Yes, that is my understanding too. It is a redundant protection. The cells don't get hurt if they overdischarge - that is a root cause solution. When you top balance you are setting up the possibility for a significant over-discharge and are completely dependent on the active lower cutoff function to protect the pack.
WIth BB'ing you may want a lower cutoff (like from a JLD404) if you want to limit the %SOC you use for long life. That is my plan anyway. I understand the JLD, I don't understand the anonymous BMS that I get with the small LFP packs that I use. The recipe for BB'in includes checking the cells or banks to see which reaches the per cell upper cutoff voltage first. Then setting the upper cutoff for the whole pack at the pack voltage when that first cell (the one with the least capacity) is topped out. As a CYA most people want to look at cell voltages periodically to see if a cell is dieing. WIth all new, good quality cells, maybe this is a waste of time. Some think so. If I run 17 cells and charge with 60VDC the upper limit is 3.53V for cells that are similar. I am not sure what happens with my salvage pack that clearly has some cells of uncertain condition. I think I may also implement an upper cut off. I don't know how BB'ing could be implemented in an OEM production process, it seems much more appropriate for DIY'ers that like the tedium. Top balancing solves needing to provide individual attention to each cell or bank that BB'ing requires. A company that has smart vendor management (I have actually seen such a thing in practice) you could get the battery vendor to provide cells of appropriate quality that are bottom balanced. Then you could omit BMS entirely. Perhaps monitoring cell voltages to flag problem cells, but not trying to adjust them. You would also need to have separate batteries for asymmetrical tasks. Like having a 12VDC system and the encapsulated higher voltage propulsion pack. I see that Jack Rickard is toying with offering "life time" LFP packs for his golf cart business. BB'd, and armored, insulated, and tamper proof. He even encapsulates them so they can't be mucked about with. I am curious to see how this works out over years. He has to charge a lot of money for them because the BB'ing is so laborious. It says something about my nerdiness and pedantry that I find this topic so interesting. On Fri, Mar 7, 2014 at 11:27 AM, Rick Beebe <r...@beebe.org> wrote: > On 03/07/2014 12:43 AM, EVDL Administrator wrote: > > My poorly-informed view of bottom balancing is that I'd better not try > it. I > > get the impression that it takes more time and attention than I care to > give > > to charging batteries. Not that I want to sound cavalier or anything, > but > > dang it, I have too many other things I like doing! Without a smart BMS > > looking after my battery, I'd probably murder it. > > Bottom balancing--in general--in a one shot deal. It's a pain in the > butt to set the pack up. You trying to make sure all the cells are > depleted to the exact same level. The idea is that once they are, they > will charge and discharge at the same rate and stay together. By > subsequently avoiding the very top and bottom of the charge curve you > compensate for any slight differences. > > Of course it assumes that the internal resistance of these cells is > close enough that you won't get substantial cell drift. You should check > the cells on occasion to make sure none have gone walkabout but the > difficult part of bottom balancing is just done once. > > The purported advantage of bottom balancing--in addition to a simple or > no BMS--is that if you drive the pack too low all your cells will run > out of juice at the same time. It becomes very obvious to you that the > pack is dead and none of the cells has enough power left in them to do > any damage to their neighbors. > > --Rick > _______________________________________________ > 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) > > -- Put this question to yourself: should I use everyone else to attain happiness, or should I help others gain happiness? *Dalai Lama * Tell me what it is you plan to do With your one wild and precious life? Mary Oliver, "The summer day." To invent, you need a good imagination and a pile of junk. Thomas A. Edison<http://www.brainyquote.com/quotes/quotes/t/thomasaed125362.html> A public-opinion poll is no substitute for thought. *Warren Buffet* Michael E. Ross (919) 550-2430 Land (919) 576-0824 <https://www.google.com/voice/b/0?pli=1#phones> Google Phone (919) 631-1451 Cell (919) 513-0418 Desk michael.e.r...@gmail.com <michael.e.r...@gmail.com> -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20140307/49f15f7b/attachment.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)
