Believe it or not, I have seen (measured) cell drift between cells that were connected to nothing and at the same temp, sitting idle next to each other in my garage, only attaching a DVM from time to time to measure them and I saw about a 2:1 spread in their self-discharge rate.
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: Paul Dove [mailto:dov...@bellsouth.net] Sent: Tuesday, August 04, 2015 3:02 PM To: Cor van de Water; Electric Vehicle Discussion List Subject: Re: [EVDL] EVLN: Offset Supercharging degradation w/ pack balancing 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 <ev@lists.evdl.org> 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 <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' > > > > > _______________________________________________ > 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/fa > bf > 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/20150804/20 > e3 > 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)
