Thanks Tomw, That was a nice video. I didn't hear any contradictions. I learned somethings about cell construction I did not know.
Dahn was lecturing to explain HPC testing, and Whitacre was lecturing to help people who assemble and manage packs, so they have a little different slant. Regarding max temperature for LFP cells. Whitacre stated 140°C for safety purposes - above that the electrolyte is turning to gas and can violently oxidize. For life of cell purposes Dahn says don't subject them to more than 40°C when fully charged. Not the same discussion at all. So much about Li ion cells is specific to the manufacturing techniques and quality, the electrolyte package and the intended use, the cost of manufacture, and so on, that lumping Li ions together or even lumping them of the same electrode chemistry is very dicey. When you get to the end the best info to a DIY'er might be group parallel cells by capacity and then by series resistance. The better you can do this the less concern about BMS function. The cell to cell balancing circuits, he stopped short of saying don't use them and that no large pack manufacturer is going to use them (too much $). Since the DIY'er may have no clout with the cell supplier, the low cost supplier may not make very consistent product, you are in a bind to be confident about exactly how to proceed. Anyway, if you are thinking of writing a check for numerous thousands for a Li ion pack, you might want to watch and understand this video. On Fri, Apr 3, 2015 at 11:58 AM, tomw via EV <[email protected]> wrote: > Side reaction rates increase with temperature, but that is only part of the > story. The rates are even more dependent on cell voltage, and Dahn > mentions > this: > > *• The biggest factor is time spent at highest voltage. > o Longer is worse > o Most of the parasitic reactions happen above 4v.* > > This was mentioned by Jay Whitacre who made this video for the guys at > Charged Car at Carnegie-Mellon which was widely discussed several years ago > by diyers: > > http://www.ri.cmu.edu/video_view.html?video_id=60&menu_id=387 > > He now has founded a storage battery company: > > Energy Storage Technology | Energy Storage Technologies | Aquion > > That’s one reason that many of us decided years ago to charge LiFePO4 cells > to only the beginning of the exponential part of the charge (V versus Ah) > curve, ~3.45 for those not using a bms with shunts, ~ 3.50V to 3.55V for > those using shunts, even though at that time some of the LiFePO4 > manufacturers specs stated full charge to 4 V. CALB has since lowered this > to 3.6 V. > > Also Venkat of LBL at his This Week In Batteries blog stated that the main > cause of premature death of laptop batteries was leaving them plugged in > all > the time. This holds the cells above 4V to maximize the amount of stored > charge – since customers rate laptops on how long the batteries last on a > charge. > > > http://thisweekinbatteries.blogspot.com/2010/02/pull-plug-your-battery-will-thank-you.html > > As Dahn points out, additives can make a large difference in cell life, so > general comments on relative lifetimes of different cell formulations – > LFP, > LCO, etc – aren’t very instructive since the lifetime of each formulation > may vary widely with additives used,and we don't know which additives > various manufacturers are using, nor their relative effects. > > The LiFePO4 cells in my car have been over 104 F a number of times during > each of the last 5 summers, and as high as 115 F. You can’t really avoid > it > without active cooling when driving on 100 F days. A couple cells were > accidently charged to ~3.95V, but none have been over 4 V. > > The pack has about 850 cycles on it now, 5 ¼ years old, 40k ev miles. No > noticeable decrease in range, but it could have decreased by 5% or so and I > likely wouldn’t notice, since the only test I do is drive it to about 28% > SoC (of the nominal 180Ah, or ~130 Ah used) once in a while, then floor it > (pulling ~3C) and see if the bms LVC alarms. Hasn’t so far. > > Coming years are going to be hotter. The positive cycle of the Pacific > Decadal Oscillation and what seems to be a beginning El Nino this year are > predicted to make this a record hot year, and the PDO typically lasts for > around a decade. Going to be tough on batteries in Phoenix AZ, as well as > some other areas. > > > http://thinkprogress.org/climate/2015/04/02/3640842/global-warming-jump-imminent/ > > > > > -- > View this message in context: > http://electric-vehicle-discussion-list.413529.n4.nabble.com/ENERDEL-Battery-Experiences-tp4674615p4674752.html > Sent from the Electric Vehicle Discussion List mailing list archive at > Nabble.com. > _______________________________________________ > 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) > > -- To invent, you need a good imagination and a pile of junk. Thomas A. Edison <http://www.brainyquote.com/quotes/quotes/t/thomasaed125362.html> A public-opinion poll is no substitute for thought. *Warren Buffet* Michael E. Ross (919) 585-6737 Land (919) 576-0824 <https://www.google.com/voice/b/0?pli=1#phones> Google Phone (919) 631-1451 Cell [email protected] <[email protected]> -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20150403/50d106ab/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)
