Understanding Depth of Discharge (DOD) for Thundersky batteries

6255 Views 28 Replies 7 Participants Last post by GizmoEV, Jan 31, 2011

corbin

Discussion Starter · Jan 13, 2011

Hi all,
I want to fully understand the Depth of Discharge (DOD) for Thundersky batteries. IE: the 200ah cells:

http://currentevtech.com/Lithium-Batteries/Thundersky/Thundersky-200ah-cell-p27.html

4.0 v max charged, and 2.8 v max discharged. So, at 4.0 v it is 0% DOD and 2.8v it is 100% DOD, correct?

To take it t a maximum of 80% DOD that would be: 3.04 volts, right? (4 - (4-2.8)*.80 = 3.04)

If I charge the cells to the maximum recommended 3.8 volts, then that is leaving off nearly 16% of the available energy, correct? I must be missing something…as I have heard that most the energy is stored around the 3.2 volt range. I want to understand exactly what this means.

If anyone has any reference papers, or URLs to refer me to I'll gladly read up on it myself.

I know some of this has been covered before on the forums, but it is hard to google out the stuff that doesn't directly talk about this.

thanks!
corbin

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spdas

· #21 · Jan 30, 2011

GizmoEV said:
Francis,

I hope you are recording the data on your tests. It may come in useful.

Well i have good charts for the discharge and basically doing this testing to form a M/O for when I put the cells into the Yaris. ( I gotta buy 28 new 200ah or 180ah but need the 180 or 160 size.
Anyway, so far my opinion is still a bit sketchy on where to set the V hi point as I think I will judge this point when I find the best resting voltage after pushing the cells to 3.6 or 3.7 or maybe even 3.5. If i see resting voltage about the same on each of the tests above, I will test ah discharge and see actually how much ah were in the battery.

But i have seen the voltage deteriorate greatly and increasingly after 3.0 volts, so will not drive the car under 3.0 volts when using about 15-20amps in speed (20-25mph?). Looking at a resting voltage at that SOC is really meaningless as you will get a surface and high phantom voltage only with no balls behind it.

Francis

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spdas

· #22 · Jan 30, 2011

Terry, I was just looking at the TS charts and wondering what the heck is Yidium?? Anyway what I noticed that there is a larger deterioration from low C to Higher C discharge with this improved battery than I see with Calb here

http://calibpower.com/ProductDetails.aspx?p=2&id=5

francis

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EVfun

· #23 · Jan 30, 2011

One thing to look at is the discharge rate. The CALB graph in your link only goes up to 1C. This graph of the current TS 160 amp hour cells shows about the same numbers at 1C and has additional discharge graphs up to 5C. I don't know if there is really a nickels worth of difference between the two. CALB says their current cells can take 10C for 10 seconds (though only 1000 amps max for the large cells) while TS publishes discharge voltage graphs up to 5C.

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T1 Terry

· #24 · Jan 31, 2011

Another scarey bit in the Calib specs is this http://www.diyelectriccar.com/forums/attachment.php?attachmentid=9058&stc=1&d=1296433372

2000 cycle life at 54amp continuous discharge rate down to 80% DoD. Well that's the way I read it anyway, I could have got that completely wrong, it's been known before.

T1 Terry

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GizmoEV

· #25 · Jan 31, 2011

spdas said:
Anyway, so far my opinion is still a bit sketchy on where to set the V hi point as I think I will judge this point when I find the best resting voltage after pushing the cells to 3.6 or 3.7 or maybe even 3.5. If i see resting voltage about the same on each of the tests above, I will test ah discharge and see actually how much ah were in the battery.

I've uploaded my spreadsheet tests of a TS-LFP100AHA cell. There are two files: one in OpenOffice.org format and the other in MS Exel format.

The Ah calculations are done using the average current for the time interval and the Wh calculations used the average voltage for the time interval. The sum is at the end of the column. Feel free to use them to enter your own data if you don't want to create the formulas your self.

I'll be interested in seeing if your results are similar to mine.

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JRP3

· #26 · Jan 31, 2011

spdas said:
Terry, I was just looking at the TS charts and wondering what the heck is Yidium??

That would be Yttrium, a rare earth element that TS added to their cells.

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spdas

· #27 · Jan 31, 2011 (Edited)

Aloha, David. I am looking at the excel sheet 3.3-3.45. What do you attribute the voltage spike up to 3.48 or so? Did you mess with the current at 775 minutes? If so, that shows the point we are talking about, very little ah will cause the voltage to greatly increase.

I have mostly been playing with charging (at the charger to 3.8v and Constant 20amps) and at the batteries 3.5 (at most) After resting they go to 3.4 or so. I drained the 400ah TS to 2.2v @20 amps and got 396ah back.

I did the same to my used TS 160ah and got 165ah back. I probably will not be testing above 3.6.

Now I have 2 x TS 160ah charging in series, #2 started at 2.86v and #3 started at 3.23. with amps at 20amps. (this was a "spread" of .37 volts at start. Now 1.5 hours later they are at #2 3.35 and #3 3.31,,,,, a difference of .04v. They are coming together in voltage, but amperage is a different story.
I will continue to charge until I reach my cutoff of 7.21v (lowered) and amperage starts to go down, then boost the voltage up and charge at 20amps for 20 minutes (I am impatient).
Then let them rest for an hour and do my 20amp discharge on each and find out how many ah are in each cell.
Now after 2.5 hours #2 is at 3.35 and #3 is at 3.33 (closing the voltage gap)

Bear in mind, the starting voltage of 2.86v is about dead and 3.23v was in my guesstimate about 1/3 to 1/2 charge. A good imbalance test.

Francis

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spdas

· #28 · Jan 31, 2011

JRP3 said:
That would be Yttrium, a rare earth element that TS added to their cells.

ok thanks. and as I noticed before looking at 2008 spec sheets against 2010 for TS the chemistry has moved along a lot. Hopefully to make a better and cheaper battery. I see new articles almost every day about Mazda, Toyota, etc etc. working on a Lithium/blablabla battery.
So there will be exciting things to come. The ev momentum with major manufacturers doing ev's not being motivated by gas prices, tells me it will work this time, not like the EV-1. Just gotta get the batteries up to a 200 mile range.

francis

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GizmoEV

· #29 · Jan 31, 2011

spdas said:
Aloha, David. I am looking at the excel sheet 3.3-3.45. What do you attribute the voltage spike up to 3.48 or so? Did you mess with the current at 775 minutes? If so, that shows the point we are talking about, very little ah will cause the voltage to greatly increase.

You must be looking at the graph. If you follow the data you can see the current in the Amps column. I put a note at 300.3min about increasing the current. I paralleled the two power supplies in my unit. There is a "blip" at 720 minutes. I think I must have taken a brake at that point. Oh, wait. Since the current is 3.531A that is all one of the two PS could put out so I must have gone back to one supply to finish the test. It made the current adjustment half as sensitive. I'll have to see if I can still find my hand written notes to be sure. At 783.2 minutes I decided to go to 3.45V rather than the original 3.425V I was headed for because I didn't see the voltage rising as fast as I thought it should so there was a current increase. Notice that the voltage went above 3.450v a few times. I turned down the current to keep the ending voltage at 3.45V when the current dropped below 500mA. Hopefully that answers your question.

spdas said:
Now I have 2 x TS 160ah charging in series, #2 started at 2.86v and #3 started at 3.23. with amps at 20amps. (this was a "spread" of .37 volts at start. Now 1.5 hours later they are at #2 3.35 and #3 3.31,,,,, a difference of .04v. They are coming together in voltage, but amperage is a different story.
I will continue to charge until I reach my cutoff of 7.21v (lowered) and amperage starts to go down, then boost the voltage up and charge at 20amps for 20 minutes (I am impatient).
Then let them rest for an hour and do my 20amp discharge on each and find out how many ah are in each cell.
Now after 2.5 hours #2 is at 3.35 and #3 is at 3.33 (closing the voltage gap)

Just don't let the high voltage cell get too high. Testing like you are doing is a great way to learn about them. It is just that using large capacity cells means it takes a long time. I wish I could get a pack of 5 or 10Ah cells like these to use as a test bench on a smaller scale. I know there are other brands that make that size but I want WB (FKA, TS) or CALB cells to play with.

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Understanding Depth of Discharge (DOD) for Thundersky batteries

corbin

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T1 Terry

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