I understand what you are saying, Don, but what is called "resistance" in the case of batteries is not the same as the resistance of a resistor. It does have an effective series resistance but it is not what a light bulb or other type of resistor would have. What Peter said is correct. Batteries, especially in the case of LiFePO4 cells, are electron storage devices.
The term SOC means exactly that, State of Charge. This is different than SOE or State of Energy. SOC depends only on the charge stored in the cell. This is why an Ah counter does not need to know the voltage applied, only the current. As you pointed out, a cell with a higher ESR than another cell would be at a lower voltage under load than the other cell. If, however, both of these cells were in the same series circuits, the each would put out the same number of Ah but different amounts of energy. That is why these LiFePO4 cells do so well in series. They do not have any theoretical self discharge mechanism as lead acid and other types of cells have so if the cells in a series string are not defective they will all fill up and discharge at the same rate, regardless of the potential each one needs to fill up or discharge. Naturally the one with the higher ESR will warm more than its neighbors but then that has the effect of lowering the ESR. The number of electrons transferred from one side of its plates to the other would still be the same as in every other cell in the series string. So, for the case of determining SOC change of a LiFePO4 cell, the "resistance" of the cell doesn't matter. Naturally this is referring to good cells, not ones which are defective. If a LiFePO4 cell discharges on its own with nothing attached to its terminals then it is a defective cell. On Fri, Jun 14, 2013 at 7:22 PM, Peter Gabrielsson < [email protected]> wrote: > While I agree that resistance is important you are unfortunately not really > correct that it causes imbalance due to energy loss in series strings. > > Batteries are primarily electron storage devices, that's why their capacity > is measured in Ah (1Ah= 5767*10^19 electrons). In a series string the > amount of electrons you shove through each battery is always the same > regardless of resistance. If you put 5Ah into a string of two empty 10Ah > cells they will both end up at exactly 50% SOC even if one has 1000 times > the resistance of the other. > > > > > > On Fri, Jun 14, 2013 at 6:20 PM, <[email protected]> wrote: > > > Hello David > > > > The statement by Jack as I recall was this. > > > > That LiFePo4 cells resistance, because it is so low, resistance is no > > longer relevant. > > > > I very strongly disagree. I believe the lower the battery resistance, > it > > can even cause a greater the impact. While resistance readings on > LiFePo4 > > cells, is much lower then many other chemistries, battery resistance is > > always relevant. > > > > I have been measuring resistance readings for years and consider it > > critical information. > > > > > > Under a load, cells with less resistance hold a higher voltage. During > > charging, the cells with lower resistance will have a lower voltage. This > > is > > nothing new and known by almost everyone on this list. > > > > Energy moves in and out of LiFePo4 with far less resistance. Since > LiFePo4 > > can have up to 1/20 resistance or even less compared to new lead or NiMH > > battery. At first take it would seem reasonable to assume with > resistance > > being so low, its just that much less of a factor over prior battery > > chemistries. > > > > Seems logical at first, lower resistance should mean less loss of energy > > to heat in the battery, so this should have all around less impact. > > > > > > However there is an error, in assuming with these very low loses, > > resistance is no longer anymore a concern. As good as the CALB cells > are, > > the > > resistance readings do vary. When you compare cell to cell, in a pack of > > 50 or > > 100 cells of LiFePo4, between the lowest and highest resistance can be > > almost 100% off from one another. Since cells are being charged in > > series, these > > losses as small as they are, impact the amount of the energy remaining > in > > each cell. > > > > Over time these small differences change the SOC of the cells, and throw > > the pack out of balance. If you have 100% differences in resistance > > readings > > in a lead or NiMH pack it would quickly become a noticeable problem. > Just > > because it may not be immediately noticeable, does not mean that slight > > differences no longer have an impact. > > > > > > The CALB cells come with test sheets showing both capacity and resistance > > readings. Overseeing these group buys I have looked at many thousands of > > the CALB CA gray cells. Compared to lead or NiMH yes they are extremely > > low. > > I believe the CALB cells are the best value for the money. > > > > > > If resistance readings were all exactly the same when new, and during > > service then none of the above would apply. However in the real world > where > > nothing is perfect, any differences between cells used in series > > regardless of > > how minor will always lead to a pack becoming out of balance. > > > > Your losses in moving energy both in and out is far less with LiFePo4, > but > > compared cell to cell the percentage of difference is even greater then > > lead or NiMH. > > > > > > Resistance readings can change a lot with temperature, SOC and cycle > life. > > > > Resistance readings are very useful information. However they can be > > frustrating at times because its a moving target. If your assembling a > > pack they > > all have to be very close to the same SOC and temperature. > > > > > > I don't claim to be an expert or know it all. However to say that > > resistance is no longer a concern in batteries, shows a clear lack of > > completely > > understanding batteries and losses used in series. > > > > David hope I gave enough context here to address your reply "makes my > > statement not correct 100% of the time." I have tried to watch three of > > Jack's > > Video's and three times he made a comment that I could hardly believe he > > said. This resistance statement just happen to be the last one I tried to > > watch. I appreciate Jack's or anyone else's testing batteries. However it > > would be better to keep random thoughts not based on real data out of > them. > > > > There is so much misinformation around in general about electric > vehicles > > and its not just EVTV. Its sad to have someone misinform others looking > to > > learn about EVs. I have tried to watched three of Jack's video's and > each > > time he made a comment that was ridiculous. I don't think I have made it > > more then half way through before I could no longer listen to nonsense > > ramblings. > > > > Don Blazer > > > > > > > > In a message dated 6/13/2013 7:38:22 A.M. Pacific Daylight Time, > > [email protected] writes: > > > > Message: 7 > > Date: Wed, 12 Jun 2013 22:38:50 -0700 > > From: David Nelson <[email protected]> > > To: Electric Vehicle Discussion List <[email protected]> > > Subject: Re: [EVDL] Jack Rickard of EVTV.me was 100ah pack on the > > cheap > > Message-ID: > > <calxn3-i_oex_ejua8wwvh95zlqlqhyh-yzq_vf9hubsvj-q...@mail.gmail.com> > > Content-Type: text/plain; charset="iso-8859-1" > > > > Don, > > > > You gave no context to the statement you said that Jack said so that > makes > > your statement also not correct 100% of the time. Depending on the > > context, > > resistance in an electrical circuit/system may or may not be a factor. > > > > I know what you mean about lack of sufficient support for some of Jack's > > statements. Like I said, however, many of them which apparently had not > > enough basis have over several weeks/months/years been finally supported > > but I would expect that a significant number of people miss it and then > > just write off practically everything he says. > > > > > > -------------- next part -------------- > > An HTML attachment was scrubbed... > > URL: < > > > http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20130614/4ef6ae6a/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) > > > > > > > -- > www.electric-lemon.com > -------------- next part -------------- > An HTML attachment was scrubbed... > URL: < > http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20130614/549e1338/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) > > -- David D. Nelson http://evalbum.com/1328 http://www.levforum.com Nokia Lumia 920 Windows Phone 8 -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20130614/2f4f8ac6/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)
