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.
> >>
>
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