That is exactly the reason for the thin welded wires, doubling as fuses in error (shorting) situations. The usual failure modes of cells are open or short. Open means less capacity and is not a problem the BMS can't handle. But a short is not something a BMS can solve, the paralleled cells will simply dump all available energy in the shorted cell, so you need a fail-safe, which is typically a fuse. The model S has 74 parallel cells so losing one cell from either open or a short and blowing the fuse wire will affect capacity but very minimal. The BMS will be able to measure a slight imbalance in capacity between the series groups comparing voltage at high and low SoC and thus warn when such an imbalance starts affecting range. Traditionally packs are made from large capacity cell, though Prismatics are often using multiple pouch cells in parallel. Even Lead acid packs have been built with buddy paired batteries and i know of at least one car burned down from a fire in such a pack and another car burned down where a manufacturing defect is suspected in the pouch cells, because the plastic cases burned away and a part of the pouches appeared folded double during insertion into the plastic case. Both cars were not charging when they burned. I currently have a double Leaf pack in my truck. Technically each Leaf module is a parallel of 2 cells and this would need a fuse between them, but i do not want to open each module, i hope Nissan did a good enough job. However, to avoid a problem in one pack to progress to the other (they are in separate box halves), i added a pack fuse in each string (mid pack, at the double pole disconnect switch) so that each pack is only connected at the full voltage and not in between. As consequence, i am running two independent (Leaf) BMSes on the two packs. Hope this gives some ideas. Cor
> On Apr 15, 2017, at 3:09 AM, Chris Tromley via EV <[email protected]> wrote: > > On Sat, Apr 15, 2017 at 3:06 AM, Cor van de Water via EV <[email protected]> > wrote: > >> Robert, >> Welding. Their 24V modules consist of 6 series banks of several dozen >> cells in parallel, >> It really is enlightening to look at some high-res pictures of a module, >> you can even find them >> On Ebay. You will see that each module consists of a "U" where >> alternatingly (starting from one >> Contact) the metal plate with the welded wires carries the current back >> and forth in 3 sections >> In each leg of the "U" and the contacts on the top of the "U". >> It is clearly visible in this ad for example: >> www.ebay.com/itm/10x-Tesla-18650/162462368115 >> Cor. > > > It seems to me the reason for each cell in a parallel group being attached > to that common sheet with a small gauge wire is that if any cell fails > short it will pop that wire as a fuse and prevent all the other cells from > shorting through that cell and setting it on fire. Am I correct in that > assumption? It this something that's missing from all the other > aftermarket BMS systems? > > Chris > -------------- next part -------------- > An HTML attachment was scrubbed... > URL: > <http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20170415/2e931f33/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)
