On Tue, Jul 31, 2018 at 9:25 PM, Tom Hudson via EV <ev@lists.evdl.org> wrote: > > Those two were ZERO VOLTS, and show ZERO OHMS resistance. > > Is this a common failure mode of lithium-ions (or the CALB modules)? I'm > thinking that I'd better set up some kind of fusing between the buddy pairs > because if one of the pair fails in this way, it's going to present a dead > short to its buddy -- and nothing good is going to come of that.
I don't know how common of a failure mode that is but I suspect it could happen. If they were placed upside down (terminals facing down) there is evidence that that has corroded the internal contacts to the terminal bolts but that caused an open circuit situation, not a dead short. Internally the cells are made up of several layers which are "buddy-paired" at the terminals. I put 20 buddy pairs of TS-LFP100AHA cells in my Gizmo back in January 2010 and they are still going strong. While the probability of any given cell failing goes up with the number of cells I claim that a 200Ah cell made up of the same size materials as the 100Ah cells would also have a higher probability of failure. Definitely give each cell a cycle test and if you have the time a capacity test. I would recommend that you then pair them up where the capacity of the pairs ends up being nearly identical. I did a rudimentary version of this for my Gizmo and it has worked out just fine. Some claim that you have to have the cells at exactly the same SOC before connecting the buddy pairs. Theoretically this doesn't make sense and I have also tested it and practice shows it really doesn't matter. As long as their voltages are similar the initial current isn't that great. Somewhere in the EVDL archives I posted the numbers from one of my tests of this. IIRC it maxed at ~100A for a second or so and quickly dropped to below 20A or so. Charging the cell pair to 3.4V or discharging to 2.8V or so brings the two cells in line with each other. The 100% SOC resting voltage of LFP cells from CALB and TS is 3.38V. Make sure you aren't hammering the cells with 3.6+V on every charge trying to equalize them. A good set of cells doesn't drift that fast. I have been running my pack top balanced without attached BMS boards for several years with no issues. I recently sold the Gizmo and I don't trust the new owner to be able to be the "human BMS" so I am going to be reinstalling the BMS boards I have. Furthermore, it is an unknown how much the cells will drift as they age. They have been fine so far but I'm not driving it now so can't do my annual voltage check. I've been charging to 3.465V/cell with my Zivan chargers and the resting voltage with no load after 12hours has been 3.36V so I know I'm not overcharging them. HTH, -- David D. Nelson _______________________________________________ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org Please discuss EV drag racing at NEDRA (http://groups.yahoo.com/group/NEDRA)
