Cor, I think that a lack of change in voltage is an indicator of lack change in state of charge - the best one can do at any rate. People talk about LiFePO4 cells self discharging, but I I don't think it happens. Maybe other formulations do, I don't know about that.
The non-linearity does make voltage very insensitive as a measure of SOC. I get that. But when you have battery and someone says, "it is self discharging," I am not sure what else to do beside actually draining it down to zero and integrating the amps to know it is losing charge; and even then the knowing is tenuous if the discharge was small. If you can't read tiny voltage well, and most of us cannot with our handheld meters, then there is a very good chance that talk of self discharge is just a reaction to measurement noise. On Thu, Dec 19, 2013 at 3:47 PM, Cor van de Water <[email protected]> wrote: > Michael, > You are quoting voltage, which is not a linear indicator of charge. > As quoted before, between 20 and 80% some battery chemistries like > LiFePO4 might only change 1mV for each % of SoC. Other chemistries react > differently and in particular LiPoly and similar have a more linear > relation between voltage and SoC. But still not completely linear so you > can still not do the calculation that you did. > > 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 Michael Ross > Sent: Thursday, December 19, 2013 12:38 PM > To: [email protected]; Electric Vehicle Discussion List > Subject: Re: [EVDL] Looking headway completed packs > > Have you ever seen a cell not connected to some parasitic load that > loses > 5% in a month? > > I think this is just a CYA, not a real spec. > > Over a month from 110/30 to 11/30 my 38120 pack lost and average of > 0.0018125V/3.3430625*100% = 0.05% > > Over the last two weeks of it only 0.007% was lost. > > The 18650 pack lost 0.128% (0.004 V) over a 2 week period. > > Both started from a fresh charge. > > Since then I have been using them so there is no longer static period. > > I think when you charge them overmuch it is more like an electrostatic > charge on top that diffuses over a bit of time and thermal cycling (a > settling period). Once that is gone you are reading the actual cell > voltage and there is almost no self discharge. > > Something to consider: 0.001V is really not so easy to read and most of > us, > I bet, don't manage lab temperature, or even get periodic calibrations. > Neither our instruments or the environment supports this sort of > accuracy. > > These single digit millivolt readings are very likely just noise, not > real > numbers worth paying attention to. Many people who are more > knowledgeable > (people who maintain calibrations and good lab practice needed for small > signal measurement) than I, say there is no self discharge; there is no > theory or evidence for how it could occur. It does not exist within > a properly made Li cell. > > > > > On Thu, Dec 19, 2013 at 2:48 PM, George Tyler <[email protected]> wrote: > > > I am repairing a headway battery from china at the moment, made a deal > with > > Conhismotor who supplied it, they supply the new cells and I do the > labour. > > I usually buy from evlithium. Dealing with china is different, > freight > > costs are so high that you have to consider the cost of sending a pack > back > > carefully, it is often not worth it. This pack has 48 38120L cells, > groups > > of 3 in parallel, 16 groups in series. I see Headway spec says up to > 5% per > > month self discharge, but most of the pack has close to zero. The BMS > can > > only bypass 50 or 100ma or so (it has SM resistors for this!), so it > stands > > little chance of equallising 5% per month difference. All the cells > may be > > in spec, but the battery does not work > > > > -----Original Message----- > > From: [email protected] [mailto:[email protected]] On > > Behalf > > Of emf-evdl > > Sent: Thursday, 19 December 2013 12:32 p.m. > > To: Electric Vehicle Discussion List > > Subject: Re: [EVDL] Looking headway completed packs > > > > Buy direct from headway. > > http://www.headway-cn.com/index.html > <http://www.headway-cn.com/index.html > > > > > > > Be warned, by the time you pay for shipping and any associated fees, > you > > get > > a pack that has little or no warranty. Don't expect China to help deal > with > > any warranty claims. > > > > After all is said and done, 38120S Headway cells aren't that much > better > > than some CALB cells. Depending on how many Ah you need, if you can do > > 40/60/100, then CALB are a much better deal. You don't have to worry > about > > all that bussbar. > > > > This is coming from someone who's built a few large packs using > headway. > > I'd > > do it differently today. > > > > > > > On December 18, 2013 at 2:08 PM Peakfoto Digital Photo Still n Video > > > <[email protected]> wrote: > > > > > > > > > Where can I find completed headyway battery packs form china or > > something? > > > > > > I'm looking for 36 volts 8-10 ah pack . > > > > > > most amaercans dealers are spendy . > > > > > > _______________________________________________ > > > 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) > > > > > -------------- next part -------------- > > An HTML attachment was scrubbed... > > URL: > > < > > > http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20131218/63795 > 1e5 > > /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) > > > > > > > -- > Put this question to yourself: should I use everyone else to attain > happiness, or should I help others gain happiness? > *Dalai Lama * > > Tell me what it is you plan to do > With your one wild and precious life? > Mary Oliver, "The summer day." > > 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) 550-2430 Land > (919) 576-0824 <https://www.google.com/voice/b/0?pli=1#phones> Google > Phone > (919) 631-1451 Cell > (919) 513-0418 Desk > > [email protected] > <[email protected]> > -------------- next part -------------- > An HTML attachment was scrubbed... > URL: > <http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20131219/cbf1 > 7898/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) > > -- Put this question to yourself: should I use everyone else to attain happiness, or should I help others gain happiness? *Dalai Lama * Tell me what it is you plan to do With your one wild and precious life? Mary Oliver, "The summer day." 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) 550-2430 Land (919) 576-0824 <https://www.google.com/voice/b/0?pli=1#phones> Google Phone (919) 631-1451 Cell (919) 513-0418 Desk [email protected] <[email protected]> -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20131219/8d3fac05/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)
