The thing is, David, while you are still going strong and not even once measured voltages from the pack during the 18k with a multimeter (naturally you have measured. right?), the BMS-less use is for people who know how to take care of their pack.
When a new born EV person reads through all this and decides to go without (cost savings) theres a good chance for disaster. Very same applies with BMS when it's installed wrong way or it's just piece of bad electronics. LFP is forgiving but it dies also by million cuts. No monitoring means "sudden accidents". It frankly is like driving without seat belts. No crash, no problem. What are the possible error scenarios which can lead to cell over discharge and/or meltdown? - loose connection on the cell pole - loose connection inside the cell (corrosion) - cracked seal to release electrolyte (internal resistance grows => capacity loss) - cracked seal can suck moist air in => more corrosion - uneven temperature conditions drive cell capacities and internal resistance (5 Celsius average over 5 years is a big thing already) - noticeable manufacturing defect (5% rejection rate at manufacturing, 0% rejection rate in sales) - developing manufacturing defect (raw material impurity, bad mixing, bad storing, interrupted process,...) - partial foil rupture at the pole assembly (sudden capacity loss) - cells from different patches (you might have seen pictures from factories how they pool cells all over the inspection floor and pick cells for delivery) There are so many things which can be done badly in the cell manufacturing. It's unskilled workers or bad management or just mistakes. While it is not hard to make cells it is hard to make good ones which serve 10-20 years. And it is not very good idea to rely solely on Chinese QC even it has taken giant leaps at most factories. In any product some defects land on users garages. If it's not Li-ion cell it might be ignition switch on the car. I believe one thing might help on this situation (why people are getting away with it for so long): delay. As people buy cells and they arrive about 12 months after manufacturing (do not believe the date on the cell) they still wait for some time before they get installed. Many do finalize the formation on all cells by paralleling them and charging up to 4v region. So I can see how some error mechanisms can be pushed down for a while. Eventually the cells will die and one can just hope they die on same cycle or 0v cell is found as it occurs (you can possibly feel the lack of power in motor and the excess power at the cell internals). I'm giving a presentation at annual insurance inspectors gathering about battery fires and sample some investigations on devastating fire incidents. BMS is one part of the equation. -Jukka http://www.google.com/profiles/jarviju#about 2014-04-22 1:38 GMT+03:00 David Nelson <[email protected]>: > On Mon, Apr 21, 2014 at 9:45 AM, EVDL Administrator <[email protected]> > wrote: > > On 21 Apr 2014 at 12:34, Michael Ross wrote: > > > >> I don't think bottom balancing is cult-ish. (It IS rude to talk bout it > >> that way.) It is just complicated. > > > > I can see why you might think it rude. > > > > The term "cult" might be a bit strong. But personally, I do see an an > > element of a sort of "religion" in some of the bottom-balancing folk. > Others > > may not feel the same way. > > In all fairness, your statement can be equally applied to the "BMS > only" crowd. As Michael and Damon point out, this is not as simple as > one thing works and another thing doesn't. There are many parameters > to consider in how a battery pack has been treated before anything can > really be said about a particular method. I was fortunate enough to > get 40 good cells. Now that I have 4 years and 18k miles on my pack > and it is still going strong and is not getting out of balance with no > balancing being done on any charge, you can see why I question the > "requirement" of having a top balancing BMS whether it is active or > passive. I purposely chose to go without the top balancing BMS I > started out with because no one could show me the theoretical reason > to have a top balancing BMS and I couldn't find one in all the > researching I did either. For my climate and setup my data is still > showing that a BMS is not necessary. Maybe if I lived where Jukka > lives I would have different results. I know I'm not the only one with > great results with no BMS. Blanket statements from either "camp" make > them appear cultish. > > It may be that, as Jukka pointed out about the formation charges, I > was actually fortunate to have very slowly (10A into 20 paralleled > 100Ah cells) charged my cells to 4.00V initially and for a short time > after installation. Now I charge to 3.455V/cell average. I don't > charge if the case temp is at or below freezing, and I rarely > discharge past 70%DOD. > > As for other types of lithium batteries besides LiFePO4, I can't say > for sure, I haven't tested or researched them enough. > > -- > David D. Nelson > http://evalbum.com/1328 > http://www.levforum.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) > > -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20140422/9f3c2e35/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)
