# Understanding Depth of Discharge (DOD) for Thundersky batteries



## corbin (Apr 6, 2010)

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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## Jimdear2 (Oct 12, 2008)

Corbin,

Be aware I am pulling numbers out of the air to make this illustration easier to understand, I'm learning this as well. the numbers are just for illustration and don't reflect an actual battery (as far as I know).

If I understand this lithuim stuff at all, you are not leaving 16% when you charge to 3.8 volts in a 4.0 volt battery. You have to look at the charge curve of the battery. Lets call this 3.8 volts you give the knee of the curve 90+% of the battery power is contained within the flat area of the discharge curve. From 3.8 volts to 4.0 volts might be only 5% of the total power stored. From 3.8 to (lets grab a figure for illustration purposes of 3.0) 3.0 volts might be the other end of that 90% of the battery power, then you his the low voltage knee where it drops from 3.0 volts to 2.8 volts might be again 5% of the power contained in the battery.

For a crude exercise take a piece of paper, turn it landscape and draw 2 verticle lines 10 inches apart 3 inches tall and call the space between them 100% of your battery power now draw a line from one verticle to the other at the 1.5 inch point of the two verticle lines. we will say that line represents 3.2 volts and 100% of the battery's power. Now erase a half inch of each end of the horizontal line. Then connect the left end of the remaining horizontal line to the top of the left line, then connect right end to the bottom of the right verticle line mark the top of the left line 4.0 volts and the bottom of the right line 2.8 volts.

You now have a VERY rough representation of a lithium charge graph. To be truely accruate the horizontal line would slope from about 3.6 to about 3.0 volts but for illustrations sake this will do. 

So from 4.0 volts to the central 96% of your power (that 5% we were talking about earlier) the voltage drops fast then hits what is called a *KNEE* of the curve then you are in the long haul of the discharge till yo get to that 3.0 volt knee at the other end where a last 5% remains, you will drop fast to 2.8 volts and 100% DOD. 

If you push a battery above 4.0 volts or below 2.8 volts you can cause irreversble damage. I don't know what the damage will be but I'm told there will probably be damage.

This is why they tell you should short charge the battery and never over discharge the batter. There is really so little power in the area above and below the knee it isn't worth going there and letting a slip of attention or equipment failure cause damage. 

Then you have to worry aput the variation between cells. If you charge your lowest cell to max you might drive your best cell over the top. 

Please tell me if what I wrote helped you. I drew that graph a while back and it sure helped me when I next looked a t a battery data sheet.

Jim


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## corbin (Apr 6, 2010)

Hi Jim -- thanks, that does help me a lot! 

So, one can not determine the SOC based only on voltage. They have to record how many amp-hours they have been drawing out of the pack and use that as a value for how much is left in the pack.

If I understand right: cells are charged to 4.0 volts, and have 100% capacity. One can keep track of how many amp-hours they take out of the pack. For instance, with a single 200ah cell, when I have reached 160ah I have taken the cell to 80% DOD. The voltage should be somewhere above 2.8 volts. If it was taken to 100% DOD, then the voltage should be around 2.8 volts.

corbin


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## Jimdear2 (Oct 12, 2008)

Corbin,

Again I'm learning as we go, but from what I understand the relationship between voltage and state of charge is quite good. It's just that the field is very narrow. You are dealing with 100th of a volt and temperature does enter into the measurements. I gather the charge graphes from the OEM are pretty accurate.

By the way if I'm off in left field here, any of you battery experts feel free to come in and squash me like a bug.

Jim


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## JRP3 (Mar 7, 2008)

Corbin, take a look at the graphs in the pdf in the link you put in your first post, it shows how voltage is fairly flat during discharge until you get near the end, the knee, and then it drops off. Same thing happens in the other direction when charging.


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## corbin (Apr 6, 2010)

JRP3 said:


> Corbin, take a look at the graphs in the pdf in the link you put in your first post, it shows how voltage is fairly flat during discharge until you get near the end, the knee, and then it drops off. Same thing happens in the other direction when charging.


Good point. Is there a reason the graphs don't show a charge voltage of 4.0? Or is it simply not necessary? I guess, as others have said, there isn't much power at the top of the cell voltage.

corbin


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## JRP3 (Mar 7, 2008)

Yes, not much capacity, better to slightly undercharge for safety, and, mostly, because as soon as you take it off the charger even if charged to 4.0 the voltage will quickly drop down to around 3.4 unloaded, and even lower as soon as you apply a load.


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## spdas (Nov 28, 2009)

JRP3 said:


> Yes, not much capacity, better to slightly undercharge for safety, and, mostly, because as soon as you take it off the charger even if charged to 4.0 the voltage will quickly drop down to around 3.4 unloaded, and even lower as soon as you apply a load.


This is frustrating me a bit. I am charging and discharging a 400ah TS and I so far get no way near 4.0 or 3.8, or 3.6, so unless I am doing something wrong or have a bad batteries (played with 4-5 so far). The voltage after resting is 3.4 or so MAX, (more like 3.35V) so why all this 4.0, 3.8 B.S and not call it a 3.4 or 3.5 battery?? And discharge down to 2.9 or 3.0 and that will be it?? 

Francis


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## Guest (Jan 30, 2011)

spdas said:


> This is frustrating me a bit. I am charging and discharging a 400ah TS and I so far get no way near 4.0 or 3.8, or 3.6, so unless I am doing something wrong or have a bad batteries (played with 4-5 so far). The voltage after resting is 3.4 or so MAX, (more like 3.35V) so why all this 4.0, 3.8 B.S and not call it a 3.4 or 3.5 battery?? And discharge down to 2.9 or 3.0 and that will be it??
> 
> Francis


Most likely your not seeing 3.8 volts because you have not filled up the battery. those are 400AH your playing with. It can take quite some time to charge up. What amperage are you using to charge up the 400AH cells? The 3.8/4.0 are absolute values not to go over. The absolute is 4.0 so the company dropped the end voltage limits to 3.8. Most will charge to even less. That voltage is not resting voltage either. It is charging volt limit. Mine settle into about 3.3 volts there abouts after resting. That is normal. I think the TS settle in at a bit higher. Mine are Hi-Power and will be a bit lower but I can take mine a bit lower too.

I have been charging mine at 15 to 20 amps. You?

Pete


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## Guest (Jan 30, 2011)

By the way it is a 3.2 volt battery.


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## spdas (Nov 28, 2009)

gottdi said:


> Most likely your not seeing 3.8 volts because you have not filled up the battery. those are 400AH your playing with. It can take quite some time to charge up. What amperage are you using to charge up the 400AH cells? The 3.8/4.0 are absolute values not to go over. The absolute is 4.0 so the company dropped the end voltage limits to 3.8. Most will charge to even less. That voltage is not resting voltage either. It is charging volt limit. Mine settle into about 3.3 volts there abouts after resting. That is normal. I think the TS settle in at a bit higher. Mine are Hi-Power and will be a bit lower but I can take mine a bit lower too.
> 
> I have been charging mine at 15 to 20 amps. You?
> Pete



It is a TS 400ah Bench charger set to constant Current of 20.4 Amps. About 18hours from 2.9 to 3.34 Noticibly quicker from 3.34 to 3.4, Now gotta go check every couple of minutes as 3.4 to 3.6 was very fast and climbing past 3.65 in the last 10 minutes. Gotta go set the charger to constant Voltage to watch the amps go down. Very interesting

Francis


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## Guest (Jan 30, 2011)

I think yours are charging just fine. You said resting at 3.4. That is what they should rest at. That last bit is in the knee area and it will quickly jump up if you don't hold it at a specified voltage until the amps drop down. I will be set at 3.6 volts there abouts and CV down to 2 amps then terminate. I have actually 10 different end of charge voltages to choose from in my charger for the car. My bench charger is not a CC CV charger. I have to watch things like a hawk near the end of charge. Your charge times are about right. Lots of time for that many AH. long run times. 

Pete


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## spdas (Nov 28, 2009)

Yeah, 400ah sure is a big bucket to fill ! I bought them used and now selling them as I decided to go with 180calb or 200TS. They test up to or over 400ah and are made in March 2008. Would be great for a larger build, not my Yaris. Even the 180's might be a bit heavy, but i am looking for 80-100mile range.

Francis


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## spdas (Nov 28, 2009)

gottdi said:


> By the way it is a 3.2 volt battery.



I was trying to find out my average voltage with "typical" current and I thought about 3.3v. Now I am more inclined to call it a 3.1 volt battery as at 40mph and about 1/2 charge, it seems to be about 3.1v. Anyway its my theory and I'm sticking to it. !! <g>


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## Guest (Jan 30, 2011)

Yeah, not bad for your guess. My Bus will have two 120 volt packs of 100 AH batteries. That way I should get my distance. I won't be hot rodding the Bus so these Hi-Power should do me just fine. For a hot rod I'd like CALB or GBS 200 AH size. 

Pete


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## spdas (Nov 28, 2009)

But I will give way and call them 3.2v, so as not to encourage a lesser important debate.

Francis


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## T1 Terry (Jan 29, 2011)

spdas said:


> Yeah, 400ah sure is a big bucket to fill ! I bought them used and now selling them as I decided to go with 180calb or 200TS. They test up to or over 400ah and are made in March 2008. Would be great for a larger build, not my Yaris. Even the 180's might be a bit heavy, but i am looking for 80-100mile range.
> 
> Francis


Hi Francis,
I didn't realise they had been making the big format LiFeP04 batteries that long. Any idea what their previous life was? Apparently they start to sag after a lot of cycles at heavy discharge but the get 400ah batteries to that high a discharge rate would be a huge load. What load rate did you test the 400ah capacity at and down to what voltage? I haven’t been able to find a lot of long term uses information over here because no one seems to have had their’s for long enough. The only deaths have been by neglect, abuse or faulty manufacture, these died very early in their career.
As far as recharging, the last 5% to 10% of capacity can be very slow, especially in a big bank because some batteries will go high voltage and require BMS equalising will others are still excepting charge. The biggest balancing cell bridges I’ve seen available over here are rated at 2 amps, if the bank of 400ah cells is 10% out of balance that could take 20hrs to get all the cells back up.
I'm guessing you are somewhere in the states, pity, I would have been interested in some of those big batteries for my house powerpack in the motorhome I'm building. The cost of freight out of the states for anything bigger than a letter is virtually prohibitive, especially to Australia. 

T1 Terry


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## spdas (Nov 28, 2009)

Aloha, I am onl testing at 20amp draw and just judging the total capacity. All the used cells i got were 3.30+ after sitting on a shelf for 6-8 months. I have no way to draw down that 400ah big boy as I am just bench testing. Every cell I tested that I did not "top off" came in at rated or slightly above rated capacity. I am topping off some TS 160ah cells now to see how much over 160 they are. My next test later today is to get 3 cells at varying SOC and series them and carefully watch and see how they behave. (why can't I just put them in the car and leave well enough alone ! !)

Francis


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## GizmoEV (Nov 28, 2009)

Francis,

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


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## T1 Terry (Jan 29, 2011)

Hi Francis,
thanks for the welcome. I think you are going to need to make a heavy discharge rig like a copper wire wound around a piece of bakelite (sp?) or something non flammable or conductive and put it in a big tub of water for the tests so it can dissipate the heat.
I hope I've got the picture attachment part right.
http://www.diyelectriccar.com/forums/attachment.php?attachmentid=9057&stc=1&d=1296428142

These are the charts from the newest Thundersky 400ah batteries with Yidium (sp?) that is supposed to improve their low temp capacity and cycle life but until someone has actually used them to confirm or deny the claims it's about the best we have.
The graph shows that even with a 200amp discharge (0.5C) the capacity of a good battery would still be over 100%. You need to be testing at the peak discharge rate your EV will draw to see if the batteries are going to be suitable or if they have passed their use by date as EV batteries. They could still be a candidate for home power batteries because the rate of discharge is much lower. 
I would love to get my hands on some large capacity used cells to see what they are still capable of delivering.

T1 Terry


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## spdas (Nov 28, 2009)

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 (Nov 28, 2009)

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 (Mar 14, 2010)

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 (Jan 29, 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 (Nov 28, 2009)

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 (Mar 7, 2008)

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 (Nov 28, 2009)

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 (Nov 28, 2009)

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 (Nov 28, 2009)

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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