Hey Mark,
I know that seems like sounds reasonable to most people. However, a battery is 
chemical and while you can measure resistance with a meter a battery has no 
resistance in the electrical sense. What appears as resistance in a Lithium Ion 
cell is really the diffusion rate of Li Ions into the graphite structure. Ohm's 
law does not apply inside a battery. You are pushing Ions into a crystal 
structure and as it gets full the cell transfers some of this energy into heat. 
So holding a battery at the set point after it is full just heats up the cell.




      From: Mark Grasser via EV <ev@lists.evdl.org>
 To: 'Electric Vehicle Discussion List' <ev@lists.evdl.org> 
 Sent: Tuesday, August 4, 2015 3:37 PM
 Subject: Re: [EVDL] EVLN: Offset Supercharging degradation w/ pack balancing
   
To be simple it is not all about just the fact that all cells get the same 
amount of current, thus a BMS is not needed. If this were true then the 6 cells 
in a FLA would not go into horible unbalance as they do.

So to be simple, as I am, it also includes internal resistance, then by math, 
volts and watts. The differneces in internal resistance will directly affect 
the cells ability to accept charge. This will directly affect balance. Please 
understand that the internal resistance during discharge will not be the same 
as during recharge.

One for the non balancing side, if you only charge to 80% and only discharge to 
30% you might never need to balance. MANY of my boating customers do this as it 
fits their needs, they simply charge underway when needed. EVs are a different 
story. Here some of us want to charge as close to 100% as possible and 
discharge as low as they can. In this instance it is important to balance as 
going to far over voltage or going negative in discharge is obviously not a 
good thing.

To say that simply because the current is the same through the entire bank of 
batteries is proof thay the cells can not unbalance, is incorrect.

Mark Grasser


 



Subject: Re: [EVDL] EVLN: Offset Supercharging degradation w/ pack balancing

Hello Cor van de Water,
My name is Paul Dove, thus dovepa.  I don't know you either. Just knowing your 
name or where you work does not make me know you.
It is not really relevant who uses a BMS. People used to use x-rays to measure 
their shoe size and it had a viewer so you could see your feet x-rayed real 
time. They figured out this was dangerous eventually but non-the-less everyone 
was doing it.
Now, with everyone is doing it argument out of the way......
The CC/CV charging methodology was arrived at by experimentation. Researchers 
were trying to determine the method that would put the most capacity in the 
cell. They tried multiple CV values and recommend the value that put the most 
into the cell.
Actual capacity has to be calculated by current times time. It cannot be 
determined by voltage. The voltage set point is part of a procedure to maximize 
the energy in the cell. 

In series the current is the same through all the cells. A balancer will 
attempt to shunt part of this current off of the cell to let other cells come 
up to the CV value while keeping this cell from exceeding the CV value. In my 
opinion, having not analyzed every BMS out there, a BMS would need to shunt 
enough energy to keep the voltage from rising above this voltage set point or 
have the ability to command the charger to lower the current. 

So the balancer would have to hold cells individually at the CV level by 
reducing current into each cell independently.

Now, even if it can achieve this feat according to the manufacturers 
recommended cc/cv procedure this does not balance the cells. Ok, maybe it's a 
semantics issue on the word balanced. What is balanced? All cells at the same 
voltage? All cells at the same capacity? I think the goal is to have maximum 
capacity not balancing.
That aside, do you actually have more capacity? If so, how much more?
When you discharge the car will cut off when the lowest capacity cell reaches 
it's cutoff voltage so by holding it at the set point voltage while the others 
fill up didn't gain any pack capacity advantage sine you still cutoff based on 
the lowest capacity cell. You pack size is the size of your lowest or weakest 
cell. No way around that. 

In addition, the amount of energy put into the cell during the constant current 
part of the charge is less than 5%.
So on 70Ah cells we are talking 3 miles range. That's if you cut off when the 
first cell reaches the set point voltage. So if you took the current down to 
C/20 based on the lowest capacity cell you would mt likely loose no capacity or 
if I'm wrong possibly 1% or less than a mile range.
The whole concept of balancing comes from other battery technology such as Lead 
Acid cells where it makes much more sense. You want all the batteries to boil 
to get maximum capacity.
I have no problem with a BMS to monitor individual cell voltage an temperature 
and controlling charge based on this.  My arguments are against shunt balancer 
circuits.
Both their effectiveness and the necessity.

      From: Cor van de Water via EV <ev@lists.evdl.org>
 To: Electric Vehicle Discussion List <ev@lists.evdl.org>
 Sent: Tuesday, August 4, 2015 10:48 AM
 Subject: Re: [EVDL] EVLN: Offset Supercharging degradation w/ pack balancing
  
Hello "dovepa at bellsouth.net" AKA "via EV", I have no idea who you are - the 
message is not signed and your name is not showing from your email.
You can believe what you want about Lithiums, but think about that even the 
Chinese invest in a BMS on every Lithium battery pack that they ship, while 
they are known to cut corners and reduce cost, so there must be a reason....

Cor van de Water
Chief Scientist
Proxim Wireless

office +1 408 383 7626        Skype: cor_van_de_water XoIP  +31 87 784 1130     
   private: cvandewater.info www.proxim.com


This email message (including any attachments) contains confidential and 
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-----Original Message-----
From: EV [mailto:ev-boun...@lists.evdl.org] On Behalf Of via EV
Sent: Tuesday, August 04, 2015 5:03 AM
To: Electric Vehicle Discussion List; Bill Dube
Subject: Re: [EVDL] EVLN: Offset Supercharging degradation w/ pack balancing

It's possible he has a defective battery. Also, I still don't believe balances 
work on Lithium cells.

Sent from Outlook




On Mon, Aug 3, 2015 at 9:40 PM -0700, "Bill Dube via EV" <ev@lists.evdl.org> 
wrote:










There are too many variables to draw any strong conclusions. The two biggest 
factors are:

1) The BMS does not have as much time to balance during a fast charge.
2) The cell temperature is typically higher (for many reasons) when you fast 
charge. The cells don't like high temperatures.

"Stale charge" is also large factor in apparent capacity change and happens in 
all chemistries to varying degrees. It may be a factor in these "tests" on 
battery pack capacity. (In nicads it can be particularly a large "stale charge" 
effect and is commonly called "memory effect".) Essentially, when you _gently_ 
and _fully_ cycle a battery, the apparent capacity becomes much greater after 
the first full cycle, and often grows a bit more with the second full cycle.

The longer it has been since you last accessed the full capacity of the 
battery, the worse the problem of "stale charge" becomes.

Batteries are very complicated chemical beasts. Simple tests often don't tell 
you the full story.

Bill Dube'




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