What is a BMS? Very good question. The small packs I have are a complete unknown. Each cell is wired with a tiny wire so I presume they are making an attempt at top balancing (I am told that is a very large assumption). They both have low cut offs, and they appear to communicate with the chargers they are shipped with so a green light comes on when "charged." In both cases if the chargers were not cutting off they would be reaching a voltage that is less than would overcharge any cell in the pack. One cuts off at 56V and the other 60V on 16 LFP cells.
The low end cutoff is a must to me, but I don't consider that to be a BMS - I think top balancing individual cells or parallel banks is minimal BMS function. I do not believe any BMS bottom balances, you need a heater to do it right. I could be wrong. Hoeberg said: "I guess the answer to the first line will be the reason of doing bottom balance, all might reach "low" at the same time, and to me THAT argument make at least some sense, (if all of them can deliver the same current in all temperatures and the same SOC(Ah left in cells.) )" Every cell or parallel bank sees the same current as every other. As long as you don't have some asymmetric load there is nothing that keeps them all from going low at pretty much the same time. As I understand it, you can take a pack to 0V and if they are all very close nothing bad happens. There isn't enough potential to heat up and burn in this case. I just brought back several cells that were showing me 0.2 and 0.1V. A couple did not come back. I see a lot of swelling in the failed ones, so I suspect abuse. I see some swelling in some that are holding 3.4V and not discharging over the winter. So who knows? You can do a thought experiment about a bottom balanced pack and it looks like if it is done right (proper torque and no depending on Nordick washers, not kidding, but let's not go there again) there is no way to mess it up. They all have the same current history, if the pack is kept tight, no peculiar loading, and a properly selected upper cut off (chosen at the limit of the lowest capacity cell which reaches that limit before any others), there is nothing to cause them to be very different at the low end. The recommendation is to take individual cell readings at some number of cycles (100? 1000?) to see if there is a noticeably bad cell. So you can say 'what if' about battery connection, but you can also say that about BMS hiccups. So I don't go that route. Plenty of failure modes however you do it. The theory is that BB should be just fine if you are careful. I use LFP so fires are pretty much not an issue. I am not saying anything about other chemistries because I am not going outside the LFP fence yet. I am replying to several different other posts which is just a recipe for confusion, so I will stop now. On Tue, Apr 22, 2014 at 1:13 PM, Hoegberg <hoegb...@hotmail.com> wrote: > Yes, I think it is a good idea. > > I did get me 8 pcs of 26650-size LiFePO4 cells, 3Ah (peak/k2/dlg -ev cells) > > Just by using 4 or 8 cells and playing with loggers and multimeters, > charging /discharging at differewnt rates and temperatures, different rates > and time at end of charge and so on, all that vill give a lot of > "experience", get to know the "soul" of a LiFePO4 -chem, and it costs > almost no money to do so. > > > /John > > ---------------------------------------- > > From: evp...@drmm.net > > To: ev@lists.evdl.org > > Date: Tue, 22 Apr 2014 12:38:01 -0400 > > Subject: Re: [EVDL] self-discharge of CALB 180Ah LiFePO4 cells > > > > On 22 Apr 2014 at 9:06, tomw wrote: > > > >> I also agree with Jukka's other comment regarding new people. That is a > >> critical time, when you don't know what you are doing, have no > experience and > >> understanding from repeated measurements on your cells. > > > > This is the reason that when I decided to try lithium, I did it on a > small > > scale and bought a fully configured battery with BMS already included. > Yes, > > I paid more for the capacity I got, but it WORKS, and I'm pretty > confident > > that I won't have any harrowing experiences with it. > > > > I don't always succeed, but I try to know my limitations. > > > > 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. To send a private message, please obtain my > > email address from the webpage http://www.evdl.org/help/ . > > = = = = = = = = = = = = = = = = = = = = = = = = = = = = = > > > > > > _______________________________________________ > > 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) > > -- 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/20140422/9583236d/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)
