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 <[email protected]>
To: Electric Vehicle Discussion List <[email protected]>
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
proprietary information of Proxim Wireless Corporation. If you received this
message in error, please delete it and notify the sender. Any unauthorized
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prohibited.
-----Original Message-----
From: EV [mailto:[email protected]] 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" <[email protected]>
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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