Hi Paul,
Thanks for putting a name to the message.
I have a bit of experience with Lithiums and the unbalance in (self)
discharge,
so I am very much in favor of using a BMS to balance cells, even if this
is the
possibly not entirely perfect matching them in max charge voltage.
For example, I have a battery operated stage light (American DJ QA-5
bar)
that has a 6Ah 24V battery to run it approx 6 hours long at full power.
I bought one second hand and its battery cut out after 1.5 hours while
displaying
that it should still have about 70% capacity left. What happened?
Simple - from the 6 series sets of 3 parallel cells, one was unbalanced
so much
that after delivering approx 30% of the expected capacity, its voltage
fell to 2.5V
and the BMS opened the output to prevent over-discharge.
I tried to recharge and cycle it several times and it seemed to gain a
little bit
but not significantly, so I wanted to know if this was a case of 2
failed cells
in the 3 parallel configuration or a badly discharged set.
I opened the pack and found that I could put 4Ah into the low cells to
bring them up
to the same 4.1V that the other cells were after a charge. Then the
battery was
working again as specified, running the light close to 6h at full power.
So, having a BMS that balances (even if at top voltage) and turns the
voltage off
when a cell threatens to go outside voltage spec, saves the pack and
avoids
fiery disasters and it might even prevent the extreme unbalance in the
first place,
because this pack was not limited by the weakest cell, it was limited by
the unbalance.
Hope this gives some insight.
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
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
proprietary information of Proxim Wireless Corporation. If you received
this message in error, please delete it and notify the sender. Any
unauthorized use, disclosure, distribution, or copying of any part of
this message is prohibited.
-----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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