Steve, the ratings are per cell. WHat else is there to do? No management is needed as with a pack. Measuring the capacity of a cell is pretty straightforward. It is up to the user to determine the needed capacity of a pack and how to take care of it.
On Sat, Mar 16, 2019 at 6:31 PM Steve Heath via EV <[email protected]> wrote: > There are lies, damn lies and battery AH data. > > Yes you are right in that even with a "100aH Liion battery" I would > derate it by 80% or so because of the BMS and so on. > The problem with many Ah figures is that you do not know how they were > measured. Is this with or without the BMS? > > Most companies take the aH from each cell in the series string and add > them up. Some pluck a figure out of the air! > > 80% of the manufacturer figure is a good place to start but that might > be a tad high in my experience with those size of currents. > > Steve > > > On 16/03/2019 21:51, Dan Baker via EV wrote: > > Wow, lots of learning here. So with Lithium, a safe BMS cut-off > typically > > kicks around when there is less than 20% remaining? So the a/h ratings > > typically (and when truthfully) displayed are actually 20% less? 100 a/h > > battery is typically only 80 a/h? This is fine as I know you can't get > > 100% of out lead either. I looked up my SBS-170f lead's datasheet, to > take > > the cells to 1.80VPc (half discharged I believe) will happen with 116 > amps > > for 1 hour. This is recommended bottom voltage and I typically see that > > with my boat as the draw is pretty constant when cruising. To take the > > leads all the way to near complete discharge (damaging but not > exploding) - > > 1.60 Vpc the amps for one hour is 125 amps so not much more. So with a > 200 > > ah pack I can expect about 1.6 hours of run time at 100 amps? In > reality > > I think I can probably get closer to 2 hours if I keep the speed the same > > and reduce the amps as the weight loss will dramatically reduce water > > drag. > > > > On Sat, Mar 16, 2019 at 6:08 PM Bill Dube via EV <[email protected]> > wrote: > > > >> No Paul, Lee is indeed referring to the rate of discharge chart, > >> however, he has chosen the cut-off to be _*3 volts*_, rather than the > >> customary cut-off of_*2.5 volts*_. (No one uses a cut-off of 3 volts, > >> that I am aware of. All the charts note that 2.5v cut-off is the > >> standard for comparison. If we picked 3.5 volts as the cut-off, we would > >> get a huge spread in the apparent capacity, but that would be silly.) > >> > >> You are correct that the 12 minute discharge (0.2C rate), the 0.5C rate, > >> and the 1C rate all show the same capacity, 3.25 mA-hr. While the 2 hour > >> discharge (2C rate) shows a slightly elevated capacity of 3.350 mA. > >> > >> I suspect that the faster rates had some unavoidable internal > >> heating, (even though the case temperature was held at a constant 25 > >> degrees Celsius,) which tends to decrease the internal resistance, and > >> tends to raise the terminal voltage under load, especially when the > >> impedance rises near the end. Thus, the apparent capacity shift is quite > >> likely due to increased internal temperature rather than ion diffusion. > >> > >> Lead acid curves would have shown a much greater sensitivity to > the > >> discharge rate. Much greater. As I said earlier, the ions can diffuse > >> perhaps 100 times more quickly in Li-Ion cells than in lead-acid cells, > >> which makes the Puekert exponent very close to unity in Li_ion. Puekert > >> is not really useful in Li_ion because the diffusion is so fast in > Li-Ion. > >> > >> Bill D. > >> > >> > >> On 3/17/2019 12:40 AM, paul dove via EV wrote: > >>> That’s not what the spec sheet says. You are reading the graph for > >> temperature variations. There is almost no difference due to discharge > >> rates. 2C is 3250 and 0.5C is 3350 according to your spec sheet. > >>> And lead acid batteries have a Puekert coefficient as low as 1.08. > >>> > >>> Sent from my iPhone > >>> > >>>> On Mar 15, 2019, at 9:14 AM, Steve Heath via EV <[email protected]> > >> wrote: > >>>> Peukert's law is not an actual law but an empirical formula that is > >> based on actual physical measurements. It gives an approximate estimate > of > >> how much capacity can be obtained. The way that it is used is that the > >> capacity is measured at different discharge rates to give a co-efficient > >> that can then be applied to other batteries. This is where the > difficulty > >> lies. The coefficient is taken by measurement and providing another > battery > >> is the same then the coefficient is applicable. If not and it isn't. > >>>> The key point is that the discharge curves for li ion batteries do > vary > >> significantly depending on the load in real life according to the > >> manufacturer data. At the 0% soc end point, the capacities are the same > >> (give or take). This is why the Peukerts coefficient is close to 1 > rather > >> than 1.2 or higher for a lead acid battery. Hence the comment that it is > >> not applicable. It is there but very small to be accurate. However at a > >> typical self preservation point e.g cutoff voltage used by BMS, the > >> capacities are different. As a result, there is a "Peukerts" effect > where > >> the amount of capacity that can be obtained is different depending on > the > >> discharge current. It is not the same Peukerts effect but the end > result is > >> the same. Discharge more, less capacity... > >>>> The data sheet for a Panasonic 18650 shows this effect very well ( > >> https://www.batteryspace.com/prod-specs/NCR18650B.pdf ) where a cut off > >> voltage of 3v gives a capacity of 2400mAh at 2c and 3300 mAh at 0.2C > . At > >> the 0% soc point they all come out at 3300 and 3400. So discharging to > 0% > >> soc, the discharge current is more or less irrelevant. Interestingly > these > >> results are taken at constant cell temperature where any overheating > >> advantage is not applicable. Without seeing the complete paper that was > >> referred to, it is difficult to know if any comparison with manufacturer > >> data was made or whether tests were done at constant temperature and > what > >> the results were. > >>>> Discharging to a lower 15-20% level to protect the battery, there is a > >> big difference. If you want to get the best capacity out of a li ion > >> battery with a BMS, either reduce the discharge rate or change the BMS > to > >> accept a lower cutoff voltage and risk battery damage. > >> -------------- next part -------------- > >> An HTML attachment was scrubbed... > >> URL: < > >> > http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20190317/1ddbc7eb/attachment.html > >> _______________________________________________ > >> UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub > >> http://lists.evdl.org/listinfo.cgi/ev-evdl.org > >> Please discuss EV drag racing at NEDRA ( > >> http://groups.yahoo.com/group/NEDRA) > >> > >> > > -------------- next part -------------- > > An HTML attachment was scrubbed... > > URL: < > http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20190316/f6f8af5b/attachment.html > > > > _______________________________________________ > > UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub > > http://lists.evdl.org/listinfo.cgi/ev-evdl.org > > Please discuss EV drag racing at 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 > Please discuss EV drag racing at NEDRA ( > http://groups.yahoo.com/group/NEDRA) > > -- Michael E. Ross (919) 585-6737 Land (919) 901-2805 Cell and Text (919) 576-0824 <https://www.google.com/voice/b/0?pli=1#phones> Tablet, Google Phone and Text -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.evdl.org/private.cgi/ev-evdl.org/attachments/20190317/c277fedc/attachment.html> _______________________________________________ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org Please discuss EV drag racing at NEDRA (http://groups.yahoo.com/group/NEDRA)
