Discharging to 0%SOC is not a realistic condition for Li ion cells in a
large pack, and I am not aware that PbSO4 are taken below 50% fro greatest
life (though I don;t know the why and wherefores of this). With Li ion you
get more usable capacity  - down to 15% or 20%SOC. Not going to 0% is about
not pushing individual low capacity cells around the bend and turning them
into a short in the pack.  An important consideration for EV manufacturers
is getting cells that are very consistent in their performance.  This
allows them to safely push the cutoff voltage lower. I understand that
Tesla packs call ~80% to be 100%, and ~20% to be 0% to ensure the range
does not drop in anything close to the near term of the pack.

Taking a Li ion cell to 0%SOC is not an issue because at that condition the
electrodes and electrolytes are not reactive at all; but, when near
100%SOC, coupled with excessive heat, this causes the very reactive de-
lithiated positive electrode to damage the electrolyte.

So a small appliance with only a few  Li ion cells, going very close to
0%SOC, is not as likely to be troubling. Cooking them in a cheap charger
that get hot and stays hot at fully charged, is a good way to ruin cells.

The cell chemistry has a lot to do with the ability to withstand high SOC.
LiFePO is particularly susceptible to damage at lower temperatures (even as
low as 95°F) with 100%SOC is going to cause trouble. Whereas the better
chemistries and with judicious use of additives to hold off damage to the
cells can handle higher temps. Managing the pack to not actually go close
to 100%SOC also speeds up charging.  They nave really float charge them, so
you average a higher C during a "full" charge.

It is fortunate that driving after a full charge quickly brings the SOC
down away from the reactive 100% SOC state. It is always best to store Li
ion cells at something less than 100%SOC. I have heard 80% suggested as a
good SOC for storage.

On Sat, Mar 16, 2019 at 2:20 PM paul dove via EV <ev@lists.evdl.org> 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.
> >
> > SNIP

-- 
Michael E. Ross
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