>From: Cor van de Water via EV <[email protected]> >I also did tests on LiFePO4 cells and while self-discharge was low, I was able >to prove from my measurements over many weeks, that there is was about a >factor 2 difference in self-discharge current between the best and worst cell. >Sample size was over 40 cells.
This is what I have found as well. Brand new cells, all bought at the same time from a quality source are very similar. Their amphour capacity, internal resistance, and self-discharge rates are very close. But cells from cheap or low-quality sources have a much broader spread in characteristics. Cells also get worse as they get old, or get cycled, or as the temperature changes. Differences between cells accumulate over time, getting worse and worse. You may get by without a BMS initially, but it gets needed more as the differences between cells grows Not having a BMS means shorter life. >This is exactly what the author of the quoted article found, that the capacity >of the cells had not degraded, but the cells had gotten out of balance. Yes. The BMS was too primitive to do its job of balancing the cells. Only having upper and lower voltage limits prevents against catastrophic failures; but does not compensate for differences between cells. -- Excellence does not require perfection. -- Henry James -- Lee A. Hart http://www.sunrise-ev.com _______________________________________________ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org Please discuss EV drag racing at NEDRA (http://groups.yahoo.com/group/NEDRA)
