Dan Baker, There is nothing wrong with taking a Li ion cell down to 0%SOC (which is not a great idea with PbSO4). The problem is when you have a multitude of them there is some variation between them. Over time, they get misaligned in terms of capacity. There are cell balancing routines to help even them out. You want them aligned at the low end of SOC so that they all come down to the same level near but not at 0%SOC. If you don't and one goes to 0% an you keep discharging the pack that low cell can reverse and short out.
That short circuit can cause damage. You don't want to risk heat and its bad effects, nor do you want to hurt other costly cells in the pack. Smart pack manufacturers like Tesla have a fusible link in series with every cell, even if there are 7000 of them, then if a cell shorts it just blows its fuse and is no longer in the pack functionally. So you only get the top 50% soc WITH PbSO4, but with Li ion you get the top 80%, or 90% of capacity, whatever you can successfully manage. Cell management is a very important piece of the puzzle for anyone using batteries for a vehicle whether DIY or OEM, lead or lithium. You said: "100 a/h battery is typically only 80 a/h?" No cell manufacturer is being disingenuous telling you the full capacity of the cells. It is up to the application and the designer to determine how, and how much of that capacity can be used. On Sat, Mar 16, 2019 at 8:20 PM Dan Baker via EV <ev@lists.evdl.org> 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 <ev@lists.evdl.org> > 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 <ev@lists.evdl.org> > > 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) > > -- 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/20190317/8006741b/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)
