I don't believe cell drift exist either. If they are unbalanced it is because there is a parasitic load on the cells that are unbalanced.
Sent from my iPhone > On Aug 4, 2015, at 4:49 PM, Cor van de Water via EV <[email protected]> wrote: > > 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 <[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 use, disclosure, distribution, or copying of any part of > this message is 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' > > > > > _______________________________________________ > UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub > http://lists.evdl.org/listinfo.cgi/ev-evdl.org > For EV drag racing discussion, please use NEDRA > (http://groups.yahoo.com/group/NEDRA) > -------------- next part -------------- > An HTML attachment was scrubbed... > URL: > <http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20150804/fabf > c5fd/attachment.htm> > > > _______________________________________________ > UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub > http://lists.evdl.org/listinfo.cgi/ev-evdl.org > For EV drag racing discussion, please use NEDRA > (http://groups.yahoo.com/group/NEDRA) > > _______________________________________________ > UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub > http://lists.evdl.org/listinfo.cgi/ev-evdl.org > For EV drag racing discussion, please use NEDRA > (http://groups.yahoo.com/group/NEDRA) > > > > -------------- next part -------------- > An HTML attachment was scrubbed... > URL: > <http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20150804/20e3 > 49b2/attachment.htm> > _______________________________________________ > UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub > http://lists.evdl.org/listinfo.cgi/ev-evdl.org > For EV drag racing discussion, please use NEDRA > (http://groups.yahoo.com/group/NEDRA) > > > _______________________________________________ > UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub > http://lists.evdl.org/listinfo.cgi/ev-evdl.org > For EV drag racing discussion, please use NEDRA > (http://groups.yahoo.com/group/NEDRA) > > _______________________________________________ > UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub > http://lists.evdl.org/listinfo.cgi/ev-evdl.org > For EV drag racing discussion, please use NEDRA > (http://groups.yahoo.com/group/NEDRA) > _______________________________________________ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org For EV drag racing discussion, please use NEDRA (http://groups.yahoo.com/group/NEDRA)
