Marcus, A reasonable question. For LFP I think you want to go lower like 1.5V maybe. I think a lot of the historically acceptable target voltages are arbitrary (adopting a RIckard opinion). 1.5V might even be higher than necessary. 1.5V is perhaps a reasonable cut off voltage for a bottom balanced LFP pack. I don't think there is anything definitive about this.
Voltage being such a crappy surrogate for State Of Charge you definitely want to be far away from the flat part of the V/SOC curve where small changes in V are large changes in SOC. Around 1.5V there a la much greater change in V for a small change in SOC. When BB'ing you want a low open circuit voltage....the way I have seen it done, the cell is loaded down, and at a target voltage the load is lifted, the voltage recovers, the load is added, the load is lifted, again and again until the voltage settles where you want it and the recovery is very small. Best to automate this if you can. You can throw a 20 foot rebar out on the sand and run 1000A through it if that works for the cell in question and the C rate you desire. I think if you put two cells in parallel, and you run them to a low target, you have the same old issue that BB'ing is trying to cure maybe? If one cell goes empty before the other , the other cell has significant charge left in it, then a loop gets going between them lots of current flows backwards through the empty cell and one or both of them are cooked. I think. FWTW On Sat, Mar 8, 2014 at 7:04 AM, Marcus Reddish <[email protected]>wrote: > Am I crazy or could bottom balancing be done at 3V with all the cells in > parallel? They will all be perfectly balanced together at 2.75V or > whatever you chose. Use a JLD and contactor with a resistive load to get > down to the target voltage. Reconnect them in series and charge. > > Please note that hooking them in parallel for balancing would only work at > the extremes (low for BB, high for TB). If you hook them in parallel at > half full (3.2v) they won't be balanced at either extreme. Obviously, this > only would work for groups of individual cells, not pre-made modules. Are > there any issues with this approach? > > > On Fri, Mar 7, 2014 at 1:36 PM, Michael Ross <[email protected] > >wrote: > > > There is a real possibility that BB'ing saves you more than you put into > > it, but this is pretty speculative. > > > > There are hidden costs too - I have to come up with a resistive load than > > can dump significant amps (rebar and an 200Ohm wire wound). Some people > > just jack up the vehicle and spin it, but that is unappealing to me when > I > > can always use an excuse to weld something. You need contactors, and > > JLD404s and shunts, stuff like that. Once you have it you can do more > > packs. The tools are general purpose. > > > > I investigated DIY BMS and this all seemed easier. > > > > > > > > > > > > On Fri, Mar 7, 2014 at 3:06 PM, EVDL Administrator <[email protected]> > > wrote: > > > > > On 7 Mar 2014 at 3:08, Michael Ross wrote: > > > > > > > So there is an appeal to bottom balancing in terms of life, cost > > > > (ignoring labor, but we are DIY'ing so that is ignored always), and > > > > ease of use. > > > > > > I see your point, but I don't agree that DIY labor cost is "ignored > > > always." > > > OK, it's not like I'm paying a mechanic, but every hour I spend under > the > > > hood or under the car is an hour I don't have for other things I like > > doing > > > just as much or more. > > > > > > But hey - if you have the time and you enjoy it, why not try? > > > > > > David Roden - Akron, Ohio, USA > > > EVDL Administrator > > > > > > = = = = = = = = = = = = = = = = = = = = = = = = = = = = = > > > EVDL Information: http://www.evdl.org/help/ > > > = = = = = = = = = = = = = = = = = = = = = = = = = = = = = > > > Note: mail sent to "evpost" and "etpost" addresses will not > > > reach me. 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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 > > > > [email protected] > > <[email protected]> > > -------------- next part -------------- > > An HTML attachment was scrubbed... > > URL: < > > > http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20140307/5fa24547/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) > > > > > > > -- > Marcus Reddish > -------------- next part -------------- > An HTML attachment was scrubbed... > URL: < > http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20140308/fba10f4c/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) > > -- 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 [email protected] <[email protected]> -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20140308/13a27b04/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)
