Exactly. This is why every lithium manufacturer recommends and uses a bms.
On Fri, Feb 15, 2013 at 10:20 AM, Lee Hart <[email protected]> wrote: > On 2/15/2013 12:12 AM, Bruce EVangel Parmenter wrote: > >> -What voltage would be a good disconnect voltage set point for that 12V >> Li-ion drop in battery? What voltage would prevent draining the Li-ion >> battery too low causing damage? >> > > With a 12v lead-acid, the traditional cutoff voltage under load is 10.5v. > I suspect that the commercial cutoff devices aren't that precise, and > probably cut off at anything from 9v to 11v. > > However, lead-acids have a useful feature. Their internal resistance goes > up as their state of charge goes down. Before the cells go dead, their > resistance gets high enough to cause an excessive voltage drop under load. > This rather sudden voltage drop it what triggers the cutoff device to > disconnect the load. > > Suppose all the cells are matched (same amphour capacity, same initial > state of charge). Then when you hit 10.5v, the 6 cells in a 12v battery are > each 1.75v under load. They aren't actually dead at this point. If you > remove the load, the resistive voltage drop goes away, and they immediately > bounce up to 1.95-2.0v. You haven't actually hurt the battery. Charge it > back up, and it will be fine (subject to whatever wear and tear a deep > cycle takes). > > Now, suppose the lead-acid cells are *not* all matched and balanced. When > it reaches 10.5v, you may have 5 cells at 2.1v, and one cell at 0v. The one > at 0v has just been *murdered*. This battery is not going to come back. > Even if it seems to recharge, it will have lost most of its amphour > capacity. > > Now let's try this same cutoff device with lithium. If you have a BMS, at > 10.5v each of the 4 lithium cells has 2.625v. That will be fine. They're > not dead yet, and they can be safely recharged and will survive. > > But suppose there is no BMS. Lithiums do not have the large resistance > increase as they approach dead. The weakest cell keeps right on trying, > right up to a few moments before it drops dead. At the 10.5v cutoff, you > may have 3 cells at 3.1v, and one at 1.2v. The 1.2v cell's voltage won't > bounce up to a "safe" voltage range. It has been seriously damaged, and > won't recover. And without a BMS, you won't *know* you've damaged it, and > may blithely keep using it, potentially leading to a fire! > > So that's the situation as I see it. When everything is fine, it works. > When something goes wrong (like the cells wandering out of balance, or > leaving the lights on, etc.), there is a risk of failure. If things go very > wrong, the failure can be dramatic! > > -- > For every complex problem, there is a solution that is simple, obvious, > and wrong. -- H.L. Mencken > -- > Lee A. Hart, > http://www.sunrise-ev.com/**LeesEVs.htm<http://www.sunrise-ev.com/LeesEVs.htm> > ______________________________**_________________ > UNSUBSCRIBE: > http://www.evdl.org/help/**index.html#usub<http://www.evdl.org/help/index.html#usub> > http://lists.evdl.org/**listinfo.cgi/ev-evdl.org<http://lists.evdl.org/listinfo.cgi/ev-evdl.org> > For EV drag racing discussion, please use NEDRA (http://groups.yahoo.com/* > *group/NEDRA <http://groups.yahoo.com/group/NEDRA>) > > -- Marcus Reddish *North Valley Systems LLC* Stevensville, Montana 406-360-8628 northvalleyev.com -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20130215/261d09d7/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)
