Generalization is bad, bad thing. NASA used Cobalt or Manganese cathodes ?
100% SOC effect is not even closely as strong in LiFePO4 than in others. I took some cells from storage last summer that were made in 2004-2005. Some of them had been charged or cycled only once since manufacturing. LFP's had much more SOC in them compared to LCP's and LMP's. Same prismatic design and manufacturer. We designed charger program that charged cells to certain end of charge voltage and then wen't to float. When the cell was full the charger waited for certain preset time and set that voltage as CV point. Current was set to 5-10%. Hysteresis took care of the current and it was just maintaining the 100%SOC. No damage was ever noticed on hundreds of cells during the years. And those cells were quite accurately monitored by killer BMS. Cells in the pack was charged with same type of chargers and pack was put together with cells that were very closely in same SOC. Helped to determine the development of capacity deviation. Floating point for each cell type and individual are a bit different. When the current is very, very low in the float, you're near. Take a notch down from there and you're safe. Did I just make oh-so-bad generalization ? :D -Jukka http://www.google.com/profiles/jarviju#about 2013/2/19 David Nelson <[email protected]>: > Who at A123 told you that? Do they have test data of holding a cell at > 3.45V for momths or years and no issues? Did they compare that with > holding the cell at 3.40V or 3.38V? What, really, is the energy storage > improvement by using the higher voltage assuming no adverse effects? > > If I were in your position I wouldn't buy that. I'd do my own testing. > I'd put a cell on at the 3.45V in a constant temperature environment > and let it sit at that terminal voltage for a few hours and then > measure any temperature difference between the cell and ambient. If the > cell is warmer then it is being overcharged. Also, carefully measure > the current going into the cell. If it is above 0µA it is being over > charged. Do the same tests at 3.40V and 3.38V and compare results. > > If you capacity test the cells after being held at the different float > voltages I'm sure you will find that there is a minuscule difference in > energy storage. Furthermore, given that the NASA research summary I > read a couple of years ago found that the calendar life of all Li based > rechargeable batteries was shortened when left at 100% SOC as compared > to a lower SOC, do you really need the cells at 100%+ all the time? > > Whatever you do please report your results. > > David D. Nelson > http://evalbum.com/1328 > http://www.levforum.com > > Sent from my Nokia Lumia 920 Windows Phone 8 From: Mark Hanson > Sent: 2/19/2013 5:37 > To: [email protected] > Subject: [EVDL] LiFePO4 float charging > > Hi Folk's,A123 told me to float charge their D size cells to 3.45V > when I was using them in a float charge UPS application.Best > Regards,markwww.REEVA.info > Message: 9 > Date: Thu, 14 Feb 2013 22:33:53 -0800 > From: "Cor van de Water" <[email protected]> > To: "Electric Vehicle Discussion List" <[email protected]>, "Electric > Vehicle Discussion List" <[email protected]> > Subject: Re: [EVDL] 12V LiFePO4 batteries > Message-ID: > <[email protected]> > Content-Type: text/plain; charset="iso-8859-1" > > Since the cells might not be perfectly balanced, > I would expect that when one cell dips to 2V, you can > expect the others still to be around 3V, so I guess that > it would be prudent to disconnect when the voltage dips > to 11V (3x3+2) > > BTW, I did not suggest to float-charge the cells at 3.45V > indefinitely, since no car is running 24/7. > Typically a car is used a short period, sometimes a few hours, > and then shut down, from which moment the battery is slowly > discharged until the car has been started again. > > I believe the highest current in a (ICE) car is starting cold. > That can run close to 1000A peak. Because AFAIK most alternators > are in the 100A region, but it is certainly good to check for > max charging current and adjust for it. > NOTE that the net charging current is the difference between > the alternator's generated current and the consumption of the car. > > Regards, > > Cor van de Water > Chief Scientist > Proxim Wireless Corporation http://www.proxim.com > Email: [email protected] Private: http://www.cvandewater.com > Skype: cor_van_de_water XoIP: +31877841130 > Tel: +1 408 383 7626 Tel: +91 (040)23117400 x203 > -------------- next part -------------- > An HTML attachment was scrubbed... > URL: > <http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20130219/cbf00a48/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)
