Active and Passive Balancing, not charging to 100%

[superox]

This isn't about my EUC per se, but... I have an e-scooter, with a custom, fairly new 16S11P battery made of Samsung 35E cells and a smart BMS. All groups of the series in it are always perfectly balanced, to < 0.01V.... Except one.

Which constantly charges faster than the other, perfectly balanced ones.

When I charge the battery from nearly empty, that one group reaches 4.20V faster, by the time the others are still at 4.16V. The BMS has active and passive balancing, and eventually catches up, but in order to avoid damage to that one group, it has to shut down charging prematurely, at just 66.7 or so volts. That's before the charger (3.5A) has even started to slow down for the last charging stage. Basically the light on the charger goes green at just 66.7 as the BMS blocks it from charging even further, then when it's done some balancing it goes red and charges again, then green again etc. If I unplug, then the BMS does it's job balancing and the excess voltage from that one group will even out with the rest.

Should I be worried? What could be the cause for such behavior?

[Tawpie]

When I bought an additional pack for my 16XS it arrived roughly handled and did the same thing... the 'new' pack told the system to stop charging early. Not exactly the same, mine was a hard shutdown of the charger (click, fans off etc.). I interpreted it as a defective pack, returned it and the replacement worked fine. If possible, I'd check with the custom builder for their opinion.

Edited October 29, 2022 by Tawpie

[Chriull]

On 10/29/2022 at 9:32 PM, superox said:

This isn't about my EUC per se, but... I have an e-scooter, with a custom, fairly new 16S11P battery made of Samsung 35E cells and a smart BMS.

Which reports single cell group voltages?

On 10/29/2022 at 9:32 PM, superox said:

All groups of the series in it are always perfectly balanced, to < 0.01V.... Except one.

Which constantly charges faster than the other, perfectly balanced ones.

When I charge the battery from nearly empty, that one group reaches 4.20V faster, by the time the others are still at 4.16V.

Which would mean very roughly something about 2% capacity deviation, if nothing else causes this difference (and not some bms peculiarity or fault) . So the 15 other groups are either 2% better or this one 2% worse.

On 10/29/2022 at 9:32 PM, superox said:

The BMS has active and passive balancing,

Both at the same time? Makes imho no real sense - but active balancing is not used by now by eucs and so i did not invest too much thoughts/reading in this topic...

Maybe it could be that passive (top) balancing is used during charging and active balancing during discharging/breaks?

On 10/29/2022 at 9:32 PM, superox said:

and eventually catches up, but in order to avoid damage to that one group, it has to shut down charging prematurely, at just 66.7 or so volts.

Which would be one time 4.2V plus 15 times 4.16 (~66.6V). 

On 10/29/2022 at 9:32 PM, superox said:

That's before the charger (3.5A)

That's about 0.1C == (very) slow charging for an 11p 3.5Ah config.

On 10/29/2022 at 9:32 PM, superox said:

has even started to slow down for the last charging stage.

So no constant voltage charging stage, as the bms seems to shut down once the first cell group reaches 4.2V.

On 10/29/2022 at 9:32 PM, superox said:

Basically the light on the charger goes green at just 66.7 as the BMS blocks it from charging even further, then when it's done some balancing it goes red and charges again, then green again etc. If I unplug, then the BMS does it's job balancing and the excess voltage from that one group will even out with the rest.

Should I be worried? What could be the cause for such behavior?

As you described once one group reaches 4.2-4.21V the bms shuts off. Once during balancing this group goes below some 4.19-4.2V charging input is enabled again. So slowly over many iterations all cells reach the ~4.2V.

Normal (as used with euc) passive (top) balancing would engage at 4.2V a bleeding resistor parallel to this cell group to slow charging a bit. At some ~4.25V the bms would shut down charging. So the other cell groups have the chance to catch up _more_ the 0.05V and after charging is cut off by the bms the on cell group is discharged by the bleeding resistor to 4.2V.

So in your case with passive balancing the battery would be nicely balanced after the normal charge cycle.

During use this one weaker group will reach lower voltages, but with charging all groups reach about 4.2V again automaticly! Small deviations will be balanced again with the before described mechanism.

As this weaker cell group has the lowest voltage once discharged this one limits overall usable battery capacity. So one should consider the battery empty once this group reaches the 3.0-3.3V and not once overall battery voltage reaches (3.0-3.3V)*16=48-52.8V. Otherwise the weakest cell could reach unhealthy low voltages, especially under burden.

So with passive balancing, which seems not be be in effect with your bms (settings?) nothing to worry. Just not ride tge batteries too empty and/or don't burden too much at lower charge states. Maybe check from time to time single cell group voltages at low charge states.

Otherwise, with active balancing in your case i'd suggest one just stops charging after the charger was cut off (turned green) the first time. Then active balancing uses the charge of the 4.2V cell to charge the other 15 cells and all end up with ~4.16V.

During discharge and (better) every break the cells get balanced again (once their voltages differ more than some threshold presumably).

So this would be the "absolute carefree package" were one can, if one wants to and needed range allows, keep the capacity most of the time roughly in the 30%-80% state of charge range and have the batteries healthy and happy for generations... 

As active balancing happens all the time there should be no worry to get the weakest cell group at too low voltage.

Just in your case with the 11p 3.5Ah config the active balancing could be a bit "weak". So if one discharges without break the battery one could still get the weakest group to too low voltages (under burden) before active balancing could recharge this weak group...

Best to watch from time to time the single cell voltages.

So - everything could/should be fine with your pack? Just watch it a bit closer to "understand" it a bit better and treat it right

Still, imho passive _and_ active balancing together makes no sense.. Seems counterproductive and a cause of more cell stress than each technique used seperately.

Edited October 31, 2022 by Chriull

[Chriull]

As you have an 

On 10/29/2022 at 9:32 PM, superox said:

e-scooter

using 

 

On 10/29/2022 at 9:32 PM, superox said:

active

 

On 10/29/2022 at 9:32 PM, superox said:

balancing

which is not used by any EUC by now i split your post here into this new topic.

@superoxpleae don't hesitate to change the topic title to your liking.

[superox]

12 hours ago, Chriull said:

Still, imho passive _and_ active balancing together makes no sense.. Seems counterproductive and a cause of more cell stress than each technique used seperately.

Yeah, I was maybe confused about that. Anyhow it's quite a featuresome ANT BMS, one of those I believe.

The battery works fine other than this issue. I'm just curious as for how can one group be consistently higher than the rest. If it had less voltage, then sure, that'd mean some cells there were busted, but more?

Using 3.5A because the scooter only has one charge port, and charging inside my apartment I'd prefer a fanless charger, and the fastest 60V fanless charger I could find was the 3.5A one by XVE. Come to think of it, I have a similar looking XVE faster fanless charger for my KS-16X (at 2.5A because of 84V), and it always goes green quite a bit later than the standard 1.5A one, which only seems to charge to 83V. So maybe it's partially down to aggressive charger?

[Chriull]

10 hours ago, superox said:

Yeah, I was maybe confused about that. Anyhow it's quite a featuresome ANT BMS, one of those I believe.

Crazy site - much written and nothing said.

My guess by now would be that it has normal passive balancing and the settings are off and causing this behaviour...

But they should have an app, so everything can be looked at?