BMS Balancing & Non Balancing??

[tongxinpete]

Can anyone tell me the difference between these BMS, first one says no balance, but lists cell max & min voltages etc, seems to do all it should?
https://www.aliexpress.com/item/8S-15A-25-6V-lifepo4-battery-BMS-PCM-battery-protection-board-bms-pcm-for-lifepo4-battery/32778363615.html?spm=2114.01010208.0.0.Utrjqh
This one is balancing
https://www.aliexpress.com/item/8S-15A-LiFePO4-BMS-PCM-PCB-battery-protection-board-for-8-Packs-18650-Battery-Cell-w/32712364286.html?spm=2114.01010208.0.0.91y1KU
Cannot work out the purpose of the first unless it uses shunts to bypass individual cells once they reach max safe volts, allowing others to catch up. Found on here affordable pouch cell to cure my problem, so more of academic interest. On Aliexpress I have not found a better deal on motors than BMSB inc shipping but worth looking at for cells

 

[Deleted member 4366]

Both have high voltage and low voltage cut off and they both have high current protection. Only the second one has balancing.

 

[Nealh]

On the first one it will cease charging once it detects any cell at the hvc, just means an unbalanced pack at some stage.

 

[dgncsk]

are there BMS circuitry which can't balance packs while charging? if so why do we need BMS then? just plug the pack and let controller cut the power when batteyr is low.

 

[Deleted member 4366]

Yes, there are some but they tend to be more expensive. There's also the smart BMS from BMSB and the new Speedict that can balance on demand from a phone app.

 

[dgncsk]

Why do i need a bms without balancing then? I was hoping that my aliexpress bms could take care of charging cells individually.

 

[Deleted member 4366]

Some cells stay in balance. Personally, I wouldn't use a BMS without the balance function.

 

[tongxinpete]

So the first just ceases all pack charging on any one cell reaching hvc if I understand correctly, & likewise discharging if any one cell reaches lvc, it seems pretty pointless not to let the other 'laggard' cells catch up & equalize them! I do not really understand why it even exists.
Do both shunting & smart BMS balance cells across a series stack? it appears the first one is neither of these? I presume that charge current is just wasted via shunt resistors on the shunting type, but again are these shunting type simpler to make or less prone to failure than the smart type?
The no balance bit did rightly ring alarm bells even to me! Thanks.
PS - what is a 'bleeding' BMS?

 

[Deleted member 4366]

A bleeding BMS is a balancing one. When any cell is over the bleed level (about 4.17v) a gate opens up a path through a resistor to ground, so the excess voltage bleeds down, which allows the others to catch up a bit. That's how nearly all BMSs work - like a line of buckets with a small hole drilled 2" from the top. Any that gets filled with more than that level will drain down.

When any cell reaches 4.2v, the charging mosfets are switched off, which closes the gate for charging.