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[Richard230]

I kind of like watching the Kill-A-Watt meter when my Zero is charging.  With my old 2014 S, I would see it bulk charge most of the time, but every 10% or so when charging the power draw would drop from about 12 amps to 4 amps for a while and then ramp up to bulk charging again.  I assumed that it was taking time off from bulk charging to balance the cells.  Interestingly, I haven't noticed that happening with my 2018 S.  It seems to bulk charge until the SOC display reads 100% and then the power draw drops to 4 amps for a few minutes, before dropping to 4 watts until I pull the plug.   

[gt13013]

OK. I would have a lot to tell, and I would do it if this forum was in French. But I am not fluent enough in English to tell what I'd like to... And it would probably be misinterpreted.

My initial questions were:
1) Is the cell balancing occurring only during the charge of the battery, or is the BMS able to do it when the bike is resting?
2) In the case where the BMS is able to do it when the bike is resting, of course it will use a little bit of energy. So the BMS probably does it only if the SOC is above some value.
3) To try to clarify that, I was wondering if the balancing activity is reported in the logs. But it does not appear clearly in my logs.
4) The string "SOC adjusted for voltage"  in the logs seems to be related with some activity of the BMS, probably updating the SOC from different algorithms, and I was wondering if it was balancing the cells at the same time

So, it seems that the way the BMS acts to balance the cells will keep some mysteries (for me at least).
Anyway, probably it does not matter a lot, since I tried to never full charge since many cycles (stopping the charge between 75 and 85% SOC), and the cell balance keeps fine at any time (4 mV).
I will now focus on building my fast charger, as stated elsewhere in this forum, and forget about self balancing that seems to work magically for me 

[BrianTRice@gmail.com]

I can't answer any of your questions definitively, only speculatively, and am tired of this thread.

I will suggest that (without commenting on your charger implementation one way or the other) it will be better if your charger's tapering strategy allows the BMS enough time at a low charge rate to balance cells.

[Doug S]

You can read at "Active balancing" here:
https://www.mpoweruk.com/balancing.htm

Here is an example:
http://www.evassemble.com/index.php?main_page=product_info&cPath=6&products_id=44
This BMS features: "Advanced capacitor balancing technique which is working at charging, discharging and static state all the time. It balances cells by transfering energy from higher cell to lower cell (by nominal 100mA). Save energy and reduce heat emission in balance process (compare with resistor bleeding bms)."
Here is its connection diagram:
http://www.evassemble.com/pdf/BMS/Capacitor-16S-BMS%20instructions%20of%20connection.pdf

But I never used these devices. Perhaps they do not work? 

I'm with Brian, I'm tired of this thread too, but I will make one last comment regarding this point. When I overstated my case before by saying that it wasn't possible to balance a series string, then retracted the statement, this sort of "flying capacitor" technique was exactly what I was thinking about. It is possible to transfer charge, a little bit at a time, from one cell to another by connecting a capacitor to the higher-charged cell, then connecting it to the lower-charged cell. MOSFET switches are used to do the switching. This technique is used a lot at low power levels when, say, you need just a small amount of current at a negative voltage when all you have available is a positive power supply.

But if you run the numbers, using any reasonable amount of capacitance and switching rates, the charge transfer is exceedingly slow, and really not worth doing with 25 amp-hour cells.

It is POSSIBLE that the part Brian identified (the PL536) uses this technique, but that's not how i read the data sheet.

[dennis-NL]

About 9 people reacted on this thread, so it's an interesting subject.
Thx all.

[Richard230]

Speaking of cell balancing, I used these cheap little BMS devices on top of the Hi Power batteries of my Electric Motorsport GPR-S when its BMS system failed.  They seemed to work OK, but of course they didn't prevent those Chinese batteries from puffing up and dying one at a time.    The Hi Power batteries just didn't seem up to the task of supplying power to my huge D&D Sepex motor.

[oobflyer]

Found this thread - hoping to find an answer to a different, but related question: Since Zero changed their recommendations on charging ("leave it plugged in all the time" vs. "charge only to 80%"), and if the active cell balancing occurs at 100% charge, then How Often Should We Charge to 100%?

On my antique Vectrix (12 yrs old!) the cell balancing occurs after 100% charge and the BMS firmware automatically pushes the cells to continue charging/balancing by shunting the excess voltage from one (full) cell to another (less charged) cell - but it was programmed to do this every 10 charges.
I swapped out the original NiMH batteries for LiFePo cells (from 30AH to 50AH - same pack voltage), using a BMS 'kit' that included the shunts that are attached directly across the terminals of the batteries. These shunts close the circuit after reaching 3.65V so the voltage can flow across the terminals and to the other cells, until they all reach the maximum 3.65V to achieve balancing.

So no 'passive' balancing. This arrangement has kept my old Vectrix alive and well for a long time (although it is showing some signs of degradation now).

Does charging to 100% every 10 charges sound about right?

[heroto]

FWIW, my SR/F gets to 100 % then charges for an additional half hour or so. The current drops steadily and slowly, fairly linearly, from 5800 down to 0 watts before shutting down. That’s cell balancing, no?

[gt13013]

FWIW, my SR/F gets to 100 % then charges for an additional half hour or so. The current drops steadily and slowly, fairly linearly, from 5800 down to 0 watts before shutting down. That’s cell balancing, no?

No, this has nothing to do with the cell balancing. The slow decay of the current is typically the characteristic of a constant voltage charging, which is a common way to end the charge.

[BrianTRice@gmail.com]

Cell balancing by Zero's BMS occurs in a certain charging regime of low current, where the BMS directs that current to a layer of the cell stack.

This often happens during the taper noted above as constant voltage charging, but can also happen on demand where the BMS wakes from long-term plugged-in storage.

There are enough notes in this thread to understand more if required, or the unofficial manual has a tiny article on the topic which I'll probably revise shortly to be clearer:
https://zeromanual.com/wiki/BMS/Cell_Balancing

[heroto]

Thanks for enlightening me.
There are tons of posts on this forum about occasionally recharging to 100%+ to rebalance the cells. IIUC, this thread tells me that those posts are erroneous. If that's correct, you will find many confused fellow forumites.
 
So is there any reason to wait that last half hour after SOC has reached 100% while the charging power slowly tapers? Is there is no gain whatsoever beyond a couple of inches of range?
Thanks

[Richard230]

Thanks for enlightening me.
There are tons of posts on this forum about occasionally recharging to 100%+ to rebalance the cells. IIUC, this thread tells me that those posts are erroneous. If that's correct, you will find many confused fellow forumites.
 
So is there any reason to wait that last half hour after SOC has reached 100% while the charging power slowly tapers? Is there is no gain whatsoever beyond a couple of inches of range?
Thanks

In the case of my 2018 S there is no charge tapering. It charges at 1350 watts until it turns off suddenly after hitting 100% SOC.  My old 2014 S did taper the charger power, but that is not the case with my new model.   

[Crissa]

According to Zero, load-balancing doesn't need to be done more than once in ten charges or once a month or so.  So very infrequently!

As long as the on-board sensors are happy, it's not going to show much tapering or balancing at the box.  I'm not seeing any tapering, either.  (maybe that's part of the newest firmware?  I have no idea what they can modify.)

-Crissa

[Doug S]

There are tons of posts on this forum about occasionally recharging to 100%+ to rebalance the cells. IIUC, this thread tells me that those posts are erroneous. If that's correct, you will find many confused fellow forumites.
 
So is there any reason to wait that last half hour after SOC has reached 100% while the charging power slowly tapers? Is there is no gain whatsoever beyond a couple of inches of range?

In my opinion, both are correct. Occasional cell balancing is a good idea, but charging up to 100% every single time does reduce the life of the battery.

The real questions are, "HOW BENEFICIAL is periodic cell balancing?" and "HOW MUCH reduction in battery life will you see if you charge to 100% every time?". I don't think the answers to either of these questions is at all clear. I don't even think Zero has a very good handle on them -- if they did, they'd give us better guidance.

Being an EE who's worked with batteries a lot, my gut sense is that neither of these considerations are hugely important. I don't think performance will degrade much if you never cell balance. I do believe you can prolong the battery's life by "short charging", but in order to achieve any real gains, you really have to limit the parameters. Toyota uses 40% minimum charge on their Prius battery, and 60% maximum. That's one thing on a hybrid vehicle, but you'd only have 20% range if you limited yourself like that on an EV.

My 2014 SR has no means to limit the charge except for unplugging it; if it did, I'd probably use it and limit the charge to 80% or so, and maybe every couple of weeks perform a cell balance. But it doesn't, so I just plug it in every day when I get home from work, and it does a full charge and full balance every night. I've got 50,000 miles on my bike with no sign whatsoever of capacity reduction.

If you feel strongly about babying your battery, go for it. If you're lazy like I am, just plug it in and forget about it. If your bike supports early charge termination, use it and feel good about it. In any case, I just don't think it's worth worrying too much about.

[NEW2elec]

Doug, looks like I'll never catch up to your mileage, unless you get a new bike.
I've started using my quiq charger in the mornings so when I hit 100% SOC it's only there for a short time.

I do have a working theory that "using" these bikes helps keep the battery in better health than sitting.  The guys on here with the really high mileage bikes seem to have the fewest issues.