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:ev-boun...@lists.evdl.org] 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)
