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  • Lithium Top or Bottom Balance

    Lot of debate which method should be used. But on the EV side the lesson is being learned the hard way as many have now destroyed cells in their EV's using Top Balance. Another one bit the dust today and learned the hard lesson of Top Balance. He is now converted to Bottom Balance. Read about it here.
    MSEE, PE

  • #2
    3.7VPC is what I charge my A123s to. I haven't had a cell fail in 5 years. I have 2 different kinds of 26550 A123 cells, one pack is the green skin cells and one was made from the paper covered cells from dewalt 36V batteries. Each is 4S4P config but only used to start/power drag bikes. I balance them once a year and they have never been more than .1-.2 apart. They are charged by the bikes onboard stator/rotor and sometimes charged with a dewalt 10A regular battery charger. Here is a pic of the one I built, I bought the green cell one.

    What do you mean by top balance and bottom balance?
    Attached Files
    1150W, Midnite Classic 200, Cotek PSW, 8 T-605s

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    • #3
      Originally posted by thastinger View Post
      What do you mean by top balance and bottom balance?
      First A123 are Nano Phosphate and not really what I am talking about. EV and Solar folks use Chi-Com LFP batteries that cost 40-cents per wh vs $2/wh for A123. However Top vs Bottom stil applies to all lithium batteries.


      When I get a minute I will answer the difference. But in a Nut shell Top Balance means you equalize voltages at 100% SOC, and Bottom Balance is equalize voltage and capacity at 0% SOC. Th eissue witrh top balance is it only equalizes the voltages at 100% SOC but does not equalize capacity. On discharge is where you get in trouble on Top Balanced systems. LFP batteries are destroyed at the bottom from over discharge. Real easy to over discharge a cell or two in a top balanced system.
      MSEE, PE

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      • #4
        Originally posted by Sunking View Post
        However Top vs Bottom stil applies to all lithium batteries.
        Ok, I have a Turnigy charger I use to do the balance via the balance circuit. I'm not aware of more than one way to balance charge the A123 cells. It does discharge them then all then charges them all up as individual cells.
        1150W, Midnite Classic 200, Cotek PSW, 8 T-605s

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        • #5
          Originally posted by Sunking View Post
          LFP batteries are destroyed at the bottom from over discharge. Real easy to over discharge a cell or two in a top balanced system.
          Which is why it is a very good idea (although expensive) to have an independent voltage cutoff for each cell or parallel group of cells in the pack.
          Boeing thought that they were doing that on their aircraft battery, smoke generator, and fire igniter combination box.
          Unfortunately cutting off outside loads is not enough when you have an internal short.

          But even then, if the intended use is to discharge the pack at or near the maximum safe short term current, properly choosing the low voltage cutoff can be a serious problem.
          A voltage that allows you to go close to full discharge under heavy load will not trip until it is too late when discharging under very light load. A programmable controller that also factors in load current might do the job, but the cost then gets really out of line with the cost of the whole battery pack.
          For solar use, there is no particular advantage to top balancing except that it can be done without taking the pack out of service to do a controlled discharge.
          But there is no inherent drawback to top balancing either as you will not be routinely discharging the pack anywhere close to 0% SOC.
          If an individual cell suddenly suffers a loss of capacity (pick a hypothetical reason) then neither top nor bottom balancing will prevent that cell from being killed when you run the pack down to whatever SOC corresponds to 0% SOC for that cell.
          Once again, active monitoring is the only way that can safeguard against most if not all contingencies.
          SunnyBoy 3000 US, 18 BP Solar 175B panels.

          Comment


          • #6
            OK let me take a quick shot at this. If you want to know ore Google it. Or I will be happy to answer specific questions.

            Let start with Top Balance. The mentality comes lead acid of keeping all cells at 100%. It also comes from manufactures trying to up sale a BMS system. 90% of all Top Balance uses a Balance Board aka Vampire Boards. That balance plug you speak of just has the Vampires built into the charger. Otherwise large format cells use the boards installed directly to th ebattery term post. One for each cell.

            The Vampire Boards are simple on/off circuits. They turn on at a specific voltage typically 3.6 volts or 100% SOC. When they turn on, they shunt some charging current around the battery while charging. The amount of current depends on the specific Vampire used but the range from .5 to 1 amp. Initially it sounds like a good way to Top Balance, but there is a problem or two with the idea.

            First and the lessor of the issue they do not prevent you from over charging the battery. On my 100 AH batteries in my EV, I have a 50 amp charger. When that first Vampire Board turns on it shunts only 1 amp around the battery, while the remaining 49 amps continues to flow through the fully charged cell. Second, third cell and so one turns on until the last battery finally reaches 100% and terminates the charge. So in my 16S pack I have 15 overcharged cells cells, and one happy cell.

            All Top Balance does is equalize voltage at 100% SOC. It does not equalize AH capacity, only voltage. If you go buy say 16 100 AH cells, what you will is a range of capacities from 100 to 115 AH and everything in between. It is very important to understand that fact.

            So here is the biggest issue for Top Balancing. Greatly increases your chances of over discharging a battery or batteries. Most who use LFP batteries in EV's and solar use either the Motor Controller LVD in EV's, and the LVD built into the Inverter. Both those use the total pack voltage to protect the battery just like you would don any lead acid battery. On a 48 volt battery LVD is set at around 40 to 42 volts. So here is the problem you start discharging or using the battery and dang it the LVD operates. No problem just recharge. But when you do you notice the voltage is 3 volts lower than normal after charging. You investigate and you find a cell at 0 volts. Game over cell is destroyed.

            What happened? Well remember I said the capacity of the batteries are not equal, and the weakest cell was 100 AH. Well when you hit 42 volts you had 15 cells with a charge left in the because they have more capacity, Those 15 cells were around 3.0 volts, but one was at 0 volts, and you destroyed it.

            Now bottom Balance when our batteries arrived, we wired them all in parallel and discharged them to 2.5 volts or roughly 0% capacity. Not only have we equalized at a know voltage, we also equalized their capacity of 0 AH.

            OK we install the batteries, and charge them up using a cell voltage battery. As soon as the first cell reaches 3.5 volts we stop charging. We look at our charge and can see we just pumped in 90 to 95 Amp Hours. We see the voltages are not equal at full charge. You wil have one at 2.5 volts and all others at something less than 3.5 volts. The battery with the highest voltage or 3.5 volts is the weakest cell of the pack. It has the exact same 95 Amp Hours as every battery in the pack has. Voltage is NOT equal at the Top. Only place they will be equal is at the bottom.

            Now we go on discharge driving along and dang it the LVD operates at 42 volts. We panic a bit and measure cell voltages and see they are all 2.6 volts which is GREAT. It means no damage, just embarrassment letting that happen. You go charge them up and pump in another 90 to 95 Amp Hours with the charger and life is good.

            • Bottom Balance is less expensive to implement and not near as complex as Top Balance with all those Vampire Boards to deal with. We did not have to buy 16 Vampire Board for $15 each.
            • We greatly reduce the risk of an over charge/discharge so we can sleep at night.
            • By never going to 100% or 0% SOC we just increased cycle life up to 50%.
            MSEE, PE

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            • #7
              Originally posted by Sunking View Post
              • By never going to 100% or 0% SOC we just increased cycle life up to 50%.
              But some of the batteries will be at a higher SOC and so may end up losing life faster. Not sure that effect really makes a difference, but it is there theoretically.
              The problem is that the cells with the lowest starting capacity will also lose capacity faster with bottom balance.
              You can minimize the problem by keeping the overall pack voltage at "full charge" low, but there will still be some cells with higher voltages than others.
              By stopping when the first cell in the pack reaches 3.5 volts you are definitely managing to keep them all out of the really fast degradation range, but there will still be some differences.
              SunnyBoy 3000 US, 18 BP Solar 175B panels.

              Comment


              • #8
                Originally posted by inetdog View Post
                But some of the batteries will be at a higher SOC and so may end up losing life faster.
                Only the weakest cell ever reaches 3.5 volts or roughly 90% SOC. No cell will ever see 100% SOC. All others will be slightly lower. But all cells will have the same capacity in a Bottom Balanced system. The only time the voltage will be equal is at the bottom. What kills lithium batteries is the stronger cells driving the weaker cells into reverse polarity when fully discharged. With all the cells having the same capacity makes it almost impossible for that to happen. With Top Balanced systems every cell except 1 or 2 gets overcharged every time and those 1 or 2 cells go to 100%. You do not want any cell going to 100%. They are not lead acid batteries and thus no need to go to 100%.

                I failed to mention the Vampire Boards have another issue. When they fail, the tend to fail shorted which will drain the cell down and discharge it without the users knowledge destroying it. The user may or may not notice it until they recharge and take notice the voltage is lower than normal at full charge. On a 16S pack full charge at rest is 54.9 volts. With a dead cell is 52.5 volts. The good thing about lithium cells is they fail shorted allowing you to use the EV or Inverter. Not so with any other battery chemistry leaving you dead in the water until you either bypass the battery or replace it.
                MSEE, PE

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                • #9
                  Originally posted by Sunking View Post
                  Only the weakest cell ever reaches 3.5 volts or roughly 90% SOC. No cell will ever see 100% SOC. All others will be slightly lower. But all cells will have the same capacity in a Bottom Balanced system.
                  No question that you are correct for all practical purposes.

                  All I am doing is stating the theoretical condition where, for example, you have 13 cells total in series to make a 48V pack and of those 12 are very close to nominal capacity while one is 5% low. Bottom balancing will get you the low capacity cell charged to ~75% SOC and the rest charged to 80% SOC. A reasonable tradeoff of cycle life and safety against total available capacity.

                  BUT, even at 80% SOC the low cell will give you fewer cycles over its lifetime than the ones which you max out at 75%.
                  Going all the way to 100% on some or all of the cells will have an even more dramatic effect on cycle life, of course.
                  SunnyBoy 3000 US, 18 BP Solar 175B panels.

                  Comment


                  • #10
                    Originally posted by inetdog View Post
                    All I am doing is stating the theoretical condition where, for example, you have 13 cells total in series to make a 48V pack and of those 12 are very close to nominal capacity while one is 5% low. Bottom balancing will get you the low capacity cell charged to ~75% SOC and the rest charged to 80% SOC. A reasonable tradeoff of cycle life and safety against total available capacity.
                    Dave maybe it is the way you are stating it and I am misunderstanding, but it is the other way around. The weakest cell goes to higher SOC than the other 12 in your example. If the lowest cell is say 100 AH and all the others are 115 AH. You would charge to 90 AH. Weakest cell is at 90% SOC and all others are 78% SOC. You have not sacrificed any capacity, and gained cycle life.

                    Whether you chose to Top Balance or Bottom balance is a personal choice. As a manufacture I would insist you Top Balance because I make a lot more money initially with the extra equipment needed to pull it off, and and make more money from more frequent battery replacement.

                    Top Balance is driven by Pb mentality and profits in my professional opinion. I was in the mind set of Top Balance initially when I was young, stupid, and most of all inexperienced. That was back when I was beer guzzling pot smoking liberal. Then I grew up.
                    MSEE, PE

                    Comment


                    • #11
                      Originally posted by thastinger View Post
                      3.7VPC is what I charge my A123s to.
                      Even though the A123's are also lifepo4, I'd still be conservative in your application and drop that back to no more than 3.6v in your Turnigy charger. The A123 setting if it has one may be convenient for consumers, but is not actually the best for longevity. 5 years is good, so you must be treating them right - namely not overdischarging them. Sounds like your bike-starting application works great with the capacity of the cells you are using. One way these die is with parasitic loads and other bike paraphenalia like heated gloves, radios, etc.

                      The big difference between the A123 cells and the CALB, Winston, GBS cells etc is that yours are the high-rate small cylindrical types, capable of 10C surge or more perhaps. That's what the nano in nano-phosphate helps out with. For us, with large prismatics, our max rate is usually like 3C surge, with operational long-term loads like 0.5C or less for both charge and discharge. Thus, to keep things simple and less costly, it is foolish to build large-capacity banks like 30ah or more with cylindricals - large format prismatics are on call here for long term loads.

                      WARNING - large prismatics are tempting for wheelchair users, but are not the most suitable types! In this instance, stick to the cylindricals like A123, Headway, etc to be able to get that large current capability when accellerating. Also see here:

                      http://www.wheelchairdriver.com/boar...php?f=2&t=1813

                      The Turnigy can only "top balance" the battery, not bottom balance as is common with EV use. Since you are only using a very quick starting application, this works out ok, but still, I'd like to see a more conservative value when you do balance. I know my iCharger will let me custom-program settings for tasks like this.

                      For more lifepo4 fun in a starting-application, you'll love this: CY is no stranger as he was part of the group that initially developed / tested the early "protected" lion cells over at Candlepower forums.

                      http://www.advrider.com/forums/showthread.php?t=757934

                      For a starting application where you seem to have calculated your capacity correctly, then don't sweat the bottom vs top balance scenario. I just suggest being a tad more conservative with your upper end voltage.

                      As long as everyone keeps in mind that different applications call for different cells, and *sometimes* you can get away with top balance if you are careful. That is what is so crazy about lifepo4 - there is no one single way to deal with them, although some might be better than others.

                      Comment


                      • #12
                        Originally posted by inetdog View Post
                        BUT, even at 80% SOC the low cell will give you fewer cycles over its lifetime than the ones which you max out at 75%.
                        On paper, this is true - the weakest, lowest capacity cell will get worked the hardest. That is one reason when you buy large expensive cells, you do so from a quality dealer who makes sure that all the cells are nearly the same in capacity and internal resistance. You may even get a printout identifying these paramaters matched to the barcode on the cells so you can keep track of this baseline measurement from the factory.

                        And, internal resistance is also a factor. You may have a cell with higher overall capacity than the rest, but it may have the highest IR of the pack, changing operational paramaters during use a bit. I think at the end of the day, just buy good quality stuff, and not try to go for absolute perfection. Being conservative either top or bottom gives one that option.

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                        • #13
                          Originally posted by Sunking View Post
                          Whether you chose to Top Balance or Bottom balance is a personal choice. As a manufacture I would insist you Top Balance because I make a lot more money initially with the extra equipment needed to pull it off, and and make more money from more frequent battery replacement.
                          No question about it for EV use - bottom balance is the way to go. I hear rumor that some powertool applications using lifepo4 may have switched to this scheme also - which makes total sense since a consumer is just going to run that high-current tool to the dregs. What is frustrating is identifying the manufacturers who actually do this.

                          Solar DIY can do this as well, but for our low-current (relatively) needs, you can top balance and at least get in the ballpark if you don't overdo it, and run conservatively. At a comparatively low current discharge rate, your lvd be it cell-based or pack based (provided you start out with quality) is a decision / risk the owner will have to choose.

                          Ideally I'd be running bottom-balance too, but all my gear is essentially a top-balance environment. Fortunately, I don't balance much with my "sub-C" application!

                          Comment


                          • #14
                            PN glad you chimed in. In all honesty you are correct the A123 cells are high C-Rate cells, but you understated them and Large Format cells.

                            The A123 cells are rated for continuous 20C, and 50C for 10 second burst.

                            CALB on the other hand is rated for continuous 3C, and 10C for 10 second burst.

                            As for using A123 cells for EV, wheel chairs, or high C-rate applications I am not in full agreement with you as that depends on AH to Controller Current ratio.

                            As an example my little golf cart hot rod uses a 650 AH controller with 100 AH cells which is more than enough. 650 amps is the peak for acceleration. For continuous the current is 190 amps. So a 100 AH large format is more than adequate for the job. I sort of wish I could use A123, but the expense, complexity of assembling and operation using 540 cells is insane IMO. Think Tesla EV 8000 cells. Heck 16 cells is enough eggs in the basket for me to worry about. 540 is unthinkable.

                            I am not sure a wheel chairs or ebikes can justify A123 cells or need the cells. A123 main advantages are higher energy density and of course higher C-Rates. I am not sure a E-Bike or wheel chair requires the higher energy density and C-Rates.

                            Don't most E-bikes use 36 to 48 volt 10 AH batteries? Don't Ebikes use 250 to 400 watt hub motors? A large format would be adequate on a 100 amp controller on a 900 watt hub motor. That would yield up to 4800 watts or 6 peak horse power. I can't see Grandma popping wheelies and burning rubber on her motorized wheel chair.
                            MSEE, PE

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                            • #15
                              Originally posted by Sunking View Post
                              I can't see Grandma popping wheelies and burning rubber on her motorized wheel chair.
                              Maybe not Grandma, but there are wheelchair users who are definitely interested in that kind of power.
                              Check out http://www.wheelchairdriver.com/ for some very interesting info ranging from ultra-high performance to conservative, hosted by an engineer who knows the details of the target application.
                              SunnyBoy 3000 US, 18 BP Solar 175B panels.

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