Active Balancer: Avoid low charge?

[pralinebuckets]

I have just hooked up a JK active balancer to my 48v 16s battery pack, and am reading a bit more on them here. When I set the pack up, I top balanced, but there is definitely some cell voltage drift since then. What I'm curious about is the following quote I've read a few times in this forum:

If you run an active balancer on cells that get down to 3.0V or below you may have some wider imbalances, and the active balancer running all the time will attempt to correct that imbalance. Doing so at the bottom can can undo a top balance, at least over time.

Can someone explain to me how or why that's the case? I had assumed that balanced is balanced is balanced. Why would balancing at the lower end of the charge curve undo balance at the high end?

 

[toms]

I have just hooked up a JK active balancer to my 48v 16s battery pack, and am reading a bit more on them here. When I set the pack up, I top balanced, but there is definitely some cell voltage drift since then. What I'm curious about is the following quote I've read a few times in this forum:


Can someone explain to me how or why that's the case? I had assumed that balanced is balanced is balanced. Why would balancing at the lower end of the charge curve undo balance at the high end?

The weaker cell in your pack has lower capacity than the others. This manifests by reaching the highest AND lowest voltages first during a complete cycle.

If you add energy into this cell at the lowest SOC, you will then have to remove this energy at the highest SOC.

This may be desirable if you intend on using every available bit of juice from your battery. Otherwise it’s simpler to keep the balance at the top, and not approach the low voltage knee of the weakest cell.

 

[Ampster]

Why would balancing at the lower end of the charge curve undo balance at the high end?

I had it explained to me one time graphically that made sense. I will try to paraphrase. Unless one buys perfectly matched cells there is a small difference in capacity in each cell. If one views capacity as the length of a chopstick imagine that when you hold them in your hand and tap them so they are all even at the bottom then there will be some that are taller at the top of the stack by a small amount. That is analogous to bottom balancing. If you wanted to balance that stack of sticks at the top you would tap them all from the top until they were even at the top but because they had different capacity (length) they will be uneven at the bottom.

 

[rhino]

 

[pralinebuckets]

Thanks all - makes sense now!