# "Short Discharge Time" is now public



## pcsrule (Dec 14, 2012)

A new supposedly more reliable metric than the infamous "C rating" is all well and good, but I'd like to know if the resulting time given by this equation pertains to anything in real life. In other words, if a cell has a short discharge time of 200 seconds, does this mean that is the minimum amount of time the cell can be discharged in? The time per amp hour? Have you experimented with this metric at all as it pertains to actual real-world discharge capabilities?

Looking at you equation it seems to provide no more information than the internal resistance of the cell. Multiplying/dividing by voltage and capacity just allows the resulting value to be compared between batteries (simply derived from V=IR).

I like the idea of being able to compare batteries this way, but I suggest adding some further explanation as to what this calculated value actually means, other than a number to compare in which lower is better, as some may take it to mean something it doesn't. Thanks for putting in the time to come up with this, it's sure nice to see something more reliable than a C rating.


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## Semper Vivus (Apr 13, 2011)

Elithion said:


> A lower Short Discharge Time indicates that the cell or battery is more efficient, which is important in power applications.


Hello Davide,

so a lower SDT indicates a lower power loss. But isn't it also important to look at the thermal behavior of the individual cell, when it comes to power applications? I.e. a "Winston style" prismatic (plastic case) can have the same (or slightly better) SDT as its cylindrical alternative. However the cylindical cell could be the better alternative for a power application, because it just can handle the heat better. I think that fact is considered by the C-Rating?

Kind regards
Tom


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## Yabert (Feb 7, 2010)

I really like power density graph (figure 7)

What quantity of samples do you tested to write your study?
One cell of each brand? Couple of cells? Or hundred of cells?


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## Elithion (Oct 6, 2009)

Hello pcsrule



pcsrule said:


> I'd like to know if the resulting time given by this equation pertains to anything in real life.


Heck, necessity is the mother of invention. It was to answer a real life necessity that I came up with this concept. (By the way, after publishing it, I am now hearing from others who came up with exactly the same concept, independently of me.) My need was this: how do I pick the best cells for a battery with a continuous 4 C discharge? How can I easily compare the efficiency of a battery built out of those cells, regardless of the capacity, voltage or chemistry of the cells?



pcsrule said:


> if a cell has a short discharge time of 200 seconds, does this mean that is the minimum amount of time the cell can be discharged in?


No.
It means that a battery built out of those cells will waste a lot of heat in a power application, compared to a battery that is built with cells with a Short Discharge Time of 20 s. Ten times as much heat, in fact.



pcsrule said:


> does this mean that is the minimum amount of time the cell can be discharged in?


Oh no! Discharging a cell through a short would be both unwise and useless.



pcsrule said:


> The time per amp hour.


I am not sure what that means, but no.



pcsrule said:


> Have you experimented with this metric at all as it pertains to actual real-world discharge capabilities?


Yes. That is how we picked Enerdel cells for our high power traction packs.
And Short Discharge Time allowed us to calculate how much heat the battery would put out, so we could design a liquid thermal management system that was just right. Experimental results matched the predictions.



pcsrule said:


> Looking at you equation it seems to provide no more information than the internal resistance of the cell.


On the contrary: the equation allows the resistance to drop out. That way, the same Short Discharge Time value can be used for, say CALB cells, regardless of them being 40 AH, 100 Ah or 200 Ah. Those cells all have different resistances, but they all have the same Short Discharge Time value: 143 seconds. Quite handy.



pcsrule said:


> Multiplying/dividing by voltage and capacity just allows the resulting value to be compared between batteries.


Exactly! That's exactly the beauty of it: it allows the resulting value to be compared between batteries.



pcsrule said:


> I suggest adding some further explanation as to what this calculated value actually means.


That's what the article describes. If you think it's better that I repeat that in this forum, instead of directing people to the article, please say so, and I will do so.

Thanks for taking the time, I appreciate it.


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## frodus (Apr 12, 2008)

Note on Headway specifically:

I'll be looking into the SDT for Headway in the near future. I've got some data that seems to rate the Headway better than his graph shows, but Davide and I would like me to do an independant test with my own equipment following his procedure before any results are revised.


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## palmer_md (Jul 22, 2011)

As was asked earlier in this thread, I'm interested in how many of the cells were tested empirically versus off of the spec sheet. Perhaps have a separate graph of all the ones tested versus the ones from the spec sheets. 

As the technology is also rapidly changing it might also be nice to have the manufacture date be listed with the part number as well. Cells built in 2009 are likely to be different from cells built in 2012 even with the same part number.


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## Elithion (Oct 6, 2009)

palmer_md said:


> I'm interested in how many of the cells were tested empirically versus off of the spec sheet.


PM me and I'll send you the source data.


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## palmer_md (Jul 22, 2011)

Elithion said:


> PM me and I'll send you the source data.


The point of my questions was not for me to have the data, but for the data you have gathered to be more meaningful for everyone. I think you are on to something here and the data is great, and I think it could be more meaningful if presented differently.

I think data from empirical testing should be separated since it is better data than anything from a generic spec sheet that may or may not have relationship to the real battery. Particularly if a company likes to under-promise and over-deliver, but you also like to know if the company does the opposite.

Also dates of manufacture on the cells are important. Like I mentioned, cells built in 2009 are not going to be as good as cells from 2012...likewise these are not going to perform as well as cells in 2015. As we move forward you might see a trend in one company versus another company staying still. It will help point out companies that are progressing with their tech.


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## Ziggythewiz (May 16, 2010)

palmer_md said:


> As was asked earlier in this thread, I'm interested in how many of the cells were tested empirically versus off of the spec sheet.


As I understand his datasheet, most of the data comes from manufacturer's curves, and from testing of a single cell each for CALB SE,CA 100AH, TS/Winston 100AH, and A123 20AH pouch.


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## mileojule (Jan 7, 2013)

I've got some data that seems to rate the Headway better than his graph shows


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## frodus (Apr 12, 2008)

mileojule said:


> I've got some data that seems to rate the Headway better than his graph shows


Yeah me too. We chatted about it and we both thing a real third-party test of IR would be a good idea.


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## palmer_md (Jul 22, 2011)

frodus said:


> Yeah me too. We chatted about it and we both thing a real third-party test of IR would be a good idea.


seems we are all on the same train of thought...glad to hear it.


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