https://www.youtube.com/user/jlghardy/videos
John Hardy's YouTube channel is extremely interesting (here) he test some of the theories out there about li ion batteries and the effect of a BMS. In the video "battery test 2" he demonstrates that a cell balancer actually damages the cells. Sent from my iPad > On Jun 27, 2015, at 2:28 AM, David Nelson via EV <[email protected]> wrote: > > 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 > <[email protected]> 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) > -------------- next part -------------- An HTML attachment was scrubbed... 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