Thank you for the reply Lee. "Far too often, the advertising is wrong; sometimes blatantly wrong!"
When you buy a small pack, and they commonly come with a built in BMS and a separate charger, there is no advertising at all, no specs, no nothing. Asking does no good, they simply have no idea how to answer questions about these things. " You could try it at 2.5v too. That would get them even closer. But at 2.5v, the evidence suggests that you have also discharged them so deeply that you're shortening their life." In my reading about battery research I have not found anything indicating that taking them low is damaging. If you have an unbalanced pack it can go bad to be sure. I am guessing now, maybe really high C discharge rates cause something different to happen at low SOC? Can you mess up by discharging at low temps (-20C)? Can you have trouble on the cathode side with too great C and too low temperatures like with charging on the anode side and plating? I haven't read anything about that. Anyway, do you have any idea what mechanism causes harm to a cell by taking it low? I am hoping for some sort of evidence as opposed to supposition about a field failure. On Mon, Mar 10, 2014 at 2:52 AM, Lee Hart <[email protected]> wrote: > Michael Ross wrote: > >> I will put my JLD 404 in to provide a lower limit (I think it can also do >> an upper limit too). The 60VDC, 6A charger I have stops at some point, >> but >> it is one of those mysterious, cheap Chinese, jackleg things with some >> LEDs >> added and stickers peeled off. It has an IEC plug to go into the BMS sent >> with it (for another pack). Very not kosher. The JLD also provides me >> with an estimate of Ah used. >> > > The first thing I do when I get a new chargers is to put my own meters on > it, and watch what it *really* does. Far too often, the advertising is > wrong; sometimes blatantly wrong! > > > I do get some extra protection if I use 17 cells, which is probable. >> > > Agreed. It's always a prudent strategy to get a couple extra batteries or > cells. If you don't, Murphy will almost certainly make sure you have bad > ones! > > It also helps extend the life of just about everything if you don't push > it right to the maximum. Don't run a "72-120v" controller at 120v. Don't > put 60 amps through a "60 amp" transistor. You get the idea. :-) > > With batteries, it means don't take 100ah out of a 100ah battery. And > don't charge it to the maximum rated charging voltage; it may maximize > amphour capacity in the short run, but will shorten life in the long run. > > > Why is 2.75V right for BB'ing a LiFePO4 pack? What is that based on? >> > > Experimentation. > > - Suppose you discharge three 100ah cells to (say) 3.0v. Are they at the > same state of charge? Let's see... *Keep* discharging them while counting > amphours to 2.5v. You'll find they deliver noticeably different numbers of > amphours (like 10ah, 15ah, and 20ah). So you haven't really bottom balanced > them. > > - Suppose you discharge them to 2.75v. Now if continue discharging them, > you get something like 2ah, 4ah, and 6ah more. They still aren't identical; > but they're close to dead and much closer together. > > - You could try it at 2.5v too. That would get them even closer. But at > 2.5v, the evidence suggests that you have also discharged them so deeply > that you're shortening their life. > > So, 2.75v is a "good enough" number; not too high, and not too low. It's a > compromise. > > -- > ICEs have the same problem as lightbulbs. Why innovate and make > better ones when the current ones burn out often enough to keep > you in business? -- Hunter Cressall > -- > Lee Hart's EV projects are at http://www.sunrise-ev.com/projects.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/20140310/c4c681de/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)
