I wasn't and I think you misread what I said. I was referring to the
bottom right graph entitled discharge characteristic. I actually said
that at 0%soc that the capacities were similar. However in real life
there will be a bms system that will prevent reaching that level and it
does this by creating an artificial cutoff voltage where the batteries
are disconnected if the voltage goes below this. To see what the
capacity actually is now, draw a horizontal line at the 3 volt level (or
whatever BMS cutoff voltage you are using) and drop a perpendicular down
where the discharge curves intersect. You will then get the different
capacities I was referring to. I actually said that at the 0% soc level
that the capacities are similar, but that because the BMS never allows
the voltage to get that low and that they cutoff at a higher voltage
e.g 3v or thereabouts, you get reduced capacity.
Yes there are so types that can get as low as 1.08 but generally the
batteries have a constant of 1.2 or higher. I did use the word typically.
BTW I did find that paper
(https://www.slideshare.net/componer/a-critical-review-of-using-the-peukert-equation-for-determining-the-remaining-capacity-of-lead-acid-and-lithium-ion-batteries)
and they tested the batteries down to their cutoff voltage so it was not
surprising that the capacities are similar with the different discharge
rates. However they did say that they suspected that if the li ion
battery had been kept at the same temperature as the low discharge i.e.
keep it at 25C instead of 50C, they expected the capacity for the higher
discharge to be less. This then raises the question of if it is good for
cycle life etc to allow this. If cooling is then used, a capacity
reduction when discharging heavier is to be expected.
On 16/03/2019 11:40, 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.
On 15/03/2019 10:20, paul dove via EV wrote:
Peukert's law was developed for Lead-Acid batteries, and works well in that
application.
It does not necessarily apply to other battery chemistries, especially
Lithium-Ion batteries. Lithium-Ion batteries tend to self-heat during rapid
discharge, and the Nernst Equation predicts battery voltage will increase with
temperature. Thus, the effect of increased resistance is offset by the
self-heating effect. This advantage of Lithium-Ion batteries is a well-known
advertised feature. In a research paper, a 50Ah lithium-ion battery tested was
found to give about the same capacity at 5A and 50A; this was attributed to
possible Peukert loss in capacity being countered by the increase in capacity
due to the 30◦C temperature rise due to self-heating, with the conclusion that
the Peukert equation is not applicable.
https://www.google.com/amp/s/www.researchgate.net/publication/245106038_A_critical_review_of_using_the_Peukert_equation_for_determining_the_remaining_capacity_of_lead-acid_and_lithium-ion_batteries/amp
Sent from my iPhone
On Mar 14, 2019, at 10:19 PM, Lee Hart via EV <[email protected]> wrote:
Michael Ross via EV wrote:
I am not sure about previous discussions and you may know this: Peukert's
Law is not applicable to Li ion cells in any way. It only relates to lead
acid cells.
I agree with the rest of what you said, but not with this. Peukert's law says
nothing about the chemistry involved; it applies to *all* types of batteries
and all chemistries.
Peukert's equation applies to any battery or cell that has internal resistance, and that has a
minimum "cutoff" voltage below which it is harmed. It simply states that the higher the
load current, the lower the apparent amphour capacity. High currents cause a larger voltage drop,
so you reach the "cutoff" voltage before the cell is truly dead.
The amphours are not "missing"; you just can't get them without reducing the
load current, or pulling its voltage below the safe minimum. If you're willing to shorten
the life of the cell, you can still get it.
Peukert matters more for lead-acids because they typically have a higher
internal resistance. In particular, lead-acid internal resistance goes up a lot
as the cell approaches dead. Most other chemistries do not have this large
change in internal resistance as a function of state of charge.
--
Any intelligent fool can make things bigger, more complex, and more
violent. It takes a touch of genius, and a lot of courage, to move
in the opposite direction. -- Albert Einstein
--
Lee Hart, 814 8th Ave N, Sartell MN 56377, www.sunrise-ev.com
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