On 19 Jun 2015 at 23:00, Michael Ross via EV wrote: > Self discharge would be where the capacity has not decreased, but the > amount available in the cell is less - very hard to measure accurately the > actual charge level with normal DIY instrumentation when the change is small.
Did you read Lee Hart's earlier post? http://www.evdl.org/archive/index.html#nabble-td4676242i40|a4676355 That is EXACTLY what he measured. If one is trying to determine self discharge by measuring voltage, that probably won't yield a valid answer. Though I'm no electrochemist, my understanding is that one can't reliably judge a lithium cell's SOC (or any other type of cell's, really) by its voltage, except in very limited circumstances. Perhaps this paper would help. Anyone here have an IEEE login? Self-discharge losses in lithium-ion cells http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=1269687&url=http%3A%2F% 2Fieeexplore.ieee.org%2Fiel5%2F62%2F28408%2F01269687.pdf%3Farnumber%3D1269687 http://v.gd/0IqfFJ "The self-discharge losses in several lithium-ion cell designs have been measured by three different methods. The losses are separated into time- dependent and state-of-charge dependent contributions. For most cycling conditions, the time-dependent self-discharge losses are dominant; however, after several months of stand on open circuit or float charge, the state-of- charge dependent losses become significant. The self-discharge rate has been found to not increase monotonically with state-of-charge, but to drop somewhat at intermediate states of charge. The implications of these measurements for maintaining balanced cell capacities in batteries and establishing optimum storage voltage levels for batteries are discussed." Or perhaps this one : Investigation on the Self-discharge of the LiFePO4/C nanophosphate battery chemistry at different conditions http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=6940762&url=http%3A%2F% 2Fieeexplore.ieee.org%2Fxpls%2Fabs_all.jsp%3Farnumber%3D6940762 http://v.gd/gfBgE2 "In this paper the self-discharge of the nanophosphate LiFePO4/C is studied at different temperature, SOC conditions and at different SOH levels of the battery. Moreover, cell to cell differences in self-discharge caused by the manufacturing tolerances are investigated." Or maybe here. http://iopscience.iop.org/0957-4484/24/42/424009/ In this paper, "Raman spectroscopy was used to study the surface phase change during charge and self-discharge on a more localized scale for three morphologies of LiFePO4 ..." Curiously the following page says that LiFePO4 has a HIGHER self discharge rate than other Li chemistries. However, it doesn't explain why nor does it give any references, so I'm skeptical. http://batteryuniversity.com/learn/article/types_of_lithium_ion The following paper discusses how to simulate the effect of self-discharge in LiFePO4 cells, but doesn't explain the chemistry which might cause it. http://www.dsea.unipi.it/Members/huriaw/journal-paper I found a paper which references yet another paper as stating that LiFePO4 self discharge is on the order of 8%(!) per month. The referenced paper is "A. Chih-Chiang Hua and B. Zong-Wei Syue, Charge and Discharge Characteristics of Lead-Acid Battery and LiFePO4 Battery.: The 2010 International Power Electronics Conference." Here is the referenced paper. ftp://213.176.96.142/ieee345c50ff-abaf-20141124102512.pdf Regrettably it simply states this as a fait accompli, but fails to explain the electrochemical mechanism for self discharge. I spent way too much of my time on this research tonight, and that was just on the web. I haven't even hit the library yet! However, I found many discussions of self discharge in lithium cells of all types, including LiFePO4. Most of these were either comparisons of LiFePO4 with other chemistries, or were discussing ways to compensate for the cell imbalance that results when these cells are used in batteries. I was able to find only one reference which even suggested that lithium cells might not have a self discharge. In fact it was very similar to the statement made recently on the EVDL that lithium cells have no specific mechanism for self discharge. http://electricvehiclesnews.com/Technology/Lithium-ion_battery.htm "According to one manufacturer, lithium-ion cells (and, accordingly, "dumb" lithium-ion batteries) do not have any self-discharge in the usual meaning of this word. What looks like a self-discharge in these batteries is a permanent loss of capacity ..." Unfortunately the author doesn't give any reference for this statement. He doesn't even identify the "one manufacturer." Of course if one desperately wants to believe that lithium batteries don't self-discharge, maybe one website with no references is sufficient. ;-) As for actual lithium cells that normal people can buy ... This spec sheet from a LiFePO4 manufacturer specifies a self-discharge rate of 3% per month. http://www.zepiaenergy.dk/ref.aspx?id=516 This manufacturer boasts a self-discharge rate of "below 0.3% per day." That would be 9%(!) per month. http://www.essexparts.com/shop/ultralight-lithium-batteries.html Headway says to expect 2-3% per month. http://www.headwaybatteryandcable.com/products/20120302586.shtml I reiterate that I'm not an electrochemist, and I can't define a mechanism for lithium cell self discharge. Heck, even if I found such a definition, I probably wouldn't understand it! But from what I've just seen, I'd have to conclude that most or all the real-world lithium cells I can buy are likely to have a self discharge of between 2% and 9% per month. David Roden - Akron, Ohio, USA EVDL Administrator = = = = = = = = = = = = = = = = = = = = = = = = = = = = = EVDL Information: http://www.evdl.org/help/ = = = = = = = = = = = = = = = = = = = = = = = = = = = = = Note: mail sent to "evpost" and "etpost" addresses will not reach me. 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