Does a BMS unbalance a pack...?

[Darren2018]

Does a BMS actually unbalance a pack via it's own resistance? Let's say you had a perfectly balanced pack and then you added a BMS in series with it would the cell group that connected to the BMS become unbalanced? Also can someone who has actually built a perfectly balanced pack confirm if this procedure will work to build a perfect pack,

charge all the individual cells to exactly 4.2V and let them rest for a week or two then group them in relation to their self discharge

discharge each cell at the max load it will see when actually in the pack and log it's Ah's/Wh's/DCIR (repackr.com looks ideal)

arrange the cells into parallel groups with as little divergence as possible from every other group

weld them all together using equal lengths of copper to keep the resistance as close as possible between the groups

add in the BMS

load the pack with the max operating current and let it get to a reasonable temp similar to what it will see when in use and note the voltage drop in each group

if needed solder in some extra copper to which ever groups need it


I have one of those smart BMS's from GBK that you can calibrate yourself along with a reasonably accurate 6000 count meter but I was wondering if the BMS would be accurate enough to let the pack stay balanced (without balancing via the resistors) or would the software/balance IC gradually unbalance the pack due to it's own inaccuracies.

I'm still learning and I know this is not common procedure for building a battery but I am interested as this is going to be my first build and I really want to make it as good as possible.

 

[billvon]

Does a BMS actually unbalance a pack via it's own resistance? Let's say you had a perfectly balanced pack and then you added a BMS in series with it would the cell group that connected to the BMS become unbalanced?

It can, if the different taps to each cell aren't balanced in their current draw. Most BMSes do a good job in keeping current draw almost identical across cells.

discharge each cell at the max load it will see when actually in the pack and log it's Ah's/Wh's/DCIR
arrange the cells into parallel groups with as little divergence as possible from every other group

Yes, you can try that. However, often you can only go so far. A cell's capacity, for example, might be different at different discharge rates - and self discharge rate (the primary driver of imbalance over time) might not track cell capacity. So at best you can reduce variations in those parameters.

I have one of those smart BMS's from GBK that you can calibrate yourself along with a reasonably accurate 6000 count meter but I was wondering if the BMS would be accurate enough to let the pack stay balanced (without balancing via the resistors) or would the software/balance IC gradually unbalance the pack due to it's own inaccuracies.

Depends on how the balancer works.

If it's a charge transfer type (rare) then you always move closer to voltage balance.
If it's a high voltage discharge type then you balance to within the accuracy of the reference on each cell. (Usually pretty accurate)

In general, a high voltage discharger will not activate at all unless voltage hits the limits. So if you have a perfectly balanced 10 cell pack, and you charge at 41 volts, the balancers will never turn on.

 

[Darren2018]

I'll edit my post to include your ideas as well.