I recently bought an e-bike build online and I need help on cell balancing. It is a 24s2p lifepo4 battery, I bought the bike knowing that 2 groups would have to be replaced because they are way to low as seen in the image below. I already have 4 new cells coming, I was wondering how I could balance the cells? I don’t think the BMS will do anything about that higher group so how could I get it lower? The battery is already assembled so id prefer not having to take it apart to balance it, any suggestions are appreciated!
Battery balancing
- Thread starter Alex_Q
- Start date
Are the new cells the exact same model and brand as the originals?
Is the BMS programmable? Some you can set the balance point. For those you could set it to below the voltage they are already at, so it drains them all down to that point to make them equal voltage. ****
Others you just have to fully charge and let it drain down high cells, leaving the charger on it so it can repeat this process until all cells are balanced. When there is a fairly large imbalance it can take days or more for this to happen. For LiFePO4 the balance point is usually up around 3.65v, for most nonprogrammable BMSes.
**** note that equal voltage at some point in the charge/discharge curve is all (top or bottom) balancing can do, which does not make them equal capacity, and cannot "fix" a pack's cell differences--that requires replacing all cells with matched cells.
Note that most of the cells are near the middle of the discharge curve (~3.2v) where voltage doesn't change very much for large differences in capacity, so there could be many of them that are very different from each other. You won't know until you either fully charge or fully discharge them and see what they all end up at. If they stay near each ohter in voltage even at full or at empty, then they are relatively close in capacity and internal resistance, etc.
If you are willing to remove the defective cells, and unbolt one cell each from the neighboring groups and temporarily put them where the defective cells were, you can use the normal BMS charge/balance procedure to balance the cells you already have, first, while waiting for the new cells.
Note also that prismatic cells like these require compression across their large faces, so the pack is not built very well to keep them working well over time. It would need to have all of them in sets that have their large faces against each other, which two of the sets do (the ones with short thin busbars) rather than the two sets that connect them edge-to-edge (the ones with large plates). Then an inflexible endplate would need to be installed on each end of each of the sets, and some form of clamp or strapping system from one end to the other that creates even tension between them, so that it applies the correct pressure across the faces of the cells (correct for those specific cells, which you would have to find out from the cell manufacturer what PSI rating that is).
Without such a system, the cells can swell during charge and discharge, separating internal layers from each other and decreasing capacity and increasing internal resistance, permanently damaging the cells (even if the swelling goes away because of venting of gases, so that you never actually *see* this swelling yourself).
This makes each cell more and more different from the others, which makes the pack less and less capable. It's bad enough that almost all ebike packs (especially DIY) start out with completely mismatched cells, but this problem makes them worse more rapidly than normal aging would. (sometimes much more rapidly).
It could be the cause of the cell failures it already has. (but that failure could also be stuck-on balancers in the BMS, or just plain defective cells).
Is the BMS programmable? Some you can set the balance point. For those you could set it to below the voltage they are already at, so it drains them all down to that point to make them equal voltage. ****
Others you just have to fully charge and let it drain down high cells, leaving the charger on it so it can repeat this process until all cells are balanced. When there is a fairly large imbalance it can take days or more for this to happen. For LiFePO4 the balance point is usually up around 3.65v, for most nonprogrammable BMSes.
**** note that equal voltage at some point in the charge/discharge curve is all (top or bottom) balancing can do, which does not make them equal capacity, and cannot "fix" a pack's cell differences--that requires replacing all cells with matched cells.
Note that most of the cells are near the middle of the discharge curve (~3.2v) where voltage doesn't change very much for large differences in capacity, so there could be many of them that are very different from each other. You won't know until you either fully charge or fully discharge them and see what they all end up at. If they stay near each ohter in voltage even at full or at empty, then they are relatively close in capacity and internal resistance, etc.
If you are willing to remove the defective cells, and unbolt one cell each from the neighboring groups and temporarily put them where the defective cells were, you can use the normal BMS charge/balance procedure to balance the cells you already have, first, while waiting for the new cells.
Note also that prismatic cells like these require compression across their large faces, so the pack is not built very well to keep them working well over time. It would need to have all of them in sets that have their large faces against each other, which two of the sets do (the ones with short thin busbars) rather than the two sets that connect them edge-to-edge (the ones with large plates). Then an inflexible endplate would need to be installed on each end of each of the sets, and some form of clamp or strapping system from one end to the other that creates even tension between them, so that it applies the correct pressure across the faces of the cells (correct for those specific cells, which you would have to find out from the cell manufacturer what PSI rating that is).
Without such a system, the cells can swell during charge and discharge, separating internal layers from each other and decreasing capacity and increasing internal resistance, permanently damaging the cells (even if the swelling goes away because of venting of gases, so that you never actually *see* this swelling yourself).
This makes each cell more and more different from the others, which makes the pack less and less capable. It's bad enough that almost all ebike packs (especially DIY) start out with completely mismatched cells, but this problem makes them worse more rapidly than normal aging would. (sometimes much more rapidly).
It could be the cause of the cell failures it already has. (but that failure could also be stuck-on balancers in the BMS, or just plain defective cells).
Last edited:
BTW, if this is this bike
endless-sphere.com
then note that most LiFePO4 generally is useful for a max of 2C, sometimes 3C if you don't mind fairly big voltage sag (wasted power). (There are some like A123 (which these definitely are not) that can do better than that, but most cannot, regardless of seller claims to the contrary). If you have the datasheet for these cells or can get it from the manufacturer (not the seller since many outright lie about batteries) then you can see what your cells are really capable of
That means that 2p of 25Ah LiFePO4, or 50Ah, is generally good for up to 100A, maybe 150A.
The BMS on the linked bike is 300A, so it will allow two to three times the amount of current that the typical cell is capable of handling.
The controller is 500A, which if the BMS allows that much would be several times the current the typical cell could handle.
Since it says the controller is a Nucular, those should be programmable current limit, so I would highly recommend turning the current limit down to below what the maximum continuous current the cells' spec sheet says they can handle. If you don't have a spec sheet, I'd turn it down to 100A or maybe 150A at most. If there's a lot of voltage sag, I'd turn it down even further. If the cells are pushed too hard it can damage them.
Voltage sag or drop of voltage under load is a good indication of cell capability. If there is no sag then the cells can easily handle it. The greater the drop in voltage as the load increases, the worse the sag, and indicates the cells are not handling the load as well--when the drop is more than say, a tenth of a volt per cell, that's fairly bad, a couple of tenths per cell is not very good and indicates the current limit should be lowered; there is probably significant heating going on inside the cells at this point, wasting power inside them instead of feeding it to your motor and wheel.
Assuming the cells are very unusual and can handle 6C easily, then at best you could set the controller current limit to 300A, which is the max the BMS can handle. Higher means either damaging the BMS, or the BMS turning the output off completely to protect itself, depending on how it's rating and limiting are done, and whether it is working correctly or not.
To use the controller at it's potential, you'd need a much better battery pack than this type of cell can usually provide, or much higher capacity cells to compensate for their usually low C-rate. (these would probably not fit in the existing battery compartment).
2007 Husqvarna TE450 USA
I started a project and got to 90% and lost interest. The frame, forks etc. are 2007 Husqvarna TE450 rolling chassis I purchased a QS 273v4 with 17" alum wheel and had stainless torque arms machined I added new shinko dual sport tires font and rear I purchased the Nucular 24F 500 amp controller...
endless-sphere.com
That means that 2p of 25Ah LiFePO4, or 50Ah, is generally good for up to 100A, maybe 150A.
The BMS on the linked bike is 300A, so it will allow two to three times the amount of current that the typical cell is capable of handling.
The controller is 500A, which if the BMS allows that much would be several times the current the typical cell could handle.
Since it says the controller is a Nucular, those should be programmable current limit, so I would highly recommend turning the current limit down to below what the maximum continuous current the cells' spec sheet says they can handle. If you don't have a spec sheet, I'd turn it down to 100A or maybe 150A at most. If there's a lot of voltage sag, I'd turn it down even further. If the cells are pushed too hard it can damage them.
Voltage sag or drop of voltage under load is a good indication of cell capability. If there is no sag then the cells can easily handle it. The greater the drop in voltage as the load increases, the worse the sag, and indicates the cells are not handling the load as well--when the drop is more than say, a tenth of a volt per cell, that's fairly bad, a couple of tenths per cell is not very good and indicates the current limit should be lowered; there is probably significant heating going on inside the cells at this point, wasting power inside them instead of feeding it to your motor and wheel.
Assuming the cells are very unusual and can handle 6C easily, then at best you could set the controller current limit to 300A, which is the max the BMS can handle. Higher means either damaging the BMS, or the BMS turning the output off completely to protect itself, depending on how it's rating and limiting are done, and whether it is working correctly or not.
To use the controller at it's potential, you'd need a much better battery pack than this type of cell can usually provide, or much higher capacity cells to compensate for their usually low C-rate. (these would probably not fit in the existing battery compartment).
Last edited:
The batteries are rated for 3c continuous. It looks like the cells might have discharged because the BMS was continuously on for a long period of time, it’s an ant BMS so I will have to see if I can program it. I will definitely take your advice and limit the max current to 150a and check the sag on it. I do see what your talking about with the expansion of the cells, I notably saw it in the group at the top. How do you think I could go about compressing them correctly? Honestly might consider upgrading to better pouch cells or get a 21700 pack built in the future
This post (and some others like it) have pics of some packs (from random image searches for cell-compression) that show packs built with prismatic or pouch cells with end-plates and tensioning systems (bolts or straps):
endless-sphere.com
The exact method you would use for your pack depends on the space you have, and whether you want to compress the entire pack (more difficult to evenly compress all of the stacks) or make (four) modules that each have their own compression (simpler but takes more space). The important thing is that the end plates be VERY stiff so they provide the same pressure across the entire surface, and are tensioned equally at each corner, side, etc., however they're connected together across the length of the modules or pack.
BTW, if the BMS drained just those cells just from being "on", they would be at one end of the pack (usually the most negative), and in consecutive series groups, and the BMS would be being powered just by those and no others.
Since they're not consecutive groups but rather "random" ones, then the BMS didn't drain them from being on.
It could have drained them by having stuck-on balancers. You can test for that by connecting another cell (from another group, for instance) in place of the drained ones, and checking for current flow from it to the BMS thru the sense wire, or by checking for any voltage across the balance shunt resistor for that channel in the BMS. (should be no voltage across it, or current into it, if it's not active).
Or if tha'ts not the problem but the BMS was on and is powered by the whole pack, then those drained cells were simply far lower capacity than the rest of the cells, so they drained long before the rest of the pack did. (it's highly likely that there's significant variation in capacity and resistance between the whole pack of cells anyway, as almost none of the sellers of these types of cells can do more than say "above xxxAh capacity" and "below xxxmilliohm resistance", so they are almost always completely non-matched and all behave differently from each other) .
Steps to safely recover low voltage NCM cells?
Hi, I have a prismatic NCM pack of 20S 92Ah 3.65V nominal cells in an electric motorbike. They have a Daly BMS set to protect them from over discharge, but somehow it didn't do that and whilst stood through winter they have run down to a range 1.8V to 2.2V. Since it's like $900 worth of cells...
endless-sphere.com
The exact method you would use for your pack depends on the space you have, and whether you want to compress the entire pack (more difficult to evenly compress all of the stacks) or make (four) modules that each have their own compression (simpler but takes more space). The important thing is that the end plates be VERY stiff so they provide the same pressure across the entire surface, and are tensioned equally at each corner, side, etc., however they're connected together across the length of the modules or pack.
BTW, if the BMS drained just those cells just from being "on", they would be at one end of the pack (usually the most negative), and in consecutive series groups, and the BMS would be being powered just by those and no others.
Since they're not consecutive groups but rather "random" ones, then the BMS didn't drain them from being on.
It could have drained them by having stuck-on balancers. You can test for that by connecting another cell (from another group, for instance) in place of the drained ones, and checking for current flow from it to the BMS thru the sense wire, or by checking for any voltage across the balance shunt resistor for that channel in the BMS. (should be no voltage across it, or current into it, if it's not active).
Or if tha'ts not the problem but the BMS was on and is powered by the whole pack, then those drained cells were simply far lower capacity than the rest of the cells, so they drained long before the rest of the pack did. (it's highly likely that there's significant variation in capacity and resistance between the whole pack of cells anyway, as almost none of the sellers of these types of cells can do more than say "above xxxAh capacity" and "below xxxmilliohm resistance", so they are almost always completely non-matched and all behave differently from each other) .
Since neither group is at the negative end of the pack, it's very very unlikely the BMS is powered from them.
I've never seen anything that powered itself from anything at the positive end of the pack unless it was the entire pack; running off one or a few groups is always done starting at the negative end.
I've never seen anything that powered itself from anything at the positive end of the pack unless it was the entire pack; running off one or a few groups is always done starting at the negative end.
Well, all the cells the BMS is being powered from will be draining (even if it's not evident at the SoC of most of them); it will only be "obvious" on a cell that is very close to or beyond the full or empty state. It could be powered just from the one cell.
Doesn't explain G15 though.
If it's a stuck balancer, then that will keep draining whether the BMS is "on" or not, as long as it is physically connected to the cells.
Hey so I’ve put in the new cells and you seen to be right about the cells draining due to them being so low, so from what I understand, all I have to do it charge it up to max and let the charger balance it on its own? The ant BMS also seems to have a auto balance feature that can balance the cells once their charged
As long as there's nothing wrong with teh BMS it should do the work for you.
appreciate the help I’ll let you know if anything else happensI have a majority of the cells balanced with most sitting around 3.285, there is one outlier with group 7 as it’s at 3.321, is there any way I can discharge this one specifically? The BMS only discharges at like 200ma so it would take forever for it to discharge.
If a group drains faster, but charges to the same voltage as all the others, then that group is lower capacity than the others. The only way to fix that is to replace it with a group that is the same capacity as all the others. If it's a minor difference, then as long as that still leaves you with enough capacity, it's easiest just to leave things as they are until the cell gets bad enough to require replacement.
In a battery that's got cells / groups of varying capacity, the only perfect way to fix this is to replace all the cells with completely new *matched* cells**** that are all exactly identical.
****It is difficult to find any place to buy such cells; the closest I have seen are used large-EV (car, truck, etc) cells such as places like Batteryhookup, etc., sell. No seller (including BH) that anyone has had this conversation with even seems to know what "matched" actually means; the best most can do is say that a cell will have "less than" some amount of resistance and "more than" some amount of capacity, neither of which is useful.
