Thank you everyone for the great response so far. I am still communicating with a few Aliexpress sellers although it has been quite frustrating. So far none have been able to give me any documentation on these packs- cell type and count, string info, bms used, etc. I have been able to weed a few out by reviews and overzealous claims. Product weight seems to be a tell tale as well, how can a pack weight half as much as other packs and have the same claimed a/h rating?
After some more digging this weekend I have found a Canadian seller of used Tesla packs. I think I can get 2 x modules of 6s74p (444 cells) from a low mileage 2015 P85D for around the same price as a 96v100ah pack on Aliexpress. I would still have to buy a BMS and likely a charger. I have saw some articles on cutting the bus bars and turning it them into 12s37p packs so I could get 2 x 48v packs and 96 volts total so I can eliminate my noisy gear set. I could also buy 4 packs but that would be way more pack that I need at this time. Anyone ever hack a Tesla pack to make it 48 volts? Cheers Dan On Sun, Mar 17, 2019 at 6:20 AM Steve Heath via EV <ev@lists.evdl.org> wrote: > Michael: totally agree. At the cell level you are right and in the end > the user has to take responsibility. The challenge facing people that a > single Liion cell is not going to cut it so they have to be assembled > into packs and that is where the differences start coming out. What > follows is that cell level properties do not automatically > follow/scale/apply to the battery pack. > > At the cell level Liion shows virtually zero Peukerts effect. At the > pack level with a BMS, it will. The cause is very different though. > > At the cell level, 0%age discharge is not damaging. At the pack level, > cells can go out of balance and potentially short out. BMS will prevent > the 0%age from being reached as part of the system level management > strategy. > > At the cell level, there is nothing stopping Liion cells from being > connected in parallel or series. At the pack level, parallel operation > is not a problem but in series, the BMS can fail. > > At the cell level, you can extract all the capacity. At the pack level, > this is difficult with the BMS applied. So if 100aH is the minimum > needed in reality, a bigger capacity battery is needed. > > I think the problem is that we have become so used to treating cells > like lego that it is easy to forget about the care and management side > of Liion. Equally it can be very confusing when cell level and system > level data/behaviour is compared because they can be almost > contradictory which makes it very confusing to people trying to > understand what is going on. > > > > On 17/03/2019 04:10, Michael Ross via EV wrote: > > 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 <ev@lists.evdl.org> > > 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 <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) > >> _______________________________________________ > >> 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) > > -------------- next part -------------- An HTML attachment was scrubbed... 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