Don, I have a pack of (older technology) Li-Ion cells. All cells are 10Ah capacity, but the internal resistance varies wildly between the cells. As soon as I start charging, some cells jump up in voltage due to the drop across their internal resistance, so I really need to taper the current to maintain a cell voltage below the max cutoff (4.25V) or risk damaging some cells further. By carefully charging until all cells are at about 4.2V resting voltage, I can start a discharge with all cells in series and by the end of the discharge, all cells will fall off at about the same moment, because all of them are very close to the 10Ah spec. So, even though their internal resistance is much different, their capacity is unaffected. *During* the discharge however, the cell *voltages* differ widely, again due to the voltage drop across the series (internal) resistance. Now this is Li-Ion, so the story can be quite different for other chemistries, but I hear similar stories for other Li-Ion types like LiFePO4. I do know that NiMH is behaving quite different.
Regards, Cor van de Water Chief Scientist Proxim Wireless Corporation http://www.proxim.com Email: [email protected] Private: http://www.cvandewater.info Skype: cor_van_de_water Tel: +1 408 383 7626 -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of [email protected] Sent: Thursday, June 20, 2013 2:35 PM To: [email protected] Subject: [EVDL] Resistance Hello David Sounds good in theory but what I find is cells with higher resistance never have as much capacity as modules with lower resistance when charged in series. Even brand new LiFePo4 cells have different resistance readings. Which slightly affects their useable capacity with each cycle. Don Blazer In a message dated 6/19/2013 11:30:57 P.M. Pacific Daylight Time, [email protected] writes: This is why, as Cor pointed out, two cells with vastly different IR but the same charge capacity, when starting at the same State of Charge, can run out of charge, and thus energy, at the same time even if the energy each delivers is vastly different. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20130620/74ea 288b/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)
