Thank you Cor! This is exactly what I needed to learn more than I do now. This amazing group of EV creators has made it possible for me to drive electric since 1999, and you are one of the members whose posts I always read.
On Thu, Jun 23, 2016 at 11:32 AM, Cor van de Water via EV <[email protected] > wrote: > Mike, > there are two approaches if your BMS has no capability to tell you which > cell is low, > one is the scientific approach of using a dummy load and measuring > time and voltage to minimum cutoff, this will give you numbers on the > current capacity of your pack. Just in case I ever wanted to do this > myself, I saved the heating element of a pool pump which is 240V 40A or > so, so its resistance is about 6 Ohms and makes a nice dummy load for a > host of tests. > > The other approach that is a lot less invasive and time consuming is to > simply drive the vehicle until it throws the low cell alarm, > park the car with the parking brake tightly set, hook up a voltmeter to > a small group of cells and blip the trottle for a second. > NOTE that when a DC motor can't turn, you must make sure it does not get > loaded with high current for more than a few seconds or it will burn up > the position that it is in! > Since these are CALB cells (LiFePO4) all good cells will still sit at > the default 3.2V while a low cell will fall through the floor and dip > below 2.5V so you can measure 10 groups of 4 cells and likely there will > be 9 groups that stay pretty solid near 13V minus wire resistance drop > and there will be one group that will drop closer to 12V minus wire > drop. > > NOTE that a higher resistance wire connection *can* cause the BMS to see > a low cell if it measures across cell + wire (which is common) so it > might turn out to be 40 perfect cells and a corroded or loose terminal. > > Success! > > 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 > > http://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 Mike Beem via > EV > Sent: Thursday, June 23, 2016 11:03 AM > To: Electric Vehicle Discussion List > Subject: [EVDL] Lithium Battery Testing > > I spent some time looking through the archive, but couldn't find what > I'm > looking for--> I think I have one weak cell in my 40 cell 100 Ah pack; > I'm > getting a low battery signal (mini BMS) under load when there is more > than > sufficient range left on the charge. These are CALB batteries which I > installed in December 2012. I recently tried going through and > individually > charging each one to 3.5v (after charging the whole pack to shut-off > point) > with my variable power supply, and at 2 amps maximum, it took me almost > 2 > weeks of not driving the EV (http://www.evalbum.com/4181) to get all the > way through, so I used a timer, which of course, would defeat the whole > process by not being able to WAIT for 3.5v on every one... > When I first started driving it with the new pack in 2012, it did have > the > 40 mile range I aimed for when I put this together. I have only driven > it > to the limit of the pack once since then, and it was in a colder winter > than we usually have, so I wasn't surprised to have much less range. > I need an easy to put together load I can use with a voltmeter to test > cells, and a range for what voltage drop on these CALB cells would be > normal or weak. I got fairly good at this with lead acid, both flooded > and > AGM, but don't have the experience or science to know if, 1) this is a > reasonable way to proceed, and, 2) what those voltage-drop decision > points > would be, and, 3) what components to use for the load? > Thank you! > Michael B > -------------- next part -------------- > An HTML attachment was scrubbed... > URL: > <http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20160623/fd3e > 157a/attachment.htm> > _______________________________________________ > UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub > http://lists.evdl.org/listinfo.cgi/ev-evdl.org > Read EVAngel's EV News at http://evdl.org/evln/ > Please discuss EV drag racing at 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 > Read EVAngel's EV News at http://evdl.org/evln/ > Please discuss EV drag racing at NEDRA ( > http://groups.yahoo.com/group/NEDRA) > > -------------- next part -------------- An HTML attachment was scrubbed... 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