunt bms' theory. Now I'm building a bms based on peter perkins slave boards and I've come up with a 'variable point balancing system' which is not a top balancer or bottom balancer but a system that balances all the cells to the lowest cells voltage before a charge. Having seen jacks videos I'm now wondering if I should carry on with this BMS and VPBS, especially since reading voltages from lifepo cells becomes problematic when they are under load, therefore a better system is to check cell voltages when the car is at rest, rather than while driving. As my ideas and theories have now been updated, what is everyone else doing? is shunt bmsing still the way for most or is bottom balancing the main theory? How do you measure voltage on cells the fluctuate wildly when under load and not under load. According to some graphs this can be up to 0.3v between load and non load voltages.

aaarrrgghhh! Thoughts??? 

D


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## Dolphyn (Nov 17, 2009)

Jack assumes that you won't know a cell is getting weak unless all of your cells get weak at the same time. If you have a system that monitors the performance of the "worst" cell, I think Jack's bottom-balancing theory goes out the window.

I use CellLog 8S to warn me when any one cell is running out of juice, and I can use the rest of the pack to replenish that one cell (using DC-to-DC converter from Ebay), so I don't want all of my cells to run out at the same time.


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## dimitri (May 16, 2008)

illuminateddan said:


> I've come up with a 'variable point balancing system' which is not a top balancer or bottom balancer but a *system that balances all the cells to the lowest cells voltage before a charge*.


This describes bottom balancing, so why do you say its not?
If you want your pack balanced, you pick either top or bottom, since those are the only practical options based on voltage reading. Bottom balancing is usually done manually and not frequently, since its time consuming. Doing it before each charge cycle is not practical because you will be wasting time and won't be able to use the car until done. Top balancing is much easier to do at every cycle and can be always skipped or cut short if you suddenly need to drive the car.

Either balancing method typicaly uses shunting, so its funny when people are against shunting, yet keep talking about balancing their packs. Any non-shunting balancing method uually involves lots of power switching components and gets too complex/expensive to be worth the trouble.

Why does it have to be a debate? There isn't one true solution for all, many are perfectly valid. The best one is what best fits YOUR needs for YOUR pack, YOUR car and YOUR wallet.




> Jack knows his shit


Now, this one we can certainly debate.


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## Guest (Sep 8, 2010)

> Jack knows his shit
> Now, this one we can certainly debate.


Can be said about anyone. Knowing ones shit is quite subjective.


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## dimitri (May 16, 2008)

gottdi said:


> Can be said about anyone. Knowing ones shit is quite subjective.


The smiley face was a hint that I was making a joke


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## Guest (Sep 8, 2010)

I can be on the light sided end of things.


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## illuminateddan (Dec 19, 2009)

dimitri said:


> This describes bottom balancing, so why do you say its not?


Bottom balancing is performing a balance when the pack is at it's lowest point, say 3v. Variable point balancing is a frequent shunt balance before every charge. Here's how my system works:



When you plug in the car the master controller -EVMS (electric vehicle management system) sends out a voltage data request to all the slave boards.
The slave boards respond with their ID and voltage.
The EVMS then looks through the data and finds the lowest cells voltage - lets call it 'nV'.
The EVMS then sends out a packet to all the slaves instructing them to set their discharge point to 'nV' and standby.
The EVMS then requests the slaves to send back their discharge point for confirmation, which they do.
The EVMS then sends a 'GO' command and the slaves discharge all the cells to the lowest discharge point 'nV'.
The slaves then deactivate their shunt discharge and the master initiates the charge sequence.
As the cells are being matched to 0.01v the pack should always stay fairly balanced. Bleeding off a tiny amount (a few hundred mAh ish) of juice is far more energy efficient than bleeding lots off the top, and having the system automated in the charge cycle saves lots of time.



As to whether Jack knows his shit or not, well that is another debate. (although the weight of imperical evidence goes in his favour as he has the cars, the equipment, the workshop, the HD TV show, the manufacturer support and the experience!) Anyone else take the trouble to make a 105min video showing a cell charging and explaining whats happening?


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## mhud (Oct 19, 2009)

illuminateddan said:


> Anyone else take the trouble to make a 105min video showing a cell charging and explaining whats happening?


This is my problem with Jack. He often has good info, but he repeats himself constantly and takes ages to explain things. 

I'd love it if he'd distill his thoughts consicely and put them out there. Instead it seems like he hits `record` and just goes. 

But, one can't deny that he has got experience. That's the problem. Do you subject yourself to 1.5 hours of Jack to see if you'll learn something? Maybe I'm just spoiled in the age of Internets.

Anyway, my $0.02 is that it's worth monitoring individual cells and wiring a dc/dc to cycle through the weakest ones while driving, propping them up. This is the only way I know of to get around the weakest cell being your range limiter.

But I'm a newbie with a lead-acid pack, so value my words accordingly.


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## DIYguy (Sep 18, 2008)

mhud said:


> Anyway, my $0.02 is that it's worth monitoring individual cells and wiring a dc/dc to cycle through the weakest ones while driving, propping them up. This is the only way I know of to get around the weakest cell being your range limiter.
> 
> But I'm a newbie with a lead-acid pack, so value my words accordingly.


Hi mhud,

I have a pack of AGM's which need to be balanced ( without boiling) My charger will do this perfectly as long as there is balance in the pack. So, I use these..... http://www.hdm-sys.com/pdf/hdm_equalizer_specs.pdf
These shunt up to 5 amps during charge, discharge and idle. 

So, the charger takes care of overcharge, the controller takes care of running them too low, and the balancers keep em all identical siblings....  The batteries will last longer, and you will get the best performance out of the pack.


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## Guest (Sep 9, 2010)

I watch Jacks show because it is informative and besides being a bit slow it is a good late night watch. Minimized and concise would make for a boring and quick show. What fun is that. Sure Jack's show is not like our evening drama shows but it's fine. I'd like to see more how to's like actually building Lee Hart Shunts and other stuff like that. I hope that stuff will come. Or maybe someone else can do a howto program. Mmmmmm. Also a show a bit more focused on those of us with far less money to spend on building an electric car.


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## mhud (Oct 19, 2009)

DIYguy said:


> Hi mhud,
> 
> I have a pack of AGM's which need to be balanced ( without boiling) My charger will do this perfectly as long as there is balance in the pack. So, I use these..... http://www.hdm-sys.com/pdf/hdm_equalizer_specs.pdf
> These shunt up to 5 amps during charge, discharge and idle.


I have AGMs too, so overcharging is a serious problem for me as well. I didn't find the HDM balancer until I'd bought my PowerCheqs. They're working well for me. A single unit like yours which could do 10-12 batteries would be awesome. 

gottdi, I'd absolutely love to see or hear someone talking about the Lee Hart system. I admired that and was considering building it instead of buying my PowerCheqs. A new system like that which could prop up individual Li cells would be great. If you wanted to limit the cost, you could balance blocks of 4 Li cells with 12V hardware and go off the existing Lee Hart documentation. When you think about it, the Pb cells are 6x2V cells that we don't bother to monitor individually...


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## illuminateddan (Dec 19, 2009)

Yup, I'd agree that he does repeat a lot and the show could do with an edit (and maybe removing the chip on the shoulder, but it does make good late night viewing while doing other things. 
The trouble is, there is still no firm answers on what is the best method for looking after lifepo packs, and there needs to be a consus on a definitive besat practice method.


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## Duncan (Dec 8, 2008)

Hi Illuminateddan

_*Bottom balancing is performing a balance when the pack is at it's lowest point, say 3v. Variable point balancing is a frequent shunt balance before every charge. Here's how my system works:

*_

_*When you plug in the car the master controller -EVMS (electric vehicle management system) sends out a voltage data request to all the slave boards. *_
_*The slave boards respond with their ID and voltage. *_
_*The EVMS then looks through the data and finds the lowest cells voltage - lets call it 'nV'. *_
_*The EVMS then sends out a packet to all the slaves instructing them to set their discharge point to 'nV' and standby. *_
_*The EVMS then requests the slaves to send back their discharge point for confirmation, which they do. *_
_*The EVMS then sends a 'GO' command and the slaves discharge all the cells to the lowest discharge point 'nV'. *_
_*The slaves then deactivate their shunt discharge and the master initiates the charge sequence. *_
I have a couple of questions, 
In the other systems I have seen the "SHUNT" does not seem to work by discharging the cell - which would require some way of dumping the energy - but works by permitting the charger to -not charge- that cell by bypassing it

If you use that system and your logic you could use the shuts to "bypass" all but the weakest cell, charge that and then remove the bypasses to cascade the other cells in sequence to charge them as well.

Does that make sense??


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## Guest (Sep 9, 2010)

illuminateddan said:


> Yup, I'd agree that he does repeat a lot and the show could do with an edit (and maybe removing the chip on the shoulder, but it does make good late night viewing while doing other things.
> The trouble is, there is still no firm answers on what is the best method for looking after lifepo packs, and there needs to be a consus on a definitive besat practice method.


Well I'd suggest a better opening and ditch that little house on the prairie style garbage. He needs something more in tune to the application. But not what he is using. The opening sucks. With all the crap and flack he gets I am surprised he does not just shut down his video for us all to view and go about his work in silence. He has the right and has earned that chip on his shoulder. He has given lots of proof and has shown his metal. He has put his money where his mouth is compared to few around here. Like he said, it's easy to type yourself smart, it's another to prove it. He has proof. Freely given proof. 

Pete


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## illuminateddan (Dec 19, 2009)

gottdi said:


> Well I'd suggest a better opening and ditch that little house on the prairie style garbage. He needs something more in tune to the application. But not what he is using. The opening sucks.


Amen to that. I go make coffee while the intro comes in.


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## illuminateddan (Dec 19, 2009)

Duncan said:


> Hi Illuminateddan
> 
> I have a couple of questions,
> In the other systems I have seen the "SHUNT" does not seem to work by discharging the cell - which would require some way of dumping the energy - but works by permitting the charger to -not charge- that cell by bypassing it
> ...


Hi Duncan.
When I refer to shunt balancing I'm actually just using the common term for 'a fat resistor to discharge'. so my bad. My system doesn't shunt current from cell to cell, it just discharges cells. I'll be charging at 30A so shunting 30A across a cell is hard and costly in heat terms.

Even with other shunt systems, all they are doing is discharging the cell while it charges, usually at about 1-5w. This seemed a little non sensical to me so I had a rethink.

One of my main issues is gather cell V data and making it relevant - i.e. v under load is miles away from V under no load.

d


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## Guest (Sep 9, 2010)

illuminateddan said:


> Amen to that. I go make coffee while the intro comes in.


I just download first then just jump past that section. 

Pete


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## JRP3 (Mar 7, 2008)

Yup, download and Fast Forward are your friends. I couldn't make it through the third drive in the Speedster.


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## mhud (Oct 19, 2009)

JRP3 said:


> Yup, download and Fast Forward are your friends. I couldn't make it through the third drive in the Speedster.


If you play videos in VLC (on any platform) you can play back at various speeds with audio... It's weird at first but pretty handy! And it corrects the pitch so it doesn't sound like listening to chipmunks.


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## JRP3 (Mar 7, 2008)

dimitri said:


> This describes bottom balancing, so why do you say its not?


Sort of. It sounds as if he would pull all cells together even if the pack is only partially depleted so it's not a true "bottom" balance. As you know there isn't much difference in the middle of the discharge curve, so balancing a pack in that range won't give you a bottom balanced pack.


> Either balancing method typicaly uses shunting, so its funny when people are against shunting, yet keep talking about balancing their packs. Any non-shunting balancing method uually involves lots of power switching components and gets too complex/expensive to be worth the trouble.


Well you can bottom balance and top balance manually by adding charge as well as removing charge. I use both to bottom balance more quickly if appropriate.


> Why does it have to be a debate? There isn't one true solution for all, many are perfectly valid. The best one is what best fits YOUR needs for YOUR pack, YOUR car and YOUR wallet.


Indeed. There is no "best practice" since the ultimate goal is to preserve cells by staying away from the ends of the curve and there is more than one way to do so.


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## illuminateddan (Dec 19, 2009)

One of my bug bears with 'shunting' is that most systems dont shunt, they bleed. Just beacause you stick a 5w resistor from positive to negative on a cell during charging it does not mean you are shunting the current, as the charge current is usually 10-30A and a 15ohm resistor at 4v will pull through 0.3A. Even taking into account a lifepos low resistance, you are still pushing most of the current through the cell.

A better term would be discharge balancing?


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## Tesseract (Sep 27, 2008)

illuminateddan said:


> ...the charge current is usually 10-30A and a 15ohm resistor at 4v will pull through 0.3A...


If you are talking about a top-balancing system then the charging current during the CV portion of cycle is usually less than 2A - it is easy to bypass most of that current (or all of it) with a shunt resistor controlled by a transistor.

If you meant to discuss a bottom-balancing system then charge current is irrelevant - the shunt in this case might be used to discharge all cells to a pre-programmed low voltage cutoff at a relatively modest current of, once again, 1-2A.

In either case, if the pack is initially balanced in some fashion, the shunting BMS has to correct relatively small imbalances, regardless of which end is chosen.

NB: I am mostly agnostic on the top balance vs. bottom balance debate. I agree that top-balancing is much easier to do and more forgiving if prematurely terminated, but I also agree that bottom balancing is much more likely to protect a pack during driving, it's just harder to do right and, of course, you can't interrupt a bottom balancing midway to go drive like you can a top balancing.

So, top balancing is more end-user friendly but bottom balancing is more cell-life friendly. 

You can also skip balancing completely, but I feel you should at least monitor cell voltages.

Finally, there are several reasons lead-acid packs rarely sport a BMS, but the main one is that without access to the individual cells it's kinda pointless. That they only last a few hundred cycles regardless of how well you treat them is another big reason...


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## illuminateddan (Dec 19, 2009)

Aha, Ok, thats the answer I was looking for! So here's a silly question...
If a lifepo cell has an internal resistance of 'x' then surely using a resistor of value 'y' would cause a combined resistance of (1/x+1/y)/(1/x * 1/y) and thus create a resistance 'anomoly' in the string of similar resistances which therefore draws more/less current and therefore causes an imbalance in the voltages across all the cells. 

Is this correct or am I missing stuff?


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## Tesseract (Sep 27, 2008)

illuminateddan said:


> ...Is this correct or am I missing stuff?


You are neglecting a few things, first of which is the "implied" or equivalent resistance of the cell voltage - that is, the voltage drop across a cell is a function of the current through its internal resistance, yes, but it mainly comes from the terminal voltage of the cell!

Let's consider an LFP cell with 10 milliohms of internal resistance. To push 10A through the cell you would need to apply a voltage which exceeds the sum of its terminal voltage (e.g. - 3.2V) and the product of the charging current through its internal resistance (10A * 0.01Ω, or 0.1V). So, the internal resistance requires that the charger supply 0.1V more across the cell to push 10A through it than would otherwise be necessary.

The "implied resistance" of this cell, though, is the equivalent resistance required to present the same load to the charger (as if there wasn't an electrochemical cell present, in other words) Just plug the relevant numbers into Ohm's law: R = V/I

Thus, the cell is equivalent to a 0.33Ω resistor because forcing 10A through it also requires a potential difference of 3.3V. At the end of the charging cycle, though, a constant voltage rather than a constant current is applied, so the cell might be at 3.6V with just 1A of current through it. The equivalent resistance now is 3.6Ω.

Starting to see how this works? The logic behind balancing is that different cells will have different Ah capacities because of manufacturing variances, etc., so if a series stack of cells is charged with a fixed number of Ah then each cell will be at a different voltage. Bottom balancing seeks to make those differing voltages converge near the fully-discharged end of the curve while top balancing seeks to make the cell voltages converge at the fully-charged end. That's it in a nutshell.

Why people get so bent out of shape over this is beyond me as it should be obvious that it is possible to have a reliable battery pack with top balancing (stay away from the bottom end of the cell voltage curve), or bottom balancing (stay away from the top end) or even no balancing (stay away from both ends!). You get the most useful capacity from bottom balancing a pack, but give up quite a bit of flexibility because if the pack is in the process of being bottom balanced you can't interrupt it - all you'll be left with is a nearly dead pack. Not balancing the pack at all demands that you give up capacity on both ends of the charge curve which has the net effect of making the pack more expensive (less usable Ah for the same $). Thus, top balancing is preferred by most DIYers because it lets you end charging early if necessary and only requires you chop off the capacity at the end of the discharge curve, where the cells start to perform less well anyway. The downside to top balancing is that it is unlikely you will notice if the weakest cell breaches the minimum allowed voltage (called LVC, for low voltage cutoff) which could result in damage to it. A BMS which signals when a single cell hits LVC can tell a controller to throttle back and/or alert the driver of the condition making it just as safe as a bottom balanced pack.

I am loathe to put words into Jack Rickard's mouth, but I think most of his antipathy towards BMSes is the result of poor design of the shunting. The shunt can fail on, either causing a fire or else discharging the cell. If a centralized system is used then unfused wires from each cell can get chafed or damage then short and cause a fire... etc. I disagree with him when he makes the jump from disliking bad engineering to disliking all BMSes because of it. Since he can't defend himself here, though, I'll leave it at that.


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## JRP3 (Mar 7, 2008)

One point about interrupting bottom balancing, it's not a regular event and even moderate planning will avoid that possibility. 
I believe Jack has said he's not actually against the concept of a BMS but isn't happy with current implementations. He mentioned a wireless bluetooth setup with a chip on each cell. I looked into the possibility of a RFID version but never got much further than it might be possible.


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## etischer (Jun 16, 2008)

60-100 wireless bluetooth devices... not exactly going to improve reliability and lower cost. 

At least Jack will be less likely to burn it down since there are fewer wires!



JRP3 said:


> One point about interrupting bottom balancing, it's not a regular event and even moderate planning will avoid that possibility.
> I believe Jack has said he's not actually against the concept of a BMS but isn't happy with current implementations. He mentioned a wireless bluetooth setup with a chip on each cell. I looked into the possibility of a RFID version but never got much further than it might be possible.


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## MN Driver (Sep 29, 2009)

gottdi said:


> I watch Jacks show because it is informative and besides being a bit *slow* it is a good late night watch. Minimized and concise would make for a boring and quick show. What fun is that. Sure Jack's show is not like our evening drama shows but it's fine. I'd like to see more how to's like actually building Lee Hart Shunts and other stuff like that. I hope that stuff will come. Or maybe someone else can do a howto program. Mmmmmm. Also a show a bit more focused on those of us with far less money to spend on building an electric car.


I noticed that he has a slower pace to his conversation as well and some of the videos can take roughly the amount of time it takes me to watch a feature film.

Here is what I do to solve both the slower talking speed and long video problem. Download VLC Media Player, it plays pretty much every video codec out there and it has a great feature that I use every time. From the menu bar on top I click on Playback and then Faster. I watch all of Jack's videos at either 1.5x speed or 2x speed. ...for the battery charge/discharge, I can make out what he is saying at 3x speed and I feel less as if I was watching grass grow as the voltage slowly climbs or drops as he tries to explain what is happening.

Here is a link, it also allows me to open the video while its still downloading and as long as I don't get beyond the point that it's already downloaded it will play through the whole way.
http://www.videolan.org/vlc/

Hope this helps!


...on to my own opinion. Which I've held back for awhile and likely will charge here and there in small amounts over time.

I see it this way with LiFePO4, as far as battery management goes. If you avoid going too high, too low in voltage, over temperature, or over current they can be damaged. The idea is simple but its all about avoiding the damage from those extremes. Usually we have enough Ah capacity to avoid over current and over temperature issues, so voltage is the typical point we all watch.

To say you don't need to balance at all doesn't make sense, you would need to have at least initially balance the cells. If the cells stay in balance well enough after this than something as simple as an HVC/LVC set at an extreme to disable the charge/discharge would protect a pack just fine but if it hits the LVC or HVC and the rest of the cells aren't in line, you are running without the full capacity of the pack so it would require at least some check up. Working with Lithium Cobalt cells in non-EV applications, I've never come across a battery pack that would stay within balance for more than a few years. I use LVC/HVC/temperature monitors on those and after I find out that I've lost about 30% of my capacity after a year, I discharge them and find they aren't balanced, charge them up with a CC/CV individually and discharge again seperately and the cells are in balance, put them back in the device and all is well. Apparently the LiFePO4 isn't as bad at self-discharge or differing internal resistance causing imbalance though as there are people here that have no issues. Until I saw peoples experience with this, based on my experience with Lithium Cobalt, I had many reasons disagree with a bottom balance or not at least monitoring a pack at the cell level. It's clear that these cells differ in this respect. ...They also differ massively from the NiMh cells I've handled that are notorious for having varying self-discharge rates amongst a large series pack, but that's not the topic of this thread.

Top balance - Top off the cells when charging. Doesn't necessarily need to happen every time and if its set up right won't waste much energy or create much heat as it should be at the end of the current taper down. Not running without a low voltage cutoff of some kind or monitoring discharge Ah with a known capacity to the cells to prevent the low voltage that will kill a battery. Not monitoring things at the cell level and waiting for the voltage to drop assuming a uniform level is where Jack got bent out of shape because he didn't understand the dynamics of batteries with slightly dissimilar capacities put in series so he didn't know how to watch them properly.

Bottom balancing - Run the batteries down to a low voltage point and match the voltages, charge them up so that way the cell with the highest voltage isn't up into stress that will damage it, find the pack voltage and use judgment to set it at this point or slightly lower to be safe.

I wouldn't mind a very simple LVC setup and I'd go from cell to cell and balance on the charged side of the SOC with a hobby charger(I already have one, only cost me about $60 and there are lower cost ones out there too) once and awhile, skipping the shunts all together. Possibly even just hitting the pack with a volt meter occasionally toward the end of the charge to see if they were all close while still being subjected to the constant voltage phase near the end and only adding a little current to ones that exhibit a lower voltage. Keeping a slightly conservative charge voltage would help prevent issues with high ending voltage in any one cell. I feel the risk is minimal and the setup could be low cost with a serially wired system that doesn't carry current and an LED and buzzer setup that goes on and stays on to easily identify the culprit if the LVC trips, which wouldn't happen with a properly planned pack designed not to drop below 70 or 80% DOD. The only other time I'd expect to see it is through lower temperatures causing heavy voltage sag or a drive where the DOD is below 80%. It's not too difficult to find the low voltage point under full acceleration at 20% state of charge at a cooler temperature and set an LVC appropriate to the point where damage would be prevented if the discharge was stopped at that point.

...your opinion may vary.


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## Elithion (Oct 6, 2009)

In the middle ages the experts debated how many angels fit on the tip of a pin. Today some debate whether you can run cars on tap water. You and I, reading about those debates, will just roll our eyes.

This BMS "debate" may be raging among hobbyists, but bring it up in circles of professional BMS designers and users, and all you will get is a knowing, condescending smile. Instead of doing that, if you'll let me, I'd like to try to address the point you raised.



illuminateddan said:


> I've come up with a 'variable point balancing system' which is not a top balancer or bottom balancer but a system that balances all the cells to the lowest cells voltage before a charge.


That's the worst possible place to do voltage based balancing of Li-Ion cells, because of the plateau in their OCV vs. SOC curve. You CAN balance at mid SOCs, but you must do so based on decisions taken when you measure the voltage at 100 % SOC or close to it.

Mid SOC balancing is how HEV manufacturers do it, and it's really tricky. They are forced to do it that way because the pack is never fully charged or discharged. You, on the other hand, with an EV, have the luxury of fully charging the pack, so you don't have to get yourself caught in the nightmare of mid SOC balancing.







illuminateddan said:


> check cell voltages when the car is at rest, rather than while driving.


Yes, that is exactly why you measure the cell voltage and make balancing decisions only at the end of charge and at zero or low current (while the charger is off, or even while charging if the charging current is much lower than the driving current).



illuminateddan said:


> How do you measure voltage on cells the fluctuate wildly when under load and not under load.


You can measure the cell voltage any old time. But, for balancing purposes, you make decision based on the cell voltage only when the current is low or zero (and the SOC is close to 100 %). 
The better BMSs are able to extract the OCV (Open Circuit Voltage) from the cell terminal voltage by knowing the current and cell resistance, and subtracting out the IR drop in the cell. That is quite complicated, so I suggest you keep it simple and measure the voltage at low or no current.



illuminateddan said:


> Having seen jacks videos I'm now wondering if I should carry on with this BMS


 Jack doesn't use a BMS. You are designing a BMS. So, Jack's comment are totally inapplicable to your work: you may safely ignore him.



illuminateddan said:


> most systems don't shunt, they bleed... as the charge current is usually 10-30A and a 15ohm resistor at 4v will pull through 0.3A.


 What you forgot is that a good BMS turns the charger off and on. The result is that the average charging current is equal to the "bleed" current of the balancing load (average current = full current * On/Off duty cycle), so no problem. As the pack gets balanced, the charge current is naturally reduced, and the charger stays on more and more of the time, until its current is equal to the balance current. At that point, the charger stays fully on, until the pack is full.




illuminateddan said:


> As the cells are being matched to 0.01v the pack should always stay fairly balanced.


1) Yes, in theory. But they will be balanced at whatever SOC happens to be at the time. As soon as you charge the cells further up from that point, or discharge them further down from that point, they will be unbalanced (because of capacity variations). 

2) No, because, for Li-Ion cells at mid SOC, 0.1 V means very little. Now, 0.1 V at either 0 % SOC or 100 % SOC means a lot, but at 50 % it's poorly related to SOC. The cells may be off by 20 % SOC. If you balance based on mid SOC voltage, the unbalance will be on the order of 20 %, which will cost you dearly in terms of total available battery capacity.




illuminateddan said:


> there is still no firm answers on what is the best method for looking after lifepo packs..


Oh yes there is. Just ask any professional who has a deep understanding of Li-Ion and BMSs, and they will all give you the same answer. It's only when you listen to charismatic amateurs that you think that this question is still open to debate.



illuminateddan said:


> If a lifepo cell has an internal resistance of 'x' then surely using a resistor of value 'y' would cause a combined resistance of (1/x+1/y)/(1/x * 1/y) and thus create a resistance 'anomoly' in the string of similar resistances which therefore draws more/less current and therefore causes an imbalance in the voltages across all the cells.


 Sorry, but that makes little sense: the internal cell resistance is not in parallel with the balance load. In any case, the two are different by 3 orders of magnitude!



illuminateddan said:


> Now I'm building a bms ...


I think that you'll benefit greatly from reading my "Battery Management Systems for Large Lithium-Ion Battery Packs". Chapter 5 tells you all about designing your own BMS, and will save a lot of time and might let you get it working well right off the bat.
http://book.liionbms.com/
http://www.amazon.com/Battery-Management-Systems-Large-Lithium/dp/1608071049/ref=sr_1_1?

Davide
________
FALCON XR6 TURBO


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## JRP3 (Mar 7, 2008)

Elithion said:


> This BMS "debate" may be raging among hobbyists, but bring it up in circles of professional BMS designers and users, and all you will get is a knowing, condescending smile.


Of course, because professionals have to assume the end user is clueless.


> Oh yes there is. Just ask any professional who has a deep understanding of Li-Ion and BMSs, and they will all give you the same answer. It's only when you listen to charismatic amateurs that you think that this question is still open to debate.


Not sure of my level of charisma but it is quite obvious that a BMS is only one way to avoid over charging or over discharging a cell. A "manual" BMS takes a little effort but there is no magic about it. If you need more than 80% of your pack's range most of the time and can't spend some time playing with your cells then you probably need an automatic BMS.


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## Crash (Oct 20, 2009)

I haven't read through all the posts on this thread, but from what I recall Jack Rickard says that the reason the BMS wouldn't work is because there's be too much EMF interference from the motor and it would effect the analog signal to the controller.

I thought about that and while it makes some sense, it doesn't make enough that you go without a BMS. In my project I'll be putting the motor in the very back of the car and the batteries in the very front. That would alleviate the issue Jack theorizes, but may not necessarily be an issue to begin with. 

Shielding the analog wires would also alleviate the issue. But I just don't see how the EMF coming off the motor would be enough to throw the controller's readings off that much.

I'm not an EET or ET so it may be over my head.


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## illuminateddan (Dec 19, 2009)

Elithion said:


> This BMS "debate" may be raging among hobbyists, but bring it up in circles of professional BMS designers and users, and all you will get is a knowing, condescending smile. Instead of doing that, if you'll let me, I'd like to try to address the point you raised.


Well, this is why I started this thread. I am an amateur in the electric car field and am not an electrical engineer, my profession lies elsewhere. My issue is that I have searched this forum hundreds of times and not found a conclusive 'this is the way its done'. Maybe we could all benefit from a few sticky threads on the basics, such as balancing and charging.

The answers provided by yourself, Tesseract and others are very helpful and are why I come to the forum. If I had all the answers I wouldn't be asking the questions and I wouldn't be coming to forums!

As for all your other points, thanks for explaining it. I am now more knowledgable!


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## Crash (Oct 20, 2009)

Thomas Edison tried over 10,000 attempts to make a light bulb. It's really a good thing he didn't give up trying.

A Battery Management System such as the complex creatures they are, are in fact going to be very important components in the electric vehicle world. Maybe someday, when purple unicorns are actually discovered, we'll have some of the battery technologies promised that won't require balancing. In the meanwhile, you're right that we should be comparing notes more and deciding what works and what doesn't.

Standards are what keep the industrial age going. Unfortunately, there aren't any standards for batteries at the moment. But if we can standardize the construction and use of a BMS system - perhaps through an open source initiative - we can start to nail down the issues at hand and fix them. In addition, it could drive the cost of a BMS down while still keeping it profitable for manufacturers (large and small) to build these units for people like me whom cannot build them on their own.

If Jack Rickard is correct, which has yet to be proven though his theory sounds correct, then it would be a good time to experiment, correct? I would say it's time to test his theory and see if it holds even enough water to redesign and figure this out.


Just as in the case of the light bulb, a BMS is new and there will be MANY ways to unsuccessfully make a BMS. It's time to start sharing information and instead of going through 10,000 iterations as Edison did, put brilliant minds together and get this done correctly and quickly. Electric vehicles are not new to the industry, but they've been in hibernation long enough to say this IS a new industry. So it's up to the DIYers to pioneer what the automakers have put off for many years.


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## JRP3 (Mar 7, 2008)

Crash said:


> Maybe someday, when purple unicorns are actually discovered, we'll have some of the battery technologies promised that won't require balancing.


I think getting manufacturers to group cells more closely would go a long way to finding those unicorns. There is no reason we can't get cells right now with the exact same ah capacity and very close resistance. My pack of 100ah cells range from 110ah - 114ah yet the data sheet for the shipment they came from showed other cells in the same shipment with similar ranges. If after testing at the factory all the 110 cells had been grouped together, and all the 111 cells, etc. etc., and the same thing with internal resistance, you'd get a pack so closely grouped you could likely get away with a once a year balance, maybe even less.


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## pm_dawn (Sep 14, 2009)

Crash said:


> I haven't read through all the posts on this thread, but from what I recall Jack Rickard says that the reason the BMS wouldn't work is because there's be too much EMF interference from the motor and it would effect the analog signal to the controller.
> 
> I thought about that and while it makes some sense, it doesn't make enough that you go without a BMS. In my project I'll be putting the motor in the very back of the car and the batteries in the very front. That would alleviate the issue Jack theorizes, but may not necessarily be an issue to begin with.
> 
> ...


Well I actually think that having the controller and motor further away from the battery pack could make things worse when it comes to the Electric noise problems. You have to consider also the cables going between the pack and the controller as a very good transmitter of noise.

I think Tesseract could explain that in more detail, or has done already in an earlier post.

Regards
/Per


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## Crash (Oct 20, 2009)

JRP3 said:


> I think getting manufacturers to group cells more closely would go a long way to finding those unicorns. There is no reason we can't get cells right now with the exact same ah capacity and very close resistance. My pack of 100ah cells range from 110ah - 114ah yet the data sheet for the shipment they came from showed other cells in the same shipment with similar ranges. If after testing at the factory all the 110 cells had been grouped together, and all the 111 cells, etc. etc., and the same thing with internal resistance, you'd get a pack so closely grouped you could likely get away with a once a year balance, maybe even less.


Interesting. I know that GM isn't so worried about balancing the batteries with the Volt's pack. They keep the SOC between 30 and 80% at all times. Even if a cell were to go out of balance, it would have to go WAY out of balance to cause a problem. GM's method is to sustain battery life and how they engineered the Volt is actually pretty smart (after really sitting down and thinking about it). Unfortunately, the buyer is going to pay 2x as much for a battery pack they're going to get 50% out of. In my case, it wouldn't matter since I don't drive more than 40 miles a day since I work from home.

When batteries get cheap enough and light enough, we may have the luxury of doing as GM does, and just keeping the SOC in a manageable range. This, like you said, may only require a balance once a year.



pm_dawn said:


> Well I actually think that having the controller and motor further away from the battery pack could make things worse when it comes to the Electric noise problems. You have to consider also the cables going between the pack and the controller as a very good transmitter of noise.
> 
> I think Tesseract could explain that in more detail, or has done already in an earlier post.
> 
> ...


Interesting. The way I want to set things up, I'd only have two large cables going to the back of the car. Positive and Negative. I'd imagine I could shield those quite easily, correct? With the controller and motor sitting in the back, my idea was to run the two power cables and coolant lines though the old torque tube tunnel. The data cable and POT sensor cable would be run through the inside of the car to the back. This would leave the charger, DC/DC converter, and accessory pumps up front under the hood with the batteries and BMS. 

Can someone chime in about what it would take to shield the cables correctly? Pros/Cons to this setup EMF-wise?


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## illuminateddan (Dec 19, 2009)

Crash said:


> Can someone chime in about what it would take to shield the cables correctly? Pros/Cons to this setup EMF-wise?


Personally I look at shielding my signal wires on my electronic systems before lookign at the "big" issue. I just used shielded cable bonded to ground and a varistor and pull down resistor on the data lines.


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## etischer (Jun 16, 2008)

Crash said:


> GM's method is to sustain battery life and how they engineered the Volt is actually pretty smart (after really sitting down and thinking about it). Unfortunately, the buyer is going to pay 2x as much for a battery pack they're going to get 50% out of.


After seeing the Volt's battery pack, I would never consider buying the volt. There are about 200 hose connections inside waiting to spring a leak. I think the pack is a bit undersized, so they resorted to extreme cooling methods to keep it cool. Their BMS boards don't appear to be anything fancy. 
http://www.diyelectriccar.com/forums/showthread.php/plug-america-2010-pictures-47631.html


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## JRP3 (Mar 7, 2008)

I thought they over sized the pack and only discharge it to 50% SOC?


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## dimitri (May 16, 2008)

Volt battery management and DIY EV battery management are apples and oranges, how did it even get in the same thread?

1. Volt is not an EV, its a hybrid, so battery management is dramatically different. Part of capacity must be reserved to serve as a buffer when ICE genset kicks in, since genset alone can't supply surge current under heavy acceleration, so battery reserve is used as a buffer.
2. Volt battery is a different Lithium chemistry, which has completely different thermal profile than LiFePo4. It needs active cooling, while LFP does not in most cases, but LFP has poor energy density, so OEM manufacturers rarely use LFP. Most OEM EVs use other Lithium chemistries. DIY EV market uses LFP because its easy to manage and has longer life cycles. OEM EVs don't care about life cycle, they want you to come back in a few years for a new battery.


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## 280z1975 (Oct 2, 2008)

I've been reading the DIY EC boards for a while now ... keeping abreast with what is going on, watched the switch from Lead Acid to Lithium over the last two years, but the BMS debate just kills me ... everyone has an opinon and there seems to be little concensus on anything.

So a few questions:

1) What do OEM EV makers do for a BMS system? 

2) Are there any set-it and forget it systems out there? Me I want to build my car and then not have to run around the trunk and under the hood once a week with a multi-meter checking each cell after the car has settled. I want a charger/bms combo that will handle those things for me with only the need to replace/repair a cell (after a warning from the system) that there is a problem. 

A lot of people here talk about charging up with small charger to bring up individual cells, monitoring this and that several hours after charging, watching packs charge. I like the idea of building my electric car, but not about spending my hours worrying over my pack.

At this point, after a lot of reading of several threads I am no closer to making a choice on a BMS system.


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## JRP3 (Mar 7, 2008)

dimitri said:


> OEM EVs don't care about life cycle, they want you to come back in a few years for a new battery.


I don't think that's true. They have to warranty the battery for 10 years I think, which is one of the reasons GM doubled the size of the pack to assure it lasts. The last thing they want is to replace any component under warranty, let alone a whole battery pack.


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## JRP3 (Mar 7, 2008)

280z1975 said:


> 2) Are there any set-it and forget it systems out there? Me I want to build my car and then not have to run around the trunk and under the hood once a week with a multi-meter checking each cell after the car has settled. I want a charger/bms combo that will handle those things for me with only the need to replace/repair a cell (after a warning from the system) that there is a problem.


Then you need a full BMS with shunting, no debate needed.


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## 280z1975 (Oct 2, 2008)

JRP3 said:


> Then you need a full BMS with shunting, no debate needed.


Cool, who offers this option? need to know so i can add that to the list and keep on saving


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## Elithion (Oct 6, 2009)

280z1975 said:


> Cool, who offers [a Li-Ion BMS with balancing]?


Here's a pretty complete list:
Analog balancers:
http://liionbms.com/php/bms_options.php#Anlg._balancers
Digital balancers:
http://liionbms.com/php/bms_options.php#Digit._balancers
________
MERCEDES-BENZ OC500LE HISTORY


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## DIYguy (Sep 18, 2008)

Elithion said:


> Here's a pretty complete list:
> Analog balancers:
> http://liionbms.com/php/bms_options.php#Anlg._balancers
> Digital balancers:
> http://liionbms.com/php/bms_options.php#Digit._balancers


Wow, nice! I havn't investigated BMS ( i use balancers on pb ) Some day I will use other chemistry.... and this a is really well done comparison! That's the way to make an informed decision! thanks man...


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## illuminateddan (Dec 19, 2009)

Elithion said:


> Sorry, but that makes little sense: the internal cell resistance is not in parallel with the balance load.



Refering to cells with balancing loads strapped across them. I understand that the resistance of a cell is a function of its voltage and its current output. Are you saying that if a cell has a resistance, lets call it X and a 'Y' ohm resistor is placed across it and I measure the resistance of the cell and resistor in parallel, taking into account the fact the cells resistance will be altered by the current, that the laws governing resistors in parallel no longer apply? Or is it that a cell cannot be treated as having a resistance in this instant.




Elithion said:


> In any case, the two are different by 3 orders of magnitude!
> Davide


Also, and this question may become moot depending on the answer of the above query, surely the orders of magnitude count for little as the combined resistance of two resistors in parallel is always less than the resistance of the lowest resistor, As stated in ohms law. True?

Please forgive my overwelming and monumental ignorance, I have high school level electronics of which very little was battery technologies nor lifepo chemistry, however I am just able to make an led or two light up without electrocuting myself.... 

Thanks for your collective patience!

For anyone who's looking for a quick series/parallel resistance run down, here's the wikipedia (  ) reference. 
http://en.wikipedia.org/wiki/Series_and_parallel_circuits

Dan


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## Elithion (Oct 6, 2009)

illuminateddan said:


> Refering to cells with balancing loads strapped across them.
> Dan


I hope this quick schematic helps.









Davide
________
Drug Testing Kit


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## major (Apr 4, 2008)

illuminateddan said:


> Refering to cells with balancing loads strapped across them. I understand that the resistance of a cell is a function of its voltage and its current output. Are you saying that if a cell has a resistance, lets call it X and a 'Y' ohm resistor is placed across it and I measure the resistance of the cell and resistor in parallel, taking into account the fact the cells resistance will be altered by the current, that the laws governing resistors in parallel no longer apply?


Hi Dan,

I hope Davide doesn't mind me chiming in. Just trying to help you understand. The cell does not break any of our basic laws governing circuits. You are apparently having trouble because you try to view the cell as discrete components. It is not. Although it is often depicted as an equivalent circuit consisting of a voltage source in series with a resistor, in actuality, the resistance is internal to the voltage source, distributed across the chemistry, plates and connectors inside the cell.

It is therefore impossible to "put" an external resistor in parallel with the internal resistance of a cell. There is no way to take the voltage source out of the circuit. That is why the simple equations governing resistors in parallel do not apply. However all the rest of the circuit laws and equations still hold. 

As Davide's diagrams show, the external resistor completes a series circuit with the cell's voltage source and internal resistance. And his equivalency circuit and values reflect this.

Regards,

major


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## illuminateddan (Dec 19, 2009)

aaaaahhhhhh. yup, gotcha. That makes sense! 

Also, Elition, thanks for all the info on your site, I had a good read through. Might be worth posting some links to your 'white papers' for the other balancing dullards like myself! 

Cheers

Dan


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## octagondd (Jan 27, 2010)

Interesting post by a BMS maker. I am curious if what they have seen with their BMS overheating the battery is a possible problem with other BMSes as well. Might explain the fires. Might not, but just thinking out loud here.

http://gwl-power.tumblr.com/post/1238013045/warning-the-risk-of-damage-of-the-cell-due-to


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