True enough, but if a 200Ah cell fails shorted, it still has 200Ah to dump all by itself, which is likely to be equally catastrophic. I think the probability of occurrence is higher with 200 1Ah cells than with 1 200Ah cell, but the end result is the same.
On the other hand, if you DO take precautions to isolate the parallel cells in a fault, you can limit the total energy released to the amount stored in a single cell (or a handful of small cells). If I recall correctly, Tesla does this, including firewalls between modules, which is how they can get away with using 1000's of cells in a single pack. -Ben On Aug 19, 2014, at 7:32 PM, Lee Hart via EV <[email protected]> wrote: > Peri Hartman via EV wrote: >> To take this to a traction pack, I would want to try a similar design >> where strings of cells are spot welded in parallel for the desired >> current load. As I understand it, you can put as many in parallel as you >> want and control them with a single BMS unit. > > Well... There are those that believe this, and it is certainly done. > > However, think about failure modes: Sooner or later, a cell will fail. What > happens if one of the cells in parallel shorts? All the rest will dump all > their stored energy into that one shorted cell. That is very likely to result > in a burst cell or even a fire. And packed like this, once one cell starts > burning, it can set the rest on fire. > > When laptop manufacturers connected their cells directly in parallel, there > were some dramatic failures and fires. Since then, reputable manufacturers > are using cells with *internal* fuses and safety devices. Hobbyists and > el-cheapo manufacturers aren't likely to use such cells. > > -- > The principal defect in a storage battery is its modesty. It does not > spark, creak, groan, nor slow down under overload. It does not rotate. > It works where it is, and will silently work up to the point of > destruction without making any audible or visible signs of distress. > -- Electrical Review, 1902 > -- > Lee Hart's EV projects are at http://www.sunrise-ev.com/LeesEVs.htm > _______________________________________________ > 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) > _______________________________________________ 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)
