That is some good info. The NASA document I read did specifically include LiFePO4 but didn't include how much degradation happened. From your results it sounds like there wasn't much.
On your 100% SOC setup what float voltage did you use with the LiFePO4 cells? On Tue, Feb 19, 2013 at 1:25 PM, Jukka Järvinen <[email protected]> wrote: > 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) > -- David D. Nelson http://evalbum.com/1328 http://www.levforum.com Nokia Lumia 920 Windows Phone 8 _______________________________________________ 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)
