Bottom balancing?

[bobc]

I guess the different battery types, chemistries, constructions, manufacturers etc will all have slightly differing characteristics - the data above is a little hobbyking lipo brick, totally raw data, and you can see the Rs is about 165mohm when the battery's full and reduces to less than half that when it empties.
And yes, any BMS that looks at each individual cell can prevent over-discharge and overcharge. The claimed advantage of the bottom balanced stack is that it is considerably (if not completely) less able to destroy itself (by cells with some juice left reverse charging the flat ones in series), because they all go flat together.
This actually suits one of my projects, and sounds like a sensible approach if you're going "no BMS". But for general ebiking a classical BMS is the easy solution to live with; and it is electrically cheaper and easier to make a dissipative top balancer.
Somebody going "no BMS" could very easily make a bottom balancing circuit to plug into each cell individually on the balance lead (just 2 pins at 0.1") and discharge it at 2A down to (say) 3.000V. This would be easier and safer than the OP's suggestion of unwiring the whole battery stack and placing every cell in parallel....

 

[Punx0r]

that almost by default leaves bottom balance as the only safe option.

Unfortunately this statement flies in the face of the opinion of (AFAIK) every single OEM cell, pack and system manufacturer going. That should act as a warning flag...

 

[Gregory]

I - the data above is a little hobbyking lipo brick, totally raw data, and you can see the Rs is about 165mohm when the battery's full and reduces to less than half that when it empties.

What data above are you referring to? Not the V/A/Speed graph?

 

[bobc]

I - the data above is a little hobbyking lipo brick, totally raw data, and you can see the Rs is about 165mohm when the battery's full and reduces to less than half that when it empties.

What data above are you referring to? Not the V/A/Speed graph?

Yeah that one - you can see the volts dip when 45A are drawn; at the discharge end it only dips half as much.

It's a minor issue because I like to compensate for that series resistance in the volts display (so the display becomes invariant with load). The LiPo above is not possible to compensate simply, in fact our lead acid batteries are better in this regard, smaller variation with SOC and in the other direction.
The compensated voltage is a attractive value to use for LVC and possibly HV alarm detections too.

ahhh - I see what you're saying - 165mohm is a typo, it should be ~65mohm...

 

[bobc]

that almost by default leaves bottom balance as the only safe option.

Unfortunately this statement flies in the face of the opinion of (AFAIK) every single OEM cell, pack and system manufacturer going. That should act as a warning flag...

My bad, I was looking at it from the narrow perspective of a particular current project; later on you'll note that I opined that in a general ebike case the traditional top balance BMS is an excellent and cost effective solution

But for general ebiking a classical BMS is the easy solution to live with; and it is electrically cheaper and easier to make a dissipative top balancer.