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[gt13013]

Some references before beginning:
https://zeromanual.com/index.php/Cell_Balance
https://zeromanual.com/index.php/Unofficial_Service_Manual#Cell_Balancing
http://batteryuniversity.com/learn/article/bu_803a_cell_mismatch_balancing

My interrogation is: in which conditions is the self balancing starting?
At which SOC? When the bike is plugged or not?

We can read things that seem contradictory:
In https://www.facebook.com/groups/zmcowners/permalink/1623588564377012/ we can read: "The battery will auto self balance at any SOC above 50%, even if not plugged in".
In http://electricmotorcycleforum.com/boards/index.php?topic=7836.0 we can read: "There is a need to charge to 100% occasionally to let the balancing happen".

I have explored my BMS logs, and there are lines with the string "SOC adjusted for voltage". I grep'ed this string in the whole log and it gives the attached file.

Question?
Does the string "SOC adjusted for voltage" log the cell balancing?
If yes, it seems that it starts at 80% SOC (I have no line with less than 80% SOC. But that is not a proof...).

[dennis-NL]

Good question, good research.
Now hoping an insideman has the correct answer, we wait!

[Doug S]

I'm no "insider", but I am an EE with tons of experience with battery-powered systems, including electric vehicles.

In https://www.facebook.com/groups/zmcowners/permalink/1623588564377012/ we can read: "The battery will auto self balance at any SOC above 50%, even if not plugged in".

I don't believe that. I don't even see how it would be possible for a series string of battery cells to be balanced by the BMS Parallel cells, yes, but not series cells. Only the charger can do that.

In http://electricmotorcycleforum.com/boards/index.php?topic=7836.0 we can read: "There is a need to charge to 100% occasionally to let the balancing happen".

That one I believe completely. The charger/BMS systems that I've seen trickle-charge each cell individually once the main charging current is turned off. It takes a while because the trickle-charging current is very low.

[hubert]

Does it switch an 4,15V source to any of the cells in need, successively (positive balancing), or is there just a resistor switched across the "too high cells" to align them with the lower ones, while the main charger ups the whole string (negative balancing)?

[gt13013]

I don't believe that. I don't even see how it would be possible for a series string of battery cells to be balanced by the BMS Parallel cells, yes, but not series cells. Only the charger can do that.

Why not? The BMS could take energy in the cells with higher voltage in order to feed the cells with lower voltage. Why would it not be possible?

[DPsSRnSD]

I don't believe that. I don't even see how it would be possible for a series string of battery cells to be balanced by the BMS Parallel cells, yes, but not series cells. Only the charger can do that.

Why not? The BMS could take energy in the cells with higher voltage in order to feed the cells with lower voltage. Why would it not be possible?

I've been told that the bike will balance the cells anytime the contactor is closed if the SOC is above about 50%. I charge my bike to 80-90% and don't recharge it until it gets down to about 25-50%. My cell inbalance stays low, about 3-5mV, despite frequent fast accelerations on my commute. Maybe I'm just lucky or tempting fate.

[mrwilsn]

The bikes 'cell balancing' function doesn't take place until the very end of the charge cycle when current going to the battery pack is less than 4 amps.  The Zero App will not show current when it goes below 4 amps so the only way to know that it is happening is if you either use a watt meter on the outlet the bike is plugged into or if you have a DVM connected to the battery (like I do) and can see that the contactor is still closed even though the app doesn't show the bike is charging.

As the battery discharges the cells slowly get out of balance as a function of the internal resistance of each individual cell.  Cells with the highest internal resistance will discharge slower and cells with the lowest internal resistance will discharge faster.  Cell imbalance will be the highest the lower the pack voltage gets.

Even if your pack is at 0% SOC this process reverses naturally the second you start charging the battery pack.  The cells with the lowest internal resistance will charge faster and the cells with the highest internal resistance will charge slower.  Since voltage of the cells with the highest internal resistance would be slightly higher at low SOC than the cells with the highest internal resistance the effect is that cell balance naturally starts to get smaller.  This doesn't require any special function of the charger it just happens as a result of chemistry.

The cells Zero uses are so well matched that cell balance will typically be very low (less than 5 or 10mV) before the actual 'cell balancing' function takes place.

Anyone can test this just by using the Zero app.  Ride your bike down to low SOC and use the app to check cell balance.  At low SOC the pack cell balance can be anywhere from 50mV to 100mV or more.  Connect your bike to the charger and within minutes you will see the cell balance start to drop.  Check on the bike every hour and you will see that every time you check the cell balance will be smaller and smaller.  By the time you get to 100% you should be less than 10mV (I frequently see 1-2mV cell balance on my bike).

In reality, it's not the charger that facilitates cell balancing the cells to be closer than what would happen naturally.  The charger has two modes.  Constant Current (CC) and Constant Voltage (CV).  The value at which the charger goes into CV can be adjusted by the manufacturer.  Once the battery pack is at 100% SOC the BMS actually facilitates cell balancing.  The charge is still coming from the charger but the BMS allows individual cells to be balanced using the balance wires and bleed resistors.  Thus it would be possible to balance the cells to the 1-2mV range at any SOC by using programming.

Tesla's for example allow you to set your max SOC value.  If, for example, you set the max charge to 60% the charger will go into CV mode once the voltage rise value hits the 60% SOC value.  As the 'resting' pack voltage hits the 60% SOC the cells can then be balanced at that voltage.  Unfortunately, Zero does not support setting a max charge and will always attempt to charge to 100%.  Changing this for a Zero would require new programming for the charger (to go into CV at a lower voltage) and the BMS (to facilitate balancing).

[BrianTRice@gmail.com]

I don't believe that. I don't even see how it would be possible for a series string of battery cells to be balanced by the BMS Parallel cells, yes, but not series cells. Only the charger can do that.

Why not? The BMS could take energy in the cells with higher voltage in order to feed the cells with lower voltage. Why would it not be possible?

I agree with gt13013 from my observations and many insiders confirming. The BMS definitely performs balancing of each set of cells in the same place in the series.

The "Battery Connections" label in the manual are a set of pins that connect to each layer of the series and are the means by which the BMS intentionally directs current to that layer of the series. The only question is what logic it applies to do this.

Ref. https://zeromanual.com/index.php/Unofficial_Service_Manual#BMS_Parts


The BMS has a function where it invokes the onboard charger to turn on periodically so it can get the power to perform the balancing, but any charger is just putting power into the battery's main DC power connections (bottom and top of the series, really).

[gt13013]

As the battery discharges the cells slowly get out of balance as a function of the internal resistance of each individual cell.  Cells with the highest internal resistance will discharge slower and cells with the lowest internal resistance will discharge faster.  Cell imbalance will be the highest the lower the pack voltage gets.

Even if your pack is at 0% SOC this process reverses naturally the second you start charging the battery pack.  The cells with the lowest internal resistance will charge faster and the cells with the highest internal resistance will charge slower.  Since voltage of the cells with the highest internal resistance would be slightly higher at low SOC than the cells with the highest internal resistance the effect is that cell balance naturally starts to get smaller.  This doesn't require any special function of the charger it just happens as a result of chemistry.

I do not agree with that. You seem to consider that the cells are in parallel, but they are in series inside a pack, so it does not work this way.

[BrianTRice@gmail.com]

As the battery discharges the cells slowly get out of balance as a function of the internal resistance of each individual cell.  Cells with the highest internal resistance will discharge slower and cells with the lowest internal resistance will discharge faster.  Cell imbalance will be the highest the lower the pack voltage gets.

Even if your pack is at 0% SOC this process reverses naturally the second you start charging the battery pack.  The cells with the lowest internal resistance will charge faster and the cells with the highest internal resistance will charge slower.  Since voltage of the cells with the highest internal resistance would be slightly higher at low SOC than the cells with the highest internal resistance the effect is that cell balance naturally starts to get smaller.  This doesn't require any special function of the charger it just happens as a result of chemistry.

I do not agree with that. You seem to consider that the cells are in parallel, but they are in series inside a pack, so it does not work this way.

Within a single brick, there is a single stack of cells that are in series.

In a long brick, there are two stacks of cells. Each stack is in series, but there are interconnects between each pair of cells at the same level of the stack.

In a monolith, there are interconnects between all available cells at the same level of the stack (IIRC for the long brick architecture it is different).

The BMS for a long brick or monolith or single power pack module has 28 pins for balancing - one for each layer of the stack, interconnected.

[gt13013]

Thanks Brian.
I was focused on my bike which has two 3.3kWh batteries, each of them with 28 cells in series.
But bigger batteries are more complex

[mrwilsn]

What I wrote is correct. I'm aware the bricks are 28S.

You don't have to believe me, as I said you can test for yourself using the app.

Sent from my SM-G950U using Tapatalk

[gt13013]

From my point of view, the cells with the higher internal resistance are the bad ones, so generally they have less capacity than those with small internal resistance. When you discharge a set of cells in series, since all the cells deliver the same Ah, those cells with high resistance will discharge faster than the other (and by the way they will heat more than the other).

[Richard230]

From my point of view, the cells with the higher internal resistance are the bad ones, so generally they have less capacity than those with small internal resistance. When you discharge a set of cells in series, since all the cells deliver the same Ah, those cells with high resistance will discharge faster than the other (and by the way they will heat more than the other).

I think that is exactly what has happened to one or more of the cells in my old 2014 S with PT, that have caused the charger to cut off at 92%.  But if you leave it plugged in for about a week, it will eventually increase the charge to 98% on the display. I don't think the lack of charging to 100% has anything to do with a charger malfunction, just a cell or two suffering from old age tricking the charger to cut off charging before the rest of the cells are fully charged. 

[Doug S]

You're making this too complicated. In any battery pack, some cells will have slightly lower or slightly higher capacity than the average, just because of manufacturing variations. That doesn't make any of them "good" cells or "bad" cells. Similarly, some will have slightly higher or slightly lower internal resistance, not necessarily the same cells that have higher or lower capacity. Some people are good at cooking AND poker, some not as good at either, right?

Again, equalizing cells in parallel is easy -- in fact, it would be hard to avoid it. Connect cells together in parallel and by definition, they're going to have the same cell voltage. That doesn't mean they're holding the same amount of amp-hours, due to the manufacturing variations, but at least the cell voltages are identical, and they will be all through any charging or discharging cycles.

Series connections are much harder to equalize. They're not hard-wired together to share cell voltage. Now, I did speak too strongly earlier when I said it's not possible to equalize a series-connected string of cells without using a charger -- anything's possible with clever circuit design, if you want to do it bad enough. But any solution would be complicated, clumsy, would itself consume power, and just not be worth implementing.

I've never seen it done, and yes, I've looked at several BMS's for several different systems. Cell equalization is done by the charger, after charging has reached "100%", by a string of resistors that trickle-charges cells that aren't quite yet up to terminal cell voltage. If someone shows me an actual schematic that does something else, I'll believe it, but until then I'm sure not going to take the word of some probably misinformed salesman who's talking up his product.