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Ninebot One S2 - BMS - Balancing?


UniCycler
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Hi everyone!

I searched the forum about information regarding the BMS of the Ninebot One S2 and found the following statements:

  • It is a "smart" BMS
  • Balancing happens at the end of the charging process when the charger indicates green light and is very slow
  • Overnight charging should help to imbalance the cells
  • Balancing current is about 50 mA because the BMS circuit uses only internal capabilities on the IC and has no external FETs
  • This BMS should be the best of the NB One series (A-S).

Can anyone confirm this?

At the moment I have the impression that the BMS doesn´t balance at all. My worst battery pack has an imbalance of 150mV, the other one 120 mV and the available capacity should be more than the cells could have when they would be new (checked with EUC World app). The wheel is 3 years old but has only 20 full charging cycles. I tried long  time charging in the green light phase but without any improvements. My charger has 62.5V on the output and I´m asking myself how balancing could work when the available voltage is not higher than 63V (15 * 4.2V) at least for passive balancing.

Would be grateful for your experiences and knowledge about this BMS. 

Thanks!

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14 hours ago, UniCycler said:

I searched the forum about information regarding the BMS of the Ninebot One S2 and found the following statements:

  • It is a "smart" BMS

It measures and reports single cell voltages and it does afair coloumb counting (showing the "real" mAh of the cells/battery)?

14 hours ago, UniCycler said:
  • Balancing happens at the end of the charging process when the charger indicates green light and is very slow

Yes at the end - about once charging changes from constant current to constant voltage stage.

The charger indicates green light once the charging current falls below some threshold - which should indicate about charging ended. some percent of C of the battery. Newer 21700 cells state some 60 mA final charge current threshold. So with 2 cells in parallel something between 100-150mA could be this threshold.

For more details about passive top balancing:

14 hours ago, UniCycler said:
  • Overnight charging should help to imbalance the cells

The longer one let's the battery on the charger, the "less" happens.

Once one cell reaches the single cell overvoltage limit the BMS cuts off and charging is stopped.

... until this cell is enough discharged by the balancing resistor so the BMS enables charging again ...

14 hours ago, UniCycler said:
  • Balancing current is about 50 mA because the BMS circuit uses only internal capabilities on the IC and has no external FETs

Seems so

But i've nowhere seen a picture of the backside of the BMS pcb... Maybe because there is nothing or all the mosfets are hiding there? ;)

14 hours ago, UniCycler said:

At the moment I have the impression that the BMS doesn´t balance at all. My worst battery pack has an imbalance of 150mV, the other one 120 mV and the available capacity should be more than the cells could have when they would be new (checked with EUC World app). The wheel is 3 years old but has only 20 full charging cycles. I tried long  time charging in the green light phase but without any improvements.

Best way for "heavy duty balancing" would be to fully charge until green lights come up, let the wheel "rest" for 1-2 hours, discharge it a little bit. Repeat this three steps ...

14 hours ago, UniCycler said:

My charger has 62.5V on the output and I´m asking myself how balancing could work when the available voltage is not higher than 63V (15 * 4.2V) at least for passive balancing.

If the voltage threshold of the BMS controller IC is programmed to 4.2V it will need some "more imbalance" for balancing to start...

But this BMS could have a firmware adopted to the 62.5V charger.

 

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Thanks for your reply. I will try the heavy duty balancing advice but during the last 2 weeks I´ve driven mostly short range trips (5-10km) each day and recharged thereafter everytime to green light, which brought absolutely no change in imbalance. Maybe the cells are already so bad that the available balancing current is not sufficient. My longest trip during these days was 22km on a flat terrain with 30% battery left. I guess the batteries couldn't be that bad yet. The BMS says battery health 98% via EUC World app. Perhaps I will try next time to drive until the BMS stops me but I don´t like to fully discharge the cells. But it could even be that it helps the BMS to recalibrate itself from time to time, at least for the capacity calculation of the cells which is in my case wrong as stated in my post before.

I wonder why all EUC manufacturers seem to neglect this topic and only implement very cheap or even no solutions (V12?) for balancing. This shows me that the EUC marktet doesn´t seem to be ready for mass market yet and is just trying to make fast money. My 8 year old eBike from a German manufacturer (ZEG/Bulls) has an excellent BMS and even after 250 full cycles the imbalance is not higher than 10mV. 

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@UniCycler some imbalance seems "ok", especially as there is no notable performance/range decline.

Full discharge, especially under burden will stress the weakest cells even more. Although from time to time this could help the coloumb counter to adjust this has much less priority than stressing cells for no real reason! No one really needs the capacity display - intact cell voltages are the primary value.

Edited by Chriull
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Posted (edited)

Meanwhile I´ve read a few about balancing strategies and it seems that the BMS of the Ninebot One S2 uses "bottom-balancing", which makes sense for a smart BMS that counts coulombs. That is presumably the reason why only the weakest cells are charged to 4.2V. The cells with lower voltages at the end of charging (green light) have more capacity than the average amount of 2850mAh. I can confirm from my own observations with EUC world app that the imbalance of my battery packs are getting lower when im reaching lower SOC (state of charge) values. My lowest SOC was 30% until now. The imbalance decreased from 150mV (at 100%) to 70mV (at 30%). I assume that at 0% the imbalance reaches 0mV. The variation in cell capacity explains why most guys here reported a higher battery capacity than the design capacity. But I guess that this is only the case for small amount of cycles (approx. lower than 100). Thereafter the actual capacity will decrease below design capacity. 

For me it seems that Ninebot was during these early days of EUC development far more ahead than the manufacturer nowadays or they cared more about longevity and security of their products. It's really a pity that they have withdrawn from the market.

Edited by UniCycler
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Posted (edited)

Sorry guys, my assumption from the previous post was wrong and Ninebot not as smart as expected in the early days of EUC development. The used AFE chip in the BMS (BQ76940) is theoretically only able to handle 5mA balancing current internally but with the cells connected via 1kOhm resistors to the AFE the max balancing current could not be more than 2 mAh. This would be so little that I doubt that any balancing algorithm at all is implemented in the battery packs of the Ninebot One S2. 

The only way to balance the pack is to open it and do it manually. How sad... :-(

 

Edited by UniCycler
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10 hours ago, UniCycler said:

(BQ76940)

Which does passive top balancing.

10 hours ago, UniCycler said:

The only way to ballance the pack is to open it and do it manually. How sad... :-(

It's not as bad - if one knows what to do it'll work out:

 

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Posted (edited)

Luckily opening the battery packs was not as hard as expected and could be done easily with a cheap smartphone opening tool without any damage. Manually balancing has worked as well and now my packs have less than 20 mV imbalance which brings me about 3 km additional range. Still wondering why Ninebot decided not to use at least the weak balancing possibilites of the BQ76940. Even 2 mAh would be better than nothing and could reduce an imbalance of 1 mV in about 1 hour. Maybe they wanted to sell replacement packs...?

Edited by UniCycler
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