Balancing lithium-ion batteries

[scootergrisen]

This is my balancing adventure.

Have been balancing my two 48V25Ah lithium-ion batteries for my electric scooter/e-bike for days now trying to get all the cell series to be the same voltage.

The BMS is using passive balancing.
It discharges the cell series that has the highest voltage in order to bring the voltage down closer to the cell series with the lowest voltage.
The balancing/discharging is quiet slow.
It stops balancering when the cell series reaches 4,00 volt.
It does not appear that charging starts automatically so i had to turn the charger off and on again to start the charging again.
And then when the charging is done it starts all over again.

Seems like that is taking one week to do for one of the batteries.
The batteries are like 3½ years old now.

The BMS has some nice LED's that flashes to show the balancing.
But you can't see them untill you take the battery apart.
So I want to find a way to figure out if the battery is balanced or not from the outside without taking the battery apart.
Seems like charging is cut off at a higher current (earlier) when the battery is not balanced. Then when better balanced it seems the charging current slowly gets lower and lower.

Also it seems that the battery can both charge and discharger (balancing) at the same time.

I have pretty much been thinking that when there is no current between the charger and battery that the battery is completly done. But turns out it can still be doing balancing for hours. Like many hours. And after the balancing i can accept some more charging but have to turn the charger off and on for the charging to start.

I suppose i can let the battery rest for an hour after it stops accepting any more current from the charger and then measure the voltage of the entire battery and maybe i can use that to know if the cells are balanced or not.
So if the BMS stops charging at 4.150 volt and i got 13 cell series then I should be able to measure 53.95 volt. If everything is ideal.

Well that was my biggest balancing adventure.

Pictures of the batteries and charger:
https://scootergrisen.org/forum/viewtopic.php?f=19&t=3612
https://scootergrisen.org/forum/viewtopic.php?f=19&t=3601

 

[amberwolf]

To know if they are balanced to the same voltage*** you have to actually measure the individual group voltages. You can't tell from any other measurement external to the pack.

If you want to be able to do this without opening the battery each time you measure, you first have to open the battery and add a second set of balance wires to measure from, parallel to the ones that go to the BMS. If you are only using it to measure from, a connector like a panel-mount female DB-15 works reasonably well, because the female connector has contacts with holes just about big enough for small meter leads to directly stick into them, and you can get covers that can be secured over them to keep stuff out of them (though not really waterproof, they'll keep from shorting out the contacts with debris, at least). Get one prewired with wires long enough to reach your cells so you can solder them one at a time next to each existing balance wire solder connection.





***which doesn't mean they are the same capacity or capability; unbalanced cells by nature tend to happen once they are *not* the same anymore, and get worse over time. Which is one use the added balance connector has, to keep an eye on this and to manually rebalance them without taking the battery apart once they get so bad the BMS can't fix them anymore in any reasonable time.

 

[scootergrisen]

There is this battery tester to communicate with the BMS via RS485 and read things like cell voltage but I dont have it.
I would like to have it but i would also like to know how i could communicate between my computer and the BMS and what software to use to read the data from the BMS.
Would be smart to be able to see the data live on my computer while the battery is charging perhaps in another room.

 

[eMark]

The BMS is using passive balancing.
It discharges the cell series that has the highest voltage in order to bring the voltage down closer to the cell series with the lowest voltage.
The balancing/discharging is quiet slow.
It stops balancering when the cell series reaches 4,00 volt.
It does not appear that charging starts automatically so i had to turn the charger off and on again to start the charging again.
And then when the charging is done it starts all over again.

Seems like that is taking one week to do for one of the batteries.
The batteries are like 3½ years old now.

DIY builders from the GET-GO would be better-off going with a so-called 'SMART' BMS. Balancing an older pack can take hours and hours depending on degree of variance. Eventually balancing the 13 parallel groups from say a variance of 0.077V (4.123V to 4.200V) to 0.005V (4.195V to 4.200V).

With a DIY build that's 3+ years old (depending on cell quality) using another set of balance leads accessible from outside the protective pack wrap may be the way to go. However, if even just one of the cells in a 13S battery is so inferior it may not be possible to manually balance the variance of that p-group closer than say 100mV indicating near EoL or replacing inferior cell(s).