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    Battery Balance Charging

    Balancing a Battery Pack.

    Anyone who uses a battery pack made up of multiple groups of cells in parallel should understand what balancing the pack means.

    Let's take a minute to refresh our memories of how an ebike pack is built.

    If you look at a regular flashlight for example it will have 2 or 3 batteries placed end to end in the barrel. This connects one battery's positive tip to the negitive base on the next one, and so on. By doing this you have a series of batteries and we call that... yep, connected in series. the voltage of the batteries is added together and in the 2 cell flashlight example you will have 3 volts going to the bulb from (2) 1.5 volt batteries.

    If we take batteries and connect them positive to positive and negative to negative,the easiest way to do that is to place them side by side parallel to each other and we get...yep you guessed it, a parallel pack. In this case the voltage remains the same as each individual battery but we have increased the capacity essentially making a large battery of the same voltage.

    Most e-bike packs are made up of cells arranged and wired in series AND in parallel, that is the only way to get enough voltage and enough capacity to meet the demands of an electric vehicle.
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    ** No, Duracells are not 3.7 volts but it was the easiest battery image to find : )



    The battery of choice right now is whats called an 18650 cell. It's about the size of a regular "C" battery but about 1 and a half times longer and has a rating of 3.7 volts (more about that in a minute). The name means 18mm x 65.0mm.

    Luna and Tesla and just about every other EV battery maker arranges enough of these cells end to end (series) to get up to the voltage desired. In Luna's case 13 will get you to about 48 volts, 14 will get you about 52 volts. That's called 13S or 14S (# of cells in series). This works great with a single string of batteries but there is a problem with doing this on a larger scale. That problem is monitoring the batteries during charge. We know these batteries are rated at 3.7 volts and that is the base for all measurements during charge or discharge, if we build say 4, 52 volt strings and connect them together in parallel to get more capacity we are effectively dealing with 4 52 volt batteries and that's no good. The solution is to build the number of parallel packs you want *then* connect them in series to get to the target voltage. In the example above we are now dealing with 14 3.7 volt packs, much better.

    The very best batteries these days (10/2016) have about 3 amp hours of capacity, that would be enough for a short ride but the problem is the short ride would drain that single string of batteries so fast that they would get hot and heat kills the lifecycle of batteries. The solution is to spread the load over more parallel cells. The newest Luna packs are the Mighty-Minis and use 2 cells in parallel to make a small 3 lb. 14S2P pack, that is enough to support normal riding and has a respectable 6-7 Ah capacity. *(You can abuse these packs with a high power 30A system like the BBSHD so some care is needed).
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    OK Battery Pack 101 is done, congratulations.

    Charging a single battery is easy, charging a string of batteries is still pretty easy. Charging a bunch of cells connected in series and in parallel at the same time is a little more problematic and here's why. Not all cells behave exactly the same. The big battery manufacturers have taken the art of building these cells to a place that was unthinkable just a few years ago, the modern battery is truly remarkable but that amazing capability comes at a cost and that cost is manufacturing variance. 500 batteries coming off the same production line at the same time will all be just a little bit different, the chemistry involved, the absolute composition of each element, the temperature, humidity the gravitational pull of the moon .... who knows. That's why its an art and an inexact science still. This is an extremely important point for the consumer since the very best manufacturers spend millions of dollars to eliminate this variance as much as possible, they then test each cell and group it with other cells that are as similar as possible before stamping a batch number on them. Do you think the low cost eBay and AliExpress sellers do this? No they take whatever cells they can find from wherever they can get them cheap, re-wrap them in pretty new labels and sell them at what looks like a great deal, til your house burns down or you replace it after 25 cycles. I won't even get into the "pouch" style batteries (think RC cars/planes) they scare me too much.

    BUY GOOD CELLS! Luna uses only the top cells from the best manufacturers and they tell you the manufacturer and the chemistry used in each pack so you can make the best decision for your build and riding style. The price and quality of the packs reflect that. Welcome to the real world.

    OK back to charging... sorry for the detour.

    So where were we? Oh yeah charging a pile of cells all at once. Now that we have split the pack into 3.7 volt parallel groups and connected those in series to get to where we wanted to be we can monitor each 3.7 group as if it were a single 3.7 cell because we have essentially built bigger 3.7 cells. That's good news since the way to keep a pack healthy for its lifetime is to make sure the cells are balanced with each other.

    In the example for the 52 volt pack we now have 14 3.7 volt groups, if we stick a test lead in between each group we can see the voltage go up as we add groups. 3.7 then 7.4 then 11.1, as we add 3.7 volts each time, this gives us a way to see what each group is looking like at any point in its charge. If we are expecting to see all cells at 3.7 volts and the first group test lead is 3.7 volts but the second group test lead is 6.5 volts something is wrong since the second group only brought 2.8 volts to the party.

    This is exactly the kind of math that the battery BMS (Battery Management System) does all the time. Balancing is the process of keeping all of the groups at the same level so the loads through the pack are distributed evenly.

    Quality chargers use a system called CCCV, Constant Current Constant Voltage, to fill up your pack. Without getting to far into it think of it as filling a bucket with a hose, you turn on the water to whatever level you can control to get the bucket filling, then when you get near the top you slowly turn down the flow so not to slop all over until you have just a trickle and can precisely get it full right to the tippy top. That is basically what CCCV does for your battery. That is a big single bucket, now think of trying to fill an empty six-pack or in our case (no pun intended) a 14 pack.

    The problem arises when one of your cans/cells does not fill at the same rate as the others or it already had some backwash/charge left in it. Each can/cell is a little different.

    Charging to 80% capacity is a great way to extend the lifecycle of your pack but over time you will never see which cells have gotten a little out of sync with the others. Balancing only occurs when doing a complete 100% charge since we know that 3.7 volt packs are completely charged when they reach 4.2 volts (yeah I know, but that's a whole 'nuther article by itself) so those are the targets we measure our packs by. Only by charging to 100% can we see which cells get there first and which ones are a little behind. At this point balancing starts to occur.

    The charger will stop the charge when the cells are at the desired voltage and the current has dropped down to about 3% of the rated charge current. If a cell reaches 4.25V before the total voltage is reached the BMS can terminate the charge and will not let the charger continue before that cell is bleed down to a safe level.

    The BMS then looks at the (in our case) 14 cells and starts to bleed off voltage from the ones that are the highest.

    Rinse. Lather. Repeat.

    This continues until all cells are so close in voltage that no further fine tuning can be done. You now have a balanced pack.This is why even if you want your pack to last as long as possible you should occasionally do a long duration 100% charge. Just remember to go for a ride as soon as possible because it is still a bad thing to store a full pack, run it down to 70% or so at least. Ideal long term storage is 40-50%.
    Any article about battery technology is obsolete by the time you hit <save>, including this one. Here are some excellent articles about batteries as related to ebikes, some are older than others, adjust as needed.

    E-Bike Charging for long battery life

    E-Bike Battery Math
    Attached Files
    Last edited by Sebz; 06-19-2017, 05:55 AM.

    #2
    Great info...& to keep as general reference. Also, appreciate the honesty of how quickly things change...or otherwise become obsolete. Kudos!

    Comment


      #3
      That is great stuff, really helped my understanding. From a practical standpoint, I have been charging my 52v battery fairly regularly for the last six months. What I have noticed is that the cheap 2a charger does a much better job balancing the pack than my original 5a charger (which broke after a few months). I recently spoke with someone who understands the technical aspects of batteries very well and rides a lot, and he found the exact same thing after his 5a charge broke. The 2a charger is much slower obviously (about 4-5 hrs to charge a 52v 11.5ah Shark) but our batteries perform better. Before the 5a charger broke, I tried a lot of different things - 3a charging, 80%, 90% etc. Nothing could get the battery back to its original capacity.

      I have no idea why this is the case but it's quite clear. I was very worried about my battery performance until my 5a charger broke, and after 2+ months with the 2a charger it's back to original strength with no declines far as I can tell.

      Comment


        #4
        Correction: "The name means 18mm x 650mm" should read "The name means 18mm x 65mm with a circular cross section".

        Comment


          #5
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          Originally posted by CraigAustin View Post
          That is great stuff, really helped my understanding. From a practical standpoint, I have been charging my 52v battery fairly regularly for the last six months. What I have noticed is that the cheap 2a charger does a much better job balancing the pack than my original 5a charger (which broke after a few months). I recently spoke with someone who understands the technical aspects of batteries very well and rides a lot, and he found the exact same thing after his 5a charge broke. The 2a charger is much slower obviously (about 4-5 hrs to charge a 52v 11.5ah Shark) but our batteries perform better. Before the 5a charger broke, I tried a lot of different things - 3a charging, 80%, 90% etc. Nothing could get the battery back to its original capacity.

          I have no idea why this is the case but it's quite clear. I was very worried about my battery performance until my 5a charger broke, and after 2+ months with the 2a charger it's back to original strength with no declines far as I can tell.
          I'm a consistent 2A charger guy. Once in a while if hurried I go to the packs limit. All ain all all 8 packs seem more consistent at 2A levels.

          Comment


            #6
            That was really a nice primer on these packs. Full of info, made understandable.

            Well done, and thanks!
            Fabrication is fun! Build something today. Show someone. Let them help. Inspire and share. Spread the desire.

            Comment


              #7
              Is there a data sheet or any info on the actual battery management system (BMS) included in the luna packs, such as the 52V sharks?

              Comment


                #8
                isn't the lower amps you charge the better??

                Comment


                  #9
                  Originally posted by Dave mc View Post
                  isn't the lower amps you charge the better??
                  The lower the charger current rate, the easier it is on the battery. All other things being equal, the battery that was charged at a lower rate of current will last longer.

                  The downside, of course, is that it takes longer to charge the battery. I charge (at a rate of 4A) overnight so I am okay with this.

                  If you charge at too high of a current rate, it will ruin the battery. The battery I have has a max charging rate of 8A.

                  Comment


                    #10
                    Given these parameters:
                    • 4.0 mile one-way commute. Mostly flat ground (one overpass), twice weekly
                    • 52v cube mighty mini 7A GA cube battery
                    • Big fat Sondors bike, or Stumpjumper with Cyclone kit
                    Would regular 80% 1A (or lowest rate that does the job) charging overnight with one 100% overnight charge per month be a workable strategy to handle the balance issue?

                    Comment


                    • Rodney64
                      Rodney64 commented
                      Editing a comment
                      Yes this would as long as you still do the 100% charges. If its not charging above 58 volts then you need to do the 100% charges more regularly as the battery is too far out of balance. Maybe start a thread in the battery section and keep this as a knowledge base. Perhaps put a link to your thread, here.

                    #11
                    How long does it take for the balancing to occur? One my 48v battery gets to 54.5 it won't go any higher (pretty sure it's full!) The 300w luna charger is set @3A @ 100%. LED 2 just stays red and fan keeps going. Is it balancing and not done yet? Or? Thanks!

                    Comment


                      #12
                      Originally posted by pweisel View Post
                      How long does it take for the balancing to occur? One my 48v battery gets to 54.5 it won't go any higher (pretty sure it's full!) The 300w luna charger is set @3A @ 100%. LED 2 just stays red and fan keeps going. Is it balancing and not done yet? Or? Thanks!
                      Balancing seems to be very slow. Mine (1 kWh) can take 24 hours+ to balance even with my fancy Satiator charger, and it's never badly out of balance. Light on my Luna 300 charger always turns off, eventually, too, but not sure how long as I use the Satiator for balance charges.

                      So, I suspect it's just not done yet, how long have you let it go?
                      Fabrication is fun! Build something today. Show someone. Let them help. Inspire and share. Spread the desire.

                      Comment


                        #13
                        Can someone clarify on a balance charge to 100% with the Luna chargers when the green light comes on (signifying it's charged) is it still balancing or is that as balanced as it's going to get?

                        Comment


                        • Sebz
                          Sebz commented
                          Editing a comment
                          Please read this: https://electricbike.com/forum/forum...ging-batteries

                          It covers who the BMS balances the pack.

                        • HutsonDave59@Gmail.com
                          Editing a comment
                          If your battery pack does not contain a BMS (battery management system) the battery cells in your pack are not being balanced when you charge it with a conventional Ebike battery charger such as the Luna models and all others for that matter. The only way to charge a battery pack and balance the cells that does not contain a BMS is with a dedicated balance charger. Your battery pack will require individual leads from the positive terminal of each cell to be balanced. These leads are in addition to the normal charging leads that go to the charger. If your pack contains a BMS and you charge it with a decent quality charger when the charger says it's done it is charged and balanced, (supposedly). Personally I don't use a BMS I charge my packs with a dedicated balance charger and monitor the cell voltage of the individual cells. I have much more confidence in the hands-on approach and seeing for myself exactly what is going on in the pack. Hope this helps

                        • Sebz
                          Sebz commented
                          Editing a comment
                          Not quite HutsonDave, if there is a BMS in the battery, the BMS will start to balance the cells only when a group reached 4.2V, Balancing occurs during charging and after charging. So always good to unplug the charger and leave the pack alone for a few hours when you are doing your balancing cycle..

                          If the pack is not fully balanced when the green light on the charger is on and you unplug and go ride, you will have redo it again.
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