BMS Balancing and Internal Resistance on mismatched cells

[Shale MacGregor]

Per Overkill BMS, they cannot account for a large difference in Internal Resistance when it comes to balancing.



Is there a general rule of thumb for accounting for differences in Internal Resistance? I have heard so little about this previously but in my first big foray into building a battery pack I am finding this to be more and more of a concern. Basically a good BMS will stop mismatched cells from over/under discharging at the cost of capacity.

If I have a mixture of higher IR and lower IR cells is it worth matching the higher IR cells into the same 16S packs so the BMS can manage them similarly?

Thanks!

 

[DThames]

The cells charge in series with the same current....not equal but the "same". The BMS in not involve in charging, but only shuts down the charge input based on cell voltage and protection settings. Some imbalance is normal and can't be avoided. If the cells of the same capacity, rating, and manufacture, I wouldn't bother much with trying to match them unless they are used and you know the capacity varies a lot. Because the charge with the same current, capacity is the real tell on what happens with the balance. If the cells of different design or Mfg, I would indeed sort them, if possible...expecting different capacity between Mfgs. Opinion of course.

 

[Shale MacGregor]

The cells charge in series with the same current....not equal but the "same". The BMS in not involve in charging, but only shuts down the charge input based on cell voltage and protection settings. Some imbalance is normal and can't be avoided. If the cells of the same capacity, rating, and manufacture, I wouldn't bother much with trying to match them unless they are used and you know the capacity varies a lot. Because the charge with the same current, capacity is the real tell on what happens with the balance. If the cells of different design or Mfg, I would indeed sort them, if possible...expecting different capacity between Mfgs. Opinion of course.

I thought the BMS helped balance / dissipate some, and that this would account for slight imbalance in terms of IR (not capacity, since capacity could be the same but never able to be reached due to taking longer to charge due to higher resistance and more energy being converted to heat rather than stored?