GET RID OF THOSE SHUNT BALANCING CIRCUITS

31 Comments / Batteries, BMS / By

As you may know, I’ve been a bit at odds with the cognescetti of the EV community on the topic of Battery Managements Systems, and particularly of the form of shunt balancing circuits.  They’re pretty much unified in the position that you have to have them or you will kill your expensive LiFePo4 batteries.

My position is that they are dangerous, a fire hazard at most and an annoyance at least.

One of the problems I have in life is that I’m often surrounded by people that are extremely sure of themselves and their positions.   I’m never quite sure.  Almost everything COULD be a couple of different ways, and most probably is, and I’ve probably got at least part of it wrong.

The oddity is that the ones most certain, are the those most certain to be in error.  And if I run into a man with a theory, who isn’t quite sure, I can often find valuable, and sometimes extremely valuable information there.

Those who most loudly voice their absolute certainty, almost inevitably lead me into something totally erroneous, and they then bleat piteously about “unintended consequences” and the simple unavailability of such information back when they were so sure.

Last week we aired a kind of a tutorial on using the Manzanita Micro PFC-75 charger.  Despite some kind of bizarre design choices, I like this device and it is undoubtedly the most powerful single phase charger on the planet at this point.  Along the way, I had a conversation with Rich Rudman, about the device of course.

But we also discussed his “Rudman Regulator” and the new MkIII device he’s working on for LiFePo4 cells.  Mr. Rudman was EXTREMELY emphatic that without some form of battery management system, I would kill numerous cells.  His solution would have been $7000 for the Mini Cooper.  He told me that they had spent THOUSANDS OF HOURS testing these cells and that balancing was imperative.

This made me feel quite badly.  In truth I have NOT spent THOUSANDS of hours testing batteries.  I’ve spent a lot of time, but it is not very rewarding work.  It takes HOURS to charge a cell.  You have to pretty much observe it closely the entire time to log any meaningful data.  It then takes HOURS to discharge it, and again, you can’t even really turn away from it.  So it’s long, boring, and tedious.

Some of it IS fascinating.  Mostly once you’ve collected all this and are going over it.  But the actual testing is pretty gruesome.

There are 2040 work hours in a year at 40 hours per  week.  I may have HUNDREDS of hours testing batteries, but certainly not THOUSANDS.  And he was so certain of his results, it rather sent me back to the lab.

The “lab” isn’t precisely so.  I can test single cells on the back workbench where I have a lot of test equipment.  And I often test 4 cell or 8 cell banks.  But a string of series cells to be tested poses some problems in dumping that much power during discharge.  So we use GEM’s and in fact have from the beginning.

[jwplayer file=”news111309-1280 – iPhone.m4v” hd.file=”news111309-1280.mov” image=”http://media.ev-tv.me/news111309.jpg” streamer=”rtmp://s3jgrp4mv5fnqx.cloudfront.net/cfx/st” provider=”rtmp” html5_file=”http://media.ev-tv.me/news111309 – iPhone.m4v” download_file=”http://media.ev-tv.me/news111309-1280.mov”]

GEM’s are Global Electric Motorcars.  I had three of them, now down to two.  Ours are like little pickup trucks.  They require no licensing.  They are Neighborhood Electric Vehicles, typically limited to 25 mph.  They have a small 7.5 HP motor and a simple GE controller.

So this week, I took some time, and brought my really pretty nice Agilent 5 1/2 digit multimeter from the back bench, along with the test device I built for the Mini.

It took 2 full days, but I very precisely hand balanced all 24 cells to precisely 4.000 volts.

The tester lets me add 3 amps of charge to a cell.  And it also lets me bleed 3 or 5 amps from a cell through some 50 watt resistors.  Although it has a built in voltmeter, I used the Agilent for precision.

Of course the problem is that if you add a bit of energy to a cell, or for that matter delete a little energy from a cell, it kind of  “bounces”.  The voltage indication will change, for example going higher when charging.  But when you quit, it will sag back down a little bit to it’s stable level.  Similarly when discharging, it will decrease the voltage alright, but when you remove the load, it bounces back up a bit.  So this “manual” balancing is a bit tedious.  But I can make sure this way that they ARE in fact balanced.

This is normally the function of these shunt regulator active cell balancing devices.  This is typically a small circuit with a voltage regulator chip controlling a larger MOSFET or transistor that “shunts” current across the cell terminal when the cell reaches a set maximum voltage.  This usually uses a current limiting resistor which dissipates some of the heat.

The theory is that you hook up your serial  string of cells to a charger.  When one cell gets to the voltage set point, 3.8 vdc or 4.0 vdc, or whatever, the shunt goes into conduction.  The rest of the cells continue to charge, but the cell in conduction is held at this maximum voltage.

Once ALL the shunts are in conduction, the cells are thought to be “balanced” in that they are all held at the same maximum voltage.

I’ve examined the cases of several fires wherein electric cars burned to the ground.  Two culprits emerge as likely causes.  Battery modules made of large numbers of small cells, and shunt balancing circuits.

So I’ve avoided them.

But after my discussion with Mr. Rudman, who has been doing electric cars for many years, has one of the most respected charger products in the community, and who personally assured me that after THOUSANDS of hours of testing, he’s utterly convinced you MUST have a battery management system, I simulated it in this fashion.

Then I went and drove the car.   Actually I went through this process THREE times this week.  And with the same result all three times – a totally destroyed battery cell.  Irrecoverably discharged to 0.0000 vdc.

So I AM in fact destroying cells.  And if I’m destroying them on this little 72 vdc 24 cell GEM system, IMAGINE how much difficulty I was going to have with 112 cells in the Mini and 72 cells in the Beck Speedster.

I was pretty depressed about it.  Not only was I murdering cells in groups, but I was apparently pathologically unable to balance them sufficiently well to prevent it.  And in fact the more precisely I balanced them, the worse the carnage became….

Uh..oh.  Is this the sound of a clue?

Finally Friday morning it all came together.  It is so obvious I’m embarassed by my own intellectual limitations and overall backwardness.  But worse, I have to go public with it because there are a LOT of people spending a LOT  of money on these shunt chargers to do precisely the same thing.

The problem is, the batteries vary in capacity.  While capacity diminishes very gradually with time, there’s really nothing you can do to change the capacity.  It is what it is and it is that for each cell.

By very carefully charging each cell to precisely the same 4.000 level, I did indeed “balance” the cells – at least at the top of the charge.

But as I discharged the cells, they reached any arbitrary point on their discharge curve at DIFFERENT times.  So at the end of the charge, where the knee of the discharge curve turns sharply down, they became more UNBALANCED at the bottom.

The graph below shows the number of seconds a cell has at a 100 amp discharge rate to 3.00 vdc from a full charge with all of the cells balanced at the top of charge.

The problem here of course is that some cells go over the knee first and start down the steep discharge wall at the end before the others. This has a very bad result. The cells still up on the plateau, making current, drive current through this smaller capacity cell and drive it down to zero volts and ultimately to destruction..

So I was repeatedly destroying cells by carefully top balancing the cells, precisely as a current shunt balancing circuit would, and then discharging past the knee of the discharge curve. The other cells turn on the weaker one and eat it like a pack of wolves.

Worse, your overall pack voltage masks all this – remaining up in the supposedly safe area.

The solution appears to be BOTTOM balancing. With all the cells discharged, I replaced the dead one, and balanced all the cells at 2.90 vdc. Then recharged the pack to 87 vdc (3.625 vdc per cell).

Now the cells are very unbalanced at the top – some slightly over 4 volts and some quite under the 3.625 average.

But I don’t care about the top. I don’t lose cells at the top, and we’re charging at 20 amps. During discharge, even the GEM can go over 200 amps of current. That is a 10x more violent event in the life of a battery. And a weak cell can drop from 2.8 to 2.0 to 1.0 to 0.0 in a matter of a dozen seconds or so at 200 amps.

This pretty much explains why I was able to lose cells on the GEM while balancing to the nth degree, but the Speedster, whose cells have never been balanced at all, wheels merrily along without problems.

In fact, we recently completed a 107 mile test drive with the new tires and really did push the little car to the limit. At the end, all of the cells measured between 2.8 and 2.9vdc in quite balanced fashion – at the BOTTOM of the discharge curve. Things were good BECAUSE we had never top balanced.

What I conclude from this is that these simple current shunt balance circuits are not only a needless expense and a fire hazard, they are doing exactly the opposite of what they purport to do. They are UNbalancing the pack at the bottom where it matters, and potentially leading to the untimely death of cells.

So we’re still in search of the perfect Battery Management/Monitoring system. But the current shunt balancers are certainly not it. Save your money, and your batteries.

Jack Rickard

http://EVTV.me

31 thoughts on “GET RID OF THOSE SHUNT BALANCING CIRCUITS”

  1. vehikelfranz

    Hi Jack,
    have a look at Mindaugas’ conversion.
    http://hr-ev.blogspot.com/

    Thats where i got the idea for my
    personal charger system.

    He uses ATiny25 for controlling each cell.
    Such an ATtiny costs about 1$ and has
    3 free pins if serial communication is used.
    one for metering input, two for output.
    At the moment i use one output for controlling
    voltage limits (low and high together,
    as in both cases everything has to stop)
    The second output drives a mosfet for the shunt.
    (i dont think, that i will remove the shunt completely, as it is an easy way to take influence to the charging )

    According to Your ideas, i would decrease
    the power that is burnt in the shunt dramatically. As every limit can be programmed individually it should not be a problem to give
    each cell its very special parameters.

    Then it should work fine.
    I like these ATtinys.
    They run from 1,8-5.5V
    Have an integrated reference of 1.1V
    that gives an input of 0-4.4V
    with a voltage divider 4:1.

    If the shunt is quite weak,
    there will not be a lot of heat,
    but the cell will softly be pushed to its intended voltage.
    After a few charging circles everything
    should fit prfectly.
    This reqires a longer testing phase to determine the weakest cells,and to get the optimal fine-tuning, with lots of
    reprogramming but then it should charge
    every single cell in the way
    You like it to be done.

    The most important thing is to have
    control over the actual voltage of every
    cell so you will be warned if anything
    goes wrong.

    Franz

  2. Michael

    Because this is my first comment, I want to praise you for all the testings and informations, that you and Brian are compile for the community :o)

    Maybe you can write down a guide “My way to balance LiFePo to buttom” or something like that.
    I own 38 SkyEnergy cells (http://www.e-beetle.de) and I’m very interested in a kind “manually balance assembly”.

  3. Anonymous

    Jack, I’ve watched your video and I’ve followed the ensuing discussion on the ThunderSky list. My first two impressions are that:

    1) You’re trying to “spin” yourself smart (to use a rickard-ism)
    2) You’ve set up a straw man for the opposition

    First, top balancing did *not* kill your cells. What killed your cells was that you changed the way you were charging (top balancing), but didn’t change the way you were monitoring discharging (pack voltage). Of course you lost cells (dare I say, “well, duh!”).

    Second, from the discussion on the ThunderSky list, people weren’t opposed to your bottom balancing method. In fact, I think you’ve convinced me and others that unless you also add active balancing during discharge, top balancing during charge isn’t necessary; range is still limited by your weakest cell, so why bother.

    What I’ve drawn from your experiment in bottom balancing is that we need to monitor cells on the individual level during discharge, not at the pack level. Normally, pack level is fine, until something unexpected goes wrong, then you’ll lose cells again (and don’t go telling me that I’m throwing in a “hypothetical”, because things *do* go wrong with cells that monitoring pack voltage won’t inform you of).

    So I think using the under voltage function of something like Cedric Lynch’s devices is the way to go. They keep any single cell from going below 2.8V, meaning you probably can’t ruin a cell due to over-discharging. During charging, use Cedric’s devices just to turn off the charger if any cells goes too far over voltage.

    James

  4. Anonymous

    I follow your work with great entusiasm, hoping that I some day will be able to drive an alectric car purchased or converted… Your discussion on battery management and balancing vas extremely interesting although my competence in this area is somewhat limited. However I stumbled upon something that might (or might not) bring the topic forward, it’s a Swedish startup claiming they’ve come up with a battery management system that actualy balances at the bottom level, thus being able to both draw that little extra from the pack as well as being able to upgrade single cells regardless of chemistry and capacity variances. Please check out the link and comment on it.

    http://www.electroengine.se/en/true-electric/unik_batteriteknologi_eng.php

    Best regards, Lars

  5. Anonymous

    Jack,
    Assuming that the bottom balance is the better way (which i agree on), you where taking about having a warning or a cut off at low voltage, a part of the problem can be solved easily i think, Thunder Sky has a battery monitoring system that will cut of when one of the cells reach a voltage that you set up 3.8 for example, knowing how many AH your smallest container has you would know how many KW on 3.8, lets say 500 W x 70 = 35 000 W, if we put a KW counter set on 35 000W, and goes down as you use the batteries, knowing that for example 7 000 W is @ 80 odo and 5 000 @ 85 odo, having a switch attached to the counter to cut off at 5 000 W. The only problem would be that the temperature of the cells would vary the KW that the cells would give. I’m not an electric engineer so don’t know if this doesn’t wake sense.
    cheers

  6. Joe

    Jack,
    Either top balancing or bottom results in a battery pack that is limited by the weakest cell. It will either hit bottom (when discharging) first when the cells are top balanced or it will reach max voltage (charging) if bottom balanced.

    The real difference in using bottom balanced cells is that they will drop at the knee at nearly the same time. This is indicated by a rapidly dropping pack voltage, making further dichaging less likely since the driver is less inclined to continue driving when they notice pack volts dropping. The thought of killing a cell would stop me! $$$$$

    If you top balance then it becomes extremely important to have a way of monitoring the weakest cell while driving since the ‘stiff’ pack voltage can easily hide the lowest cell falling over the knee.

    Bottom line: If you can monitor the weakest cell and take action to prevent overrange at either end then you have solved the problem. A battery shunt can’t do this by itself. It only protects the pack at the top of the range.

    Jack, your speedster stategy of bottom (mid, random, as delivered, or whatever you’d like to call it) is a reasonable compromise since it is likely that the pack will start dropping near the time the lowest cell is reaching the discharge knee. Again this signals the driver the pack is getting low reducing the probability of damage.

    My personal concern is that I’d get cells that are not in sync with the rest and I’d kill some before they get balanced. While doing this I will also be looking for the weakest cell. I’d trim the cells so the weakest cell hits the knee at nearly the same time as the rest and I’ll adjust the final pack charge voltage so the weak cell tops just under the limit. I’ll let the rest of the cells partial charge, no big deal.

    As proven by the speedster, this status is likely to remain consistent for quite some time. I will periodically check cell voltages to make sure top and bottom are remaining safe.

    On another topic: Jack, how much voltage drop do you get under throttle/discharge? I’m looking for numbers that I can compare to my lead acid battery powered car. For example, 144 volts no load, put on a 100 amp load and voltage sags to 136 volts, 200 amp load voltage sags to 130 volts. As long as we haven’t discharged too long let off the throttle and the voltage should rebound back to nearly 144 volts.

  7. Joel Clemens

    I agree that bottom balancing is most critical of this, but I also agree that if you can monitor each cell and disable/reduce the load when the voltage drops to a certain point, then you won’t need to balance at all.
    It would help to know what the sag voltage is of a cell that is almost completely drained.

    also, excellent work on the videos, please keep them comming. Unlike most people in the internet, I actually have the attention span to get through these, thus I enjoy them a lot.

    Another tip; you can save a lot of bandwidth and money by re-encoding the videos to be smaller resolution and smaller bitrate. They are very pretty to look at, but it really isn’t nessesary.

    I really would like to know if you have receieved any comments/feedback from the people with “1000’s of hours of experience with batteries’ in regards to your clear explanations on lithium ion balancing. I agree that the balancing methods that are main stream right now are far too expensive, and seem to be based on lead acid thinking. (old)

  8. Anonymous

    Jack, I was hoping you might be able to provide some crucual advice.

    I do not have a BMS on my electric motorcycle, though I did hook up a wire to each battery and hook all the wires into euro style terminal strips that reside under my (fake) gas tank so that I can check, charge, and drain individual batteries without going through the nasty business of removing the battery trays from the frame.

    Actually I am slowly reaching the conclusion that my main problem is that I live on a hill 3 miles up a reasonably steep incline. That 3-mile incline is always at the end of whatever trip I take with the bike, and the voltage sags a lot on that incline, to the point where the controller’s low voltage cutoff can kick in quite early.

    For instance, my 24 Thundersky 60AH batteries might still be settling in at a total of 79.6 volts at rest, yet still they will sag down low enough going up that hill so that the low voltage cutoff (set at 60 volts) is triggered, and the bike starts limping (a bit dangerous on a road where traffic can be quite aggressive and speedy).

    I understand that damage sets in on these batteries when they are discharged to under 2.5 volts. But what about when they sag to under 2.5 volts temporarily (though they will soon settle back up to 3.3 volts when they get a chance to rest)?.

    I often read specifications on motors and controllers where a distinction is made between how many amps or volts the motor/controller can handle continuously, and how many amps/volts the motor/controller can handle in bursts — should the same distinction be made for batteries? Do I have some leeway as far as bringing my low voltage cutoff number down to something less than 2.5 volts per battery?

    Thanks very much for any feedback you would be willing to provide…

  9. Jack Rickard

    Yes, of course you do. I cannot seem to get it across to these people. They want to set a low voltage monitor. I’m asking them all the time, WHAT are you monitoring and WHAT will you do with the information. Your cells are at 3.31 v each on average – about 5 % discharged. They can go to ZERO volts under some load situations and be PERFECTLY fine. If you can reprogram your controller, simply change it. I had the same think happen with my Kelly controller when I first put it in the car. And indeed, I was a little confused myself over these voltages. I’ll try to attach a graph of a test I did today. It shows a 180 second 1C load followed by a 60 second rest all the way across the 100% discharge.

    A couple of things jump out. First, using voltage to detect charge state is a little iffy. It appears to work for me because I’m always poking around at the 95-100% end and it works fine there. But for the “good” 80%, it truly appears almost useless.

    Second, there is a big swing in voltage under even 1C load. This gets much worse of course at 3C or 4C that you might encounter on your hill.

    You need an Amp Hour counter that you reset each time you charge and actually gives you a count of spent AH as you ride. That’s the only “fuel gage” that will ever be accurate. BUt 79V is extreme. You’re charged.

    Hope this helps.

  10. Anonymous

    Thanks Jack. I will go ahead and re-program it a bit (Alltrax). My cycle analyst does show amp hours and last night, for instance, I got the low voltage cutoff at around 22 amp hours into the cycle (60 AH batteries).

    However, I suspect that the low voltage cutooff might not be my only problem. Cycle analyst stats I have saved on and off since September show a number of days when I made the exact same commute to and from work. There is a pretty steady pattern in the statistics showing my remaining voltage (at the end of the cycle) to be around 9 or 10 volts higher than the minimum voltage I reached during the commute. I would generally start with around 76 volts (I had only 23 batteries at the time), then at some point drop to a minimum of 66 or 67 volts.

    At the beginning of November I took too many chances and actually drove the motorcycle for 60 miles on a cycle, eating up a whopping 76 amp hours. 2 cells were outright dead and I replaced them, but ever since I abused the batteries, the difference between my remaining voltage and my minimum voltage shot up drastically (13 to 19 volts difference rather than 9 or 10).

    Even coming 9 miles to work this morning, which is all down hill or flat, brought about a voltage sag from 79 down to 65 at some point. I have read that the colder weather can increase voltage sag, but I am living in Soutghern California and I wonder if the temperatures here could be considered “cold”. I am wondering if there are more cells in the pack that are damaged, even though they can apparently hold a charge…

    Thanks very much for sharing your knowledge and data.

  11. Anonymous

    Jack, thanks so much for your sharing all your work here.
    I am about to build a conversion; a Porsche 914 with TS 100s, and a curtis controller.
    After extensive reading and thinking, I like your apporoach, as it involves less spending and a liitle more learning.
    I plan to simply use and learn the pack til I figure out which are the lowest two or three cells, and then put whatever monitoring I have on those cells.
    I have an amp hour meter, and plan to use that to know when the batteries are close to discharged, possibly resetting the lower end alarm to reflect what I learn about the capacity of the laziest battery.
    What pitfalls do you see with this approach?
    I plan to use a peukert’s equation number of 1.05 or less, but was only able to find one mention of the PE# of a TS batt, do you have any idea?
    Anyway, thanks again, and good luck!
    Anton

  12. Jack Rickard

    Mostly the Curtis controller. What controller and motor are you using in a Porsche 911?

    Your approach sounds good. I would not worry about Peukert. There IS a little bit but it’s so small it really just doesn’t matter as best I can tell.

    The best way to determine SOC is with an accurate AH count. What meter are you using?

    It all sounds like a good plan. And a 911 is about as good a candidate as there is in my book. I’ve eyed them for some time. The time, effort, and component costs of these projects point toward doing a car you really want – whatever the drive train. A 911 is a great choice to my tastes.

    Jack Rickard

  13. Anton B

    First, while it’s a Porsche, it’s a 914, which is not considered a Porasche by some, but I like them, and enjoy drving them. I weird that way.
    As for the meter, I’ve gotan old E-Meter (now discontinued), which is supposed to be pretty accurate. That along with a few simple bolt on BMS unit with low voltage alarm, maybe high, not shunts. Just to let you know when I am getting a bit low. When it starts beeping on accel, start thinking baout batt capacity. That should teach me what the AH rating of the lowest capacity battery is, and then it’s easy to look out for, as long as it doesn’t cahnge too fast.
    I figure I use two or three of those on the lowest few cells and I can keep tabs on them without spending too much, or relying on them for batt cutoff. With only a few of them, there’s fewer of them to fail 😉
    As for the controller, it come with a pile of parts I got cheap, so when it fails I can just swap it out.
    One more thing: could you tell me wherre you got your submarine rotary switch? I can’t find anything with 42, or even 20 terminals to be able to check voltage on the go.
    Regards,
    Anton

  14. Garth Irwin

    I read your article and commend you on taking huge amounts of time/effort to educate EVers – I have learned from your blogs/videos and am much appreciated.

    I do not entirely agree with your “no BMS needed” approach and top vs bottom balancing ideas however.

    I think that if you do not have any way of monitoring each cell voltage (ie using a BMS) then you are in danger of losing a cell using either top or bottom balancing.

    With top balancing, you can over discharge a cell (as you have experienced).

    With bottom balancing, you can over charge a cell.

    You can minimize (but not completely avoid) the problems with top or bottom balancing (without a BMS) by using conservative high and low voltage thresholds during charging or discharging, but then you leave some energy “on the table” either way.

    If you had a BMS to monitor each cell (forget about the balancing circuits that may come with them) then you would be able to stop charging or stop dis-charging and prevent damage to any cell (regardless of whether they are top balanced, bottom balanced, or manually balanced). You would still be protected even if the pack is grossly unbalanced.

    I think this is the key – monitor and protect.

    My 2 cents. I also don’t know it all, but this is where I am being pointed (I have converted a RAV4 with 46 200AH TS cells).

    Respects, Garth

  15. Jack Rickard

    Well Garth, that’s the power of doing it yourself and at your own expense. You get to do it your way.

    We now have a couple of years and a couple of tens of thousands of miles on a variety of cells. It just isn’t really a theory anymore. BMS’s are expensive, they pose a fire hazard in a vehicle, and they save nothing.

    I have nothing against monitoring, and we’re always looking for a way to do it better. But the monitoring gear can’t burn the car to the ground trying to tell me about cell voltages that never were a problem. Yes, we leave energy on the table, and charge very conservatively. We actually discharge probably beyond what is healthy, but it works. And it preserves the cells without all that hardware, wiring, and expense. I have files full of input from others who desperately wanted to protect their investment, and who ruined their cells and in some cases their cars all with the very best of intentions.

    I cannot shout down the clamor from dozens of would be engineers and entreprenuers who have hitched their wagon to this high margin gold mine. But I’ve encountered nothing to support your view, and a lot to support mine.

    Always subject to change with the next product announcement….

    Jack Rickard

  16. John Hardy

    This is a really interesting discussion – thank you for your efforts in kicking it off.

    It seems to me (I’d be interested in your views) that the scenario you describe, and illustrate so clearly, is imbalance due to variations in cell capacity. Here you are clearly correct: if you balance at the top and then just discharge to minimum pack voltage you will lunch the weakest cell or cells.

    It seems to me that there is however another sort of imbalance: imbalance due to variations in initial state of charge, which might in turn result from variations in self discharge rate between cells in the same pack.

    If you conduct the extreme mental experiment of putting a 30% SOC cell into a pack of cells at 10% and then charge the pack as a whole without overcharging any cell, one cell will be at 100%, and all the rest will be at 80%. If the weakest cell in terms of capacity is one of the ones charged to only 80% then your pack (which is limited by the weakest cell) will only give you 80% of what it could have done.

    In that scenario, top balancing, bottom balancing or anywhere-at-all balancing would presumably help?

  17. Jack Rickard

    It might help, and then again, it might hurt. Kind of depends on how you did it.

    IF the cells were all received from the manufacturer at almost perfectly the same state of charge, and you top balanced it to marry in this errant cell, which on occasion you are driven to do, ie when expanding your pack size for example, you’ve now rearranged them all so that they are only at the same state of charge at the top of the charge cycle.

    If, on the other hand, you discharged your pack until all cells were under 3v and showing differences in capacity pretty spritely, and you took some readings and determined a more or less average cell voltage, and discharged your errant cell to THAT level, and then charged the entire pack as a series, things would probably go much better.

    Ask me how I know….

    Jack Rickard

  19. Jack Rickard

    Because we’ve done it otherwise and lost cells. And redone it and they worked perfectly.

    So I set up an experiment to replicate it and did it precisely the same way each of three times. We lost one cell each of the three times.

    Kind of explensive testing…. but now I know.

    In spite of this, I’m pretty much outnumbered by people who know better, by having typed a lot into a computer screen. Very frustrating….

    Jack Rickard

  20. GerhardRP

    Hi Jack,
    Do you know whether a single Lithium cell is killed by discharge to zero, say by strapping a resistor across it? Or is it only killed by reverse charging when residing within a series string that is overdischarged?
    Gerhard

  21. moxbox

    I think this whole discussion is confusing the concepts of “Cell Balancing” with “Secondary Battery Protection”. What kills batteries (of any chemistry) is either charging them above their maximum rating, or discharging them below their minimum rating. There are several devices specifically designed to handle this – for example the following Seiko parts are used extensively in Lion and LiFePO4 battery protection circuits (i.e. one per cell) :
    http://datasheet.sii-ic.com/en/battery_protection/S8261_E.pdf

    The CO output is used to disconnect the charging source during overcharge, and the DO output is used to disconnect the load during discharge. This is “secondary battery protection”. “Primary battery protection” is usually a PPTC (Positive Polymeric Temperature Coefficient) device attached to each cell by the cell manufacturer.

    “Cell Balancing” on the other hand, whether active or passive, is a mechanism to allow
    a more uniform distribution of charge across all cells of the battery pack.

    You absolutely need “Secondary Battery Protection”, however “Cell Balancing” is optional…

    😉 my two cents, anyway…

    1. Rickyboy

      Thanks for this valuable information. I just know one friend who recently built an electric car. He has a bms, guess what he has already lost 4 cells! ( he is blaiming the batteries)….he has 80 batteries in his pack. I don’t believe that the battery factory has a battery failure rate of 5% ( 4 cells out of 80 going bad) . This is not a factory defect…this is caused by the BMS!!!!

  23. Enginer Inc

    Thank you for the valuable advise. Based on this, we developed an inductive active balance BMS to conduct bottom balance on the go. Low capacity cells are being charged up from the entire pack. We cut off charger power at high cell voltage of 3.8V without shunting circuit.

    The BMS circuit is away from the cell terminals for safer operation. Max balancing current per cell is 2A.

    We have over few hundreds Enginer conversion kits on the road at this time. It works very well.

    Thanks again.

  24. Pingback: In the Balance « Electrifying a Porsche 914

    1. mjrickard

      This is nonsense. They have parted the baby neatly down the middle, and in an impressive thought experiment, devined nothing. This guy should drive a hybrid. Six of one kind is half a dozen of another.

      There is no middle ground between these two camps. ANd his assumptions on the weaknesses of bottom balancing are simply made up. None of that happens.

      The only time you need to “trim up” your bottom balance is if you change something in your pack.

      Jack Rickard

  25. Andrew Lamb

    Hello
    I can confirm that wisdom of bottom balance. we don’t care when the battery is full but we do care how long until empty.

    It is possible to detect drop off of a single cell in a string.

    I have run up to 200 LiPO in a pack with very good results.

    Andrew

  27. Toro

    I agree that bottom ballancing is simple and works great. We normally buy very expensive lipo4 battery systems with custom top shunt ballencers. Battery packs always fail due to bms issue. Now just use bottom ballance and pack undervoltage cutout. Avoid over charge by only charging to 3.5V per cell. Not much energy lost by not charging past the top knee point

  28. Gregor Scott

    If you top balance your cell to gotta have a cell level ow voltage cutout, so you don’t over discharge weaker cells. I guess this was written a while ago but nowadays doing this is easy and cheap even on large packs.

Leave a Comment

This site uses Akismet to reduce spam. Learn how your comment data is processed.