BMSs

Base mechanics. 10s3p battery pack or 10 groups of three cells, in series.

Qustion: How do each of the eleven BMS balance wires or the BMS detect which cell is a problem, or, is it more general, meaning the contact is generally referring to the group, nothing so specific?

So, for example, if a cell or group voltage is below specs, the BMS, shuts the pack, down, machine stalls and one must check all groups with meter...?????

If so, then what is the point of the BMS, aside from popular promotions?

parallel groups - series strings

parallel groups behave

as 1 large cell

wired in series to form a string

to achieve desired pack voltage

bms requires wire connection

to each series string point

which protect the parallel group


example

2p14s describes

2 cells in parallel (capacity Ah/Wh)

wired in 14 series strings (pack voltage)

bobbill said:

Base mechanics. 10s3p battery pack or 10 groups of three cells, in series.

Qustion: How do each of the eleven BMS balance wires or the BMS detect which cell is a problem, or, is it more general, meaning the contact is generally referring to the group, nothing so specific?

So, for example, if a cell or group voltage is below specs, the BMS, shuts the pack, down, machine stalls and one must check all groups with meter...?????

If so, then what is the point of the BMS, aside from popular promotions?

Click to expand...

I think you are OVERTHINKING this. I have 8 or so battery packs and none have ever "stalls" or needed checking with a multimeter.

bobbill said:

Base mechanics. 10s3p battery pack or 10 groups of three cells, in series.

Click to expand...

Hopefully one string of 10 groups, at the 1S level each group 3 in parallel.

For good performance and reliability, you need to start with a top quality make & model, from a supplier trusted to sell only new Grade A from the factory.

The BMS is there to

protect the pack from you, get adjustable LVC so you don't murder them drawing SoC% too low

protect the pack from a bad charger, HVC set just above the charger setpoint

and maybe balance the cell/groups, but usually cheap BMS do that pretty poorly for when the cells are wearing out and getting very unbalanced - make sure the cell-level voltages are visible so you can tell when balancing is done.

A good BMS lets the user adjust its setpoints, disable balancing if you want to bottom balance, get a better dedicated balancer and/or use a balancing charger.

The only way to test individual cells with parallel groups is to atomize the pack, which IMO is the key advantage to building packs using solderless/no-weld gear.

Otherwise a bad cell will reduce performance of its group, and you will notice the group's lower capacity when overseeing the balancing process.

If using second-hand cells, odds increase of a cell actually shorting out, damaging the others

starting with lots more than you need, diligently testing each cell before assembly and recycling the worse ones will help

but as cells get near their EoL point (70-75% SoH) this testing needs to be one regularly, moe frequently

in order to prevent "unexpected events" replace your packs proactively.

Over thinking? Perhaps. But my experience says maybe not. Built two Vruzends, one of which was problem, could not get components (not listed; email requests ignored).

Maybe a single wire shows low current condition of group generally or records unusual events....maybe not?

Same if a large parallel group...say 6 or 8 "plated" parallel group?
One wire detects low/high surge or heat and stops pack function?
Makes sense, but is that what happens?

Seems basic (to me) so answer should be readily available, but not the case.

The above disappeared for a time and sent abbreviated version in meantime...

@ John61ct +1; agree...makes sense to me. All cells are high end Sams or NCR; all top end stuff, even wires.

What we call "a BMS" is really just a collection of functionalities

certainly just a subset of what would be needed for fully automated protection

usually several key factors are just ignored.

Especially true for a BMS costing under $100, compared to one costing $5000, the latter will offer more protection features, be more accurate, more reliable and more adjustable.

An owner can replicate those features selecting components off the shelf known to be accurate and reliable.

Technically "no BMS" but still protected.

A knowledgeable user can go completely bareback just monitoring gear, and act as a "human BMS", maybe most reliable of all in simplicity, but that strategy relies on an individual's attention, memory and diligence.

My point is, learn the details. No BMS is a magic bullet, they have limited functionality, obviously can fail, thus are not to be blindly trusted.

But, however you do it, the cells do need protecting!

John, thanks! I am a newbie who has had some flakey experiences, an wonders about stuff, but simple lsogic seems to work more. Was not attempting to villify BMS use. Their ubiquity speaks for itself. Just needed answers. Someone says "x" which my experience has shown opposites, I need to know why and what I must be missing in my work. All seems to be as expected, thanks to contributions...

I forgot to mention, I believe, that after the pack shut down, my cell examination had cells with voltages that differed by as much as a volt from cell to cell, which sort of told me why BMS shut all down...imbalance...and wondered why.

Still, if say 6 cells are in parallel, and there are a few such groups, and the wire is generally placed in the group, "how do it know?" Of course, it makes sense a group of cells = x in current sum, deviation y = kaput, ? Correct ? which makes sense at this point..."group total."

BMSs are usually designed to maintain balance, but you need to be able monitor their work, cell/group level voltages

keep the delta from getting so great that it takes days of float charging to give them time to finish.

A full volt shows extreme neglect, usually 30-60mV requires a rebalance session.

This only pertains to a string, IOW cells or groups in series

Cells in parallel will always self-balance, stay at the same voltage, act as a single cell.

_______
This is not clear:

bobbill said:

Of course, it makes sense a group of cells = x in current sum, deviation y = kaput, ? Correct ? which makes sense at this point..."group total."

Click to expand...

Your last sentence and the "unclear" comments by moi are really similar...and your comment makes perfect sense to me.

Merci.

Has always seemed (to me) why my pack ran, quit, and cells v all over the place later.

bobbill said:

I forgot to mention, I believe, that after the pack shut down, my cell examination had cells with voltages that differed by as much as a volt from cell to cell, which sort of told me why BMS shut all down...imbalance...and wondered why.

Click to expand...

You told me you never received the barrel bolts with either your V1.6 or V2.1 kit. That explains why the cell voltages differed so much as apparently not all the cells were making contact with the stainless steel spring inserts in the colored caps.

One problem is that even when both ends with the barrel bolt screws are tightened as far as possible the stainless steel contacts are still not fully compressed against the head of the threaded steel stud (at anode can end and cathode end). Thus additional resistance through the stainless steel spring inserts instead of just the thickness of the stainless steel cap contact if it were fully compressed against the head of the threaded steel stud at both ends of a cell. I understand why Micah designed it that way, but it does minimize the advantage of the nickel plated copper bus bars.

bobbill said:

Has always seemed (to me) why my pack ran, quit, and cells v all over the place later.

Click to expand...

Toll only added, IMO, to keep up with the competition...Vruzend has no clue...it is all about your Benjamins...am out and will stay out. I will say Toll's ebook is decent, though somewhat contratictory in places. But, used I-net to figure out...might be fun to craft packs to learn, but more sensible to buy Wallen-made packs and ride, IMO.