BMS Questions

[tmho]

Recently, after changing the BMS of my 10S4P battery pack, I found that the voltage of group 7 will gradually decrease (much more when compared with other groups) even if the battery is not being used. When I disconnect the balancing connector, then the voltage of group 7 will stay just like the other groups. Will this mean the BMS is drawing current from only group 7 which is a bad design?
Will there be a BMS which will draw current from all group of cell (not just some or one group of cell)? And if yes, how can I know it is from looking at the BMS?

 

[john61ct]

If you are talking cheap BMS then you need to test, can't trust the seller's assurances.

Use a good quality DC ammeter accurate for low currents like Fluke.

For me the ideal is being able to easily remove / replace the BMS, so store the cells 100% isolated.

 

[amberwolf]

THe BMS is probably damaged. That group's balancer is probably stuck on--not an uncommon failure mode, based on damaged-pack troubleshooting threads over the years.

Easy to check if you can use a current-meter between the cell group and the BMS sense line. If the current never changes whether the BMS is balancing that cell (with it at a higher voltage than others) or not (with it at a lower voltage than others), then the balancer is simply stuck on. If the current changes then there is a parasitic currnet draw that is the constant, and balancing current is on top of that.


Regarding which BMSs are powered from the whole pack, the only certain way to know is to check current draws on each cell sense line, which should all be zero except when balancing. Only the main pack positive, or the sense line for the positive end of the most positive cell, should have current on it (except for main pack negative).

 

[tmho]

Thanks for your replies.

Do you mean BMS should draw current from the whole battery pack instead of from just certain group of cells, except while balancing? And in my case, the BMS is damaged.

 

[amberwolf]

Do you mean BMS should draw current from the whole battery pack instead of from just certain group of cells, except while balancing?

Depends on teh BMS design...but if it doesn't draw from the whole pack (at the main positive) it will draw from *all* of the cells below whatever point it starts--not just a single cell (unless it is only the most negative cell).

Meaning, if it's not running off the whole pack you might detect current thru just one of the sense wires (the most positive of the set it runs on), but *all* of the cells more negative than that cell will be dropping in voltage by about the same amount, because the whole set of groups between that cell and battery negative are all supplying power to the BMS.

So since yours is just group 7 changing voltage, it's not running off group 7 and the rest of the cells more negative than that, so it is almost certainly running off the whole pack.