How Do You Balance.

[Craig]

I'm wondering what peoples thoughts are on how active and passive balancing works. When do you do it and how many amps do you need to move to be practical. When I ask when. Is it while charging, discharging, resting, also at what voltage range do you start. And how far difference are you comfortable with.

I may come back with some strong questions depending on what people say but I'm hoping we can all learn a thing or 2.

 

[FilterGuy]

I'm wondering what peoples thoughts are on how active and passive balancing works. When do you do it and how many amps do you need to move to be practical. When I ask when. Is it while charging, discharging, resting, also at what voltage range do you start. And how far difference are you comfortable with.

Thanks for asking the question!! My understanding (and opinions) on this is 'evolving' as I learn more. Hearing/learning from others would be good.

Here are the answers I would give right now, but I probably could not defend all of them:

What voltage range do you start: I don't have a specific number, but I would guess someplace in the upper half of the charge range of the cell.

Is it while charging, discharging, resting: I would say anytime it is in the right voltage range but if I have to pick one I would say charging. (Note: Ballencing while resting is probably best, but if you have an active system powering a small house, it may never be resting, Consequently, you almost have to balance during charge, discharge or both.)

How many amps do you need to move to be practical: This seems very dependent on the use pattern. Big systems probably need more current than small systems and if you want fast balancing you will need more. If you spend a lot of time in the voltage range you have set for balancing, perhaps a few hundred mili-amp would be fine.

Active or passive: Active is nice but for systems that arn't pushing the charge and discharge hard, passive is probably fine.

Bonus Question: Do you need balancing at all? I will always implement it. However, if your top and bottom voltages are set conservatively you might be able to get away without it. If you decide to not have balancing, you are making a contract with yourself to keep an eye on your cell voltages. The penalty clause in the contract is possible battery damage.

I may come back with some strong questions depending on what people say but I'm hoping we can all learn a thing or 2.

Like I said, I may not be able to defend my answers. Some of them are based more on impressions than facts and logic. Since I joined this forum, I have learned that a lot of the things I thought were trivial are actually quite complex.

 

[Maast]

I'm wondering what peoples thoughts are on how active and passive balancing works. When do you do it and how many amps do you need to move to be practical. When I ask when. Is it while charging, discharging, resting, also at what voltage range do you start. And how far difference are you comfortable with.

I may come back with some strong questions depending on what people say but I'm hoping we can all learn a thing or 2.

Trouble is, there are about as many different answers as there are models of BMS's

I'm no expert by any means but I've been researching for over a year and I've learned this in general:

Active - even for big banks 5A is probably the most you'd ever need, and thats if you're correcting for one crappy cell and you go past the 'knees'.
There are two methods of active balancing I'm aware of: One like the QNBBM moves charge around the entire pack as needed regardless of state of charge, the other is it moves charge just between neighboring cells and it the balancing cascades through the pack. (terrible way IMO)

Passive: usually burn off extra charge on weak cells at the top of the state of charge OR the bottom of SOC, a few can be set to balance througout the entire charge cycle.