Michael, Exactly the problem: the average cell charging voltage was 3.75V but without BMS you had no clue how high each individual cell was getting. Due to the behavior of LFP to quickly run up in voltage once the cell is full, the one cell that was the highest in the pack (a slightly lower self-discharge will do this - remember the earlier quoted available cell capacity above 3.45V of less than 2Ah which is only 1% of capacity? So, with one only 1% higher cell, its charging voltage can easily have run up to 4+ Volts. Lithium really needs per-cell monitoring and charge shutoff or you run an unknown risk...
Cor van de Water Chief Scientist Proxim Wireless office +1 408 383 7626 Skype: cor_van_de_water XoIP +31 87 784 1130 private: cvandewater.info www.proxim.com This email message (including any attachments) contains confidential and proprietary information of Proxim Wireless Corporation. If you received this message in error, please delete it and notify the sender. Any unauthorized use, disclosure, distribution, or copying of any part of this message is prohibited. -----Original Message----- From: EV [mailto:[email protected]] On Behalf Of Michael Ross via EV Sent: Monday, June 01, 2015 12:27 PM To: tomw; Electric Vehicle Discussion List Subject: Re: [EVDL] Success! "No one I know of charges fully." The eBike and light EV crowd are not so fortunate. (The area I am most concerned with.) They typically buy no name packs with, BMS of unknown function and provenance, and chargers with all the stickers removed and unknown internal tweaks. My first off the reservation pack, would probably have been a nice unit were it not for the charger which poured it on a 60V for 16 cell series of LFPs. I couldn't tell you what exactly it did, but I would plug it in with the vehicle sitting in the hot sun and let it charge until finally I thought about it. Potentially running the average cell voltage to 3.75V. Admittedly, with the charger removed the resting voltage was, 56V ==> 3.5 / cell. I have to think that the charger running at 60V was bad just the same, creating the conditions for activation of the electrolyte damage. Net result, pack did not last very long. On Mon, Jun 1, 2015 at 1:46 PM, tomw via EV <[email protected]> wrote: > Yes, I am aware of that. For about 5 years I have been advising > people not to charge to over around 3.45V which is about where the > exponential increase in the curve starts at typical charge currents. > I published a number of cell measurements and charge curves around > that time showing there is less than 2 Ah charge between 4.45V and > 3.55V on 180Ah cells, so not much gain in charge by going there unless > you use a shunt balancing BMS which I do. > Shunt turn-on varies from 3.48V to 3.52V over my 36 cells, so every 4 > - 6 charges I charge the highest cells to 3.54 - 3.55V and check the > shunt LEDs to ensure they are all on giving me peace of mind that the > pack is still balanced. The rest of the time I just do partial > charges. Many people only charge to less than 3.45V every charge. No > one I know of charges fully. > The manufacturer's spec is final CV charge at 0.05C to 3.6V which is > below the maximum voltage for the cells. You also have to keep in > mind that although there are significant differences in the rates of > side reactions in cells of different chemistries, the rates are fairly > low, so the effects accumulate slowly over years unless you > significantly over charge or discharge a cell. I don't think anyone > expects these prismatic cells to last 10 or more years with less than > 10% capacity loss. I'll be happy if my pack still has 90% nominal > capacity in 7 years, which is only 1 1/2 years away. Then I'll > replace it with a wrecked Leaf 30kWh pack :^) > > > > -- > View this message in context: > http://electric-vehicle-discussion-list.413529.n4.nabble.com/Success-t > p4675905p4675947.html Sent from the Electric Vehicle Discussion List > mailing list archive at Nabble.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) > > -- To invent, you need a good imagination and a pile of junk. Thomas A. Edison <http://www.brainyquote.com/quotes/quotes/t/thomasaed125362.html> A public-opinion poll is no substitute for thought. *Warren Buffet* Michael E. Ross (919) 585-6737 Land (919) 576-0824 <https://www.google.com/voice/b/0?pli=1#phones> Google Phone (919) 631-1451 Cell [email protected] <[email protected]> -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20150601/a728353c/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)
