Thanks Paul. Yeah not quite the same but somewhat similar. I'm assuming that the Alibaba battery would have a higher finishing voltage ~108 volts vs the Tesla pack which would be 24s or 100.8. I should also be able to push a lot more amps out of a Tesla pack if required down the road. I like the fact that the Tesla pack is likely much more configurable as the voltage/ amperage is what I use for BMS/ cell layout but also dont like it for the same reason lol. I can be great to dangerous in my projects. It's down to the unknown turn-key solution vs DIY. using known good (but used) products ,
On Mon, Mar 18, 2019 at 12:39 PM paul dove <[email protected]> wrote: > Well, I am not sure you are comparing apples to apples. All Lithium Ion > batteries are not equal in power density or energy density. > > A Tesla model S battery cell goes up to 4.2 volts. > 2modules x 4.2V x 444cells x 2.8Ah = 10.4kW > So 2 x modules of 6s74p (444 cells) =10.4kWh > > While your 96v100ah pack on Aliexpress may be Lithium Iron Phosphate > 96 x 100 = 9.6Kw and may have lower discharge rates. > > On Monday, March 18, 2019, 7:19:01 AM CDT, Dan Baker via EV < > [email protected]> wrote: > > > 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 <[email protected]> > 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 <[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) > > >> > > >> > > _______________________________________________ > > 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/20190318/eaded1dd/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... 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