I have a question about the self-discharge data Cor (below) and Lee has listed and/or referred to in various posts on the EVDL and more recently referred to in the various self-discharge threads.
On Fri, Jun 19, 2015 at 2:48 PM, Cor van de Water via EV <ev@lists.evdl.org> wrote: > I have shared that I have a pack of > used LiFePO4 cells in my garage and I have measured its self discharge and I > have seen > the cells go down towards zero and obviously I did not want to destroy the > pack, so > as soon as they went below 2.5V due to self-discharge, I recharged them. > Capacity was > good, so it looks like it is not the capacity fade that causes self-discharge > apparently. > How do we know that what Cor is seeing is self-discharge and not something else? My question comes from the fact that these are used cells and there is no information about how they were used. Were they abused or treated properly? What are the specifics to how they were treated so that others can review the information to see if a cell that was treated properly/abused was in fact abused/treated properly? In the case of Cor's cells which discharged below 2.5V with no external circuit I would remove them from my pack as they are defective. If the cells Cor measured were in fact abused (in the true sense whether or not any one knows what that is) then the measurements don't point to self-discharge of the type of cell but to discharge due to some form of internal damage to the cell. If these data are not compared to data from good cells then we have an inaccurate picture of what might be happening. I'm thinking of a paper I was reading in the early '90s about the Space Shuttle Challenger disaster. The decision to launch was based only on data from launches where there was some issue so the model gave an incorrect outcome. I found it interesting that when all data from all launches was put in the model the outcome was don't launch. We need to be careful we don't fall into the same kind of thing. I started out thinking that LiFePO4 cells had a self discharge similar to how lead acid, NiCd, and NiMH batteries do. It has been through multiple research papers and presentations that I found out otherwise. I and others have posted links and references to some of these reports including one from a university in Canada that Tesla has recently signed a contract with on battery related development. Just because a company decides to put a cell level BMS on their battery pack doesn't mean that the batteries they are using have a self-discharge. Maybe they are ignorant, maybe they are concerned that over time cells will develop internal damage and behave like the used cells Cor has tested, maybe they have decided to push the limits on the cells and want to protect against the inevitable, maybe they don't expect any need for the BMS but don't trust that 100% of the cells were manufactured perfectly. Maybe they didn't do destructive tests to really find out what causes cells to die and what their failure mechanism is. I believe it is John Hardy, in the UK, who built a device to monitor cell voltages which put exactly the same load on each cell so as to not introduce an external imbalance in the set of test cells. After hundreds (over 900 IIRC) of cycles he hasn't found any drift between cells which implies that whatever capacity loss mechanisms are in the cells they aren't showing up as varying between cells after long term cycling. Even with Cor's data, it doesn't prove that there is a self-discharge mechanism in the cells. There are multiple independent sources which have clearly stated that there isn't a self-discharge mechanism built into the cells that using that argument as to why a cell level or sub pack level BMS is a requirement is disingenuous at best. I can think of many other reasons why one might want one, however. In these discussions lets not forget that there are many variables which can affect outcomes and cause one to come to the wrong conclusions. It is even possible to come to the right conclusion for the wrong reasons. That doesn't make the reasons right. Also, just because someone doesn't agree with your interpretation of data doesn't make them someone who doesn't want to learn or a troll. Maybe go study the references they list to see what can be learned. Maybe they interpreted the information incorrectly, maybe you did, or maybe you both did. It could even be as simple as both are working from a different set of assumptions and don't know it. Everyone can learn something from everyone else. -- David D. Nelson http://evalbum.com/1328 http://www.levforum.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)
