# LiFePO4 Cell Drift Rumor



## Guest (Jul 18, 2011)

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## Jan (Oct 5, 2009)

I've read in several online scientific tests/reviews that the self discharge of the popular lithium chemistrys is actually the aging process. And that is a real real slow process, especially if they're stored at a modest SOC. So, nothing new to me.


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## MN Driver (Sep 29, 2009)

Being able to pull 98.5Ah out of a 90Ah cell that is 2.5 years old suggests that capacity loss doesn't occur over time. It doesn't mean anything regarding cell drift, which is a pack of cells having different cells differ in SOC relative to eachother over many cycles.

98.5Ah after 2.5 years is promising and the voltage sag under low is still in check which is a good thing in terms of calendar life. ...although 2.5 years I wouldn't expect any though. True, you can't do that with lead-acid. NiMh you could though, I've got some 11 year old NiMh here that can still put out its rated capacity and discharge amperage as it originally could without deeper sag of any significance as it originally did. Yes, NiMh does drift due primarily due to self-discharge but it demonstrates the same thing as your quote does that its possible that the capacity can remain stable over time with a small sample size.

Cell drift in a cell without significant self-discharge is a function of cycling, not rest. Exacerbated by deeper discharges and a differential in internal resistance. Deeper voltage sag due to higher internal resistance is more energy wasted internally inside the cell as heat. As lithium cobalt cells age the internal resistance rises and not in a uniform fashion amongst cells from my own experience, if the same occurs with LiFePO4 down the line this might become more important. Check out the voltage profile under different loads on a specific cell(manufacturers provide this and you've likely seem them), you get less Ah capacity(even more so kwh) out of a cell at a higher discharge rate than a lower one. With a higher internal resistance you are effectively subjecting a cell to a higher relative discharge rate, in effect, with the cell producing more heat(wasted energy) with that additional voltage sag. The discharge rate is the same however, but the added heat and deeper voltage sag is what is making the cell burn off energy internally.

So the real questions are: How much and does it matter? By keeping discharge rates low that is minimized. If your cells have very similar capacities and internal resistances it doesn't matter nearly as much as dissimilar cells. Does this matter in the long run, possibly, does it matter in a 1 year period, probably not unless the cells are being abused. Does it matter over a few months, no. Does it matter for someone who doesn't drive below 20% SOC, absolutely not. Will it at a 5% SOC a few years down the line after a few detours and getting a little lost on the way home, could possibly be an issue.

Does all this warrant having a BMS? ...IMHO a simple LVC system is really about all you would need. Wouldn't even need something on every cell to see a difference, split pack monitoring could do it and would be cheap. Better yet, save your batteries the stress and treat 20% SOC as 0% and save the money.

I agree though, these cells are awesome.


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## DavidDymaxion (Dec 1, 2008)

I have measured cell drift, of roughly 0.1 V per week. It was in 2 cells that had vented. I also had another cell that vented a bit (just one drop of electrolyte) that did not drift.

Anyway, I suppose it is possible a real life pack could get a situation in between these two extremes, where a cell vents a little bit so the cell slowly drifts. A BMS in this case could keep such a cell limping along.

Just thinking out loud here, back in the days when folks were charging LiFe to 4.x volts, maybe they were getting small ventings that would lead to drift. Many reported bulging cases and the need to mechanically restrain the sides. Now that the collective wisdom is to charge to just 3.6ish this has gone away (cells don't bulge), which would eliminate/lessen the need for corrective action by a BMS.

To sum it up, I'm hypothesizing that overcharging could lead to a "need" for a BMS (opposite of lead acid floodies, where overcharging, called "equalization," balances the batteries).


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## EVfun (Mar 14, 2010)

gottdi said:


> Well this rumor is busted. There is no cell drift in these cells what so ever.


That particular test doesn't measure drift. That test looks at self discharge and capacity loss. 

I have not seen any evidence of cell drift unless they are connected to a cell level BMS. Then there is some drift is consistent with the slight difference in current draw from the different BMS modules. I measured a bunch of my cell modules and found a difference of 82 microamps at just over 3 volts. That would be about 3/4 amp hour per year.


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## coryrc (Aug 5, 2008)

gottdi said:


> Well this rumor is busted. There is no cell drift in these cells what so ever.


You going to source that and explain the procedure or just assert it because it jives with what you think?


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## Guest (Jul 19, 2011)

Well I guess I should rephrase that. There is no self discharge. And no it is not because I it jives with what I think.


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## coryrc (Aug 5, 2008)

gottdi said:


> Well I guess I should rephrase that. There is no self discharge. And no it is not because I it jives with what I think.


Then:

1. Under what parameters was 98.5Ah measured? looking for at least discharge amps and to what voltage it was discharged

2. What equipment was used to measure 98.5Ah? How is it calibrated and has it been verified to be accurate?

3. Are you saying the cells were tested with the last full charge being 2.5 years ago?

Extraordinary claims require extraordinary proof.


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## Guest (Jul 19, 2011)

I think you need to ask Jack about the equipment. As long as equipment is consistent it is accurate. I'd say that his equipment is just fine for what he is doing. At least he is doing and not THINKING. Ask Jack the details. 

The cells were measured directly from the box. If the presumed self discharge of 3% per month were in play then after two years the voltage would be noticeably different. The cells are pretty much at 50% when shipped. No damage was done to the cells just sitting either at that 50% state of charge. Then they fully charged the cell and measured the capacity. Got more capacity as expected and just the same as the cells he tested over 2 1/2 years ago. Same voltages from the cells from the package and the same capacity as well. 

One can't THINK themselves smart nor can the TYPE themselves smart. You must TEST to make yourselves smart. It is not even about smarts. It's knowledge. One is SMART if they do the testing. Get the idea. 

I trust the test over the thought. 

Pete


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## Jozzer (Mar 29, 2009)

Hmm, I get very different results from you guys with any other LifePo4 cell (never had TS cells to test).
A123's, LfeBatts and PSI cells ALL self discharge, they also all lose capacity with time (My A123's lost 5% capacity in 12 months, LifeBatt and PSI nearer to 10% in 12 months). I have some 3 year old PSI cells with very few cycles that now return only 7.5AH too.
They also self discharge at different rates. 10 boxes (80 cells) of 10AH PSI cells were charged in parallel to 3.65v 2 years ago, then put on the shelf for storage. 18 months later they were taken down and checked, recharged to 3.65v and notes were taken of the capacity needed to recharge fully. Maximum was 7AH needed, minimum was 4.5AH. 5 cells had discharged themselves to below 1v and were discarded.
After 5 cycles some of the lost capacity is restored (1 or 2% only).

Either TS behave very very differently, or test methods are not consistant (it is after all hard to ensure exact test conditions after a meaningful period of storage).

I now have some A123 pouch cells on the shelf to join the experiment, must pick up a couple of TS cells to complete the set!


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## Guest (Jul 21, 2011)

> There is new thread on ES about LifeBatt/PSI cells and packs.
> There are some very serious accusitions against LifeBatt.
> Ypedal dropped real bomb by claiming that not all 10Ah LifeBatt and PSI cells contain Pchostch Lithium powder. This powder is what supposed to give those cells 120 A continous capability.
> Misteriously in the middle of posting by various members to this thread ES web site went down.
> ...


So maybe the ones you mentioned are not true LiFePO4 cells. And maybe it is because of how they are put together in the cylinders. I do know that most who use the cells mentioned above do seriously abuse them in the RC arena and I am not surprised to see that they loose power or capacity fast. My brother uses cells like this with his copter and with the charger provided for charging these he really gets crappy life from them. He keeps good records of how many charges. He has tried many and most just crap out way too fast. I suspect they are NOT LiFePO4 cells. 

Pete 

The topic is of course the LiFePO4 cells like TS, CALB, Winston, Hi-Power, GBS, and the like. It is not a topic of Kokam, A123 or the like.


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## Jozzer (Mar 29, 2009)

I am indeed aware of these acusations, and the facts surrounding them, but am not at liberty to discuss this here.
Suffice it to say, I am sure that the PSI cells I had were Phostek, and even if they werent they would still be LiFePo4 cells. As far as I am aware, TS don't use Phostek powder either..
To say non Phostek LifePo4 cells are not really LifePo4 is like saying that non Optima Lead acids are not really Lead acids...


The A123 cells are genuine, new, and were not used in RC applications.



FYI, A123 ARE LifePo4 cells, in fact, they are the most famous LifePo4 cells

Steve


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## Guest (Jul 21, 2011)

Ok. So maybe the mystery powder is the culprit. Maybe the electrolyte is different and the culprit. Something must be different if the cylindrical cells exhibit a self discharge. Since the AH rating is quite small compared to the large cells I guess it would be apparent over a shorter period of time. Maybe I will go buy one new and just let it sit. Measure the voltage from the package and then in a year measure it again. If there is self discharge I expect it to be pretty much flat by that time. Hasn't someone already done this with these cells? I mean totally new and unused sitting on a shelf to see if they just up and die or self discharge to zero. Someone should have done this by now.


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## rwaudio (May 22, 2008)

gottdi said:


> Ok. So maybe the mystery powder is the culprit. Maybe the electrolyte is different and the culprit. Something must be different if the cylindrical cells exhibit a self discharge. Since the AH rating is quite small compared to the large cells I guess it would be apparent over a shorter period of time. Maybe I will go buy one new and just let it sit. Measure the voltage from the package and then in a year measure it again. If there is self discharge I expect it to be pretty much flat by that time. Hasn't someone already done this with these cells? I mean totally new and unused sitting on a shelf to see if they just up and die or self discharge to zero. Someone should have done this by now.


The problem with that is a cell could go from 70% charge to 30% charge and be within a few mv of the same voltage reading in an unloaded state.

A better test might be to fully charge, then discharge a cell to a certain SOC (IE pull 50ah out of a 100ah cell) or put in 50ah from a fully discharged 100ah cell. Then after a year or two discharge the cell and see what you can get out of it. If you can pull 50ah out of a cell in two years that you put exactly 50ah into, then you can make a statement about LiFePO4 "cell drift" or "self discharge"

It would be a good idea to cycle the cell when new a couple times to get a very accurate capacity to start with, then you know if you can only pull out 30ah or 40ah after a year then the cell DOES have some self discharge. Open terminal voltage just doesn't mean anything when the cell is around 50% SOC.


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## JRP3 (Mar 7, 2008)

Self discharge may be higher in cells stored near full at a high SOC, which may be why some of the cells showed high self discharge, and may be part of the reason that manufacturers recommend storing cells around 50% SOC.


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## coryrc (Aug 5, 2008)

gottdi said:


> Then they fully charged the cell and measured the capacity.


So what does that have to do with self-discharge?



gottdi said:


> One can't THINK themselves smart nor can the TYPE themselves smart. You must TEST to make yourselves smart. It is not even about smarts. It's knowledge. One is SMART if they do the testing. Get the idea.
> 
> I trust the test over the thought.
> 
> Pete


Apparently you keep trying to "type yourself smart" by parroting your god-hero instead of doing any testing yourself (which many of us, including myself, have done). You didn't even interpret his work correctly, either. It showed very, very low LOSS OF CAPACITY when maintained at ~50% state of charge. That was already known, but it's good to have another data point.

It shows NOTHING about CELL DRIFT or SELF-DISCHARGE, besides that the TK cells do not self-discharge from 50%->0% in 2.5 years, which is not surprising either.


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## Guest (Jul 22, 2011)

coryrc said:


> So what does that have to do with self-discharge?
> 
> 
> 
> Apparently you keep trying to "type yourself smart" by parroting your god-hero instead of doing any testing yourself (which many of us, including myself, have done). You didn't even interpret his work correctly, either. It showed very, very low LOSS OF CAPACITY when maintained at ~50% state of charge. That was already known, but it's good to have another data point.


I'd be the last one here trying to type myself smart. Ok smarty pants then why don't you post your results. By the way. Jack is no GOD. His does however test and post his results and many on these forums try to discredit his work because his results counter their perceived knowledge. I trust his work over yours any day of the week unless you have and can prove otherwise. He also has not trouble falling on his own sword where many here would not dare. I post his stuff because he stopped posting here because of all the garbage. He continues and continues to actually do things rather than sit around typing themselves smart. Have you seen the latest videos of Elithion BMS? 

Oh yeah! LiFePO4 cells are not lead or NiMH or NiCAD. They do not behave like them. PERIOD. 

So I will take a small cylinder style cell that has a 2.5 AH capacity and charge it and let it sit for the next 5 years unused and then do a capacity check. With that It may put to rest this stupid argument once and for all. You willing to wait that time frame to get your answer? Good so while you sit and wait I will continue to move forward.


JRP

Yes if a cell was fully charged to it's max then maybe some perceived self discharge was taking place as the cells surface charge slowly dissipates since it is not used. That is a possibility.


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## coryrc (Aug 5, 2008)

gottdi said:


> I'd be the last one here trying to type myself smart. Ok smarty pants then why don't you post your results.


First, I have a long thread about cells that turned out to be not be good enough:
http://www.diyelectriccar.com/forums/showthread.php?t=59330

Second, I've parallel charged groups of cells, left them sit, then charged them again. I wasn't too meticulous about keeping records, but they always sat at CC for a while before going above 3.45V when they were left a month or three.

Third, here's Jozzer's experience:



Jozzer said:


> They also self discharge at different rates. 10 boxes (80 cells) of 10AH PSI cells were charged in parallel to 3.65v 2 years ago, then put on the shelf for storage. 18 months later they were taken down and checked, recharged to 3.65v and notes were taken of the capacity needed to recharge fully. Maximum was 7AH needed, minimum was 4.5AH.


All of these experiments actually demonstrated SELF-DISCHARGE.

Jack's test did NOT test SELF-DISCHARGE. It tested LOSS OF CAPACITY, because he charged the cells before discharging.

Do you not agree?



gottdi said:


> So I will take a small cylinder style cell that has a 2.5 AH capacity and charge it and let it sit for the next 5 years unused and then do a capacity check.


No need to wait 5 years. Just do the following:
1. Take a few cells of the same type. Charge all to full.
2. Measure the full discharge, in amp-hours, of each cell at a low C-rate (to avoid any heating effects). Record.
3. Charge each to full again.
4. Store for 4 months
5. Fully discharge each cell the same as in step #2 WITHOUT CHARGING FIRST. The difference between the cell now and four months ago is the self-discharge. It will be non-zero.
6. Charge and fully discharge each cell again. Its capacity will be greater than in step #5, and possibly measurably below step #2. The difference between capacity now and in step #2 is the Loss Of Capacity. Probably not a measurable amount in only four months.

The amounts we are talking about are fairly small, so please keep the temperature constant between the tests.

Then maybe you will finally know what the rest of us do.


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## Guest (Jul 22, 2011)

> Then maybe you will finally know what the rest of us do.


Yeah! No self discharge. No degradation of the cell sitting for 2 1/2 years either. I don't need to wait either. I suppose you did that test you are outlining me to do, right? If you did then you should have full documentation and should be able to present your results for all to see and learn rather than just say something. I go with Jack because he does test. Where are your tests? Documentations? Results? 

Charging in parallel does nothing to balance the cells. It only combines the cells into a single cell when you parallel. So each section still has it's base capacity. Open a TS cell and spit it into two separate cells and you will find that they have different capacities. Parallel charging is not how you balance a pack. you must balance a pack as a single unit. Parallel all the cells into one unit and you only have one huge AH unit. You do not have a balanced cell. 

One cell of 100 AH size is actually two 50 ah packs of multiple plates paralleled within one package to make one cell. It charges as a single unit.

http://greenev.zapto.org/GreenEV/GreenEV/GreenEV_Productions_Photo_Albums/Pages/Hi-Power_Photos.html


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## JRP3 (Mar 7, 2008)

coryrc said:


> Jack's test did NOT test SELF-DISCHARGE. It tested LOSS OF CAPACITY, because he charged the cells before discharging.
> 
> Do you not agree?


Not necessarily. He compared the voltage of a brand new cell with an old cell and they were the same within one thousandth of a volt. The idea that the discharge curve is perfectly flat and that voltage has no meaning in the middle really is not accurate. The curve is not flat, it is sloped, and differences in resting voltage do correlate to SOC. So if TS consistantly ships it's cells at 50% SOC, and they seem to, old cells showing the same voltage as new cells would indicate no to very little self discharge.


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## JRP3 (Mar 7, 2008)

gottdi said:


> One cell of 100 AH size is actually two 50 ah packs of multiple plates paralleled within one package to make one cell. It charges as a single unit.
> http://greenev.zapto.org/GreenEV/GreenEV/GreenEV_Productions_Photo_Albums/Pages/Hi-Power_Photos.html


Actually it's a number of cells paralleled together, they just split the grouping to make it easier to connect. Each of those 50 amp hour stacks is a bunch of paralleled cells, so it's just a bunch of cells paralleled to make 100 amp hours. Cells paralleled and charged together do balance, otherwise all those internal cells would not be balanced together.


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## frodus (Apr 12, 2008)

JRP3 said:


> Not necessarily. He compared the voltage of a brand new cell with an old cell and they were the same within one thousandth of a volt. The idea that the discharge curve is perfectly flat and that voltage has no meaning in the middle really is not accurate. The curve is not flat, it is sloped, and differences in resting voltage do correlate to SOC. So if TS consistantly ships it's cells at 50% SOC, and they seem to, old cells showing the same voltage as new cells would indicate no to very little self discharge.


That's great, except that Voltage is not an indication of the state of charge. You can have a cell at 50% SOC and it measures the same nominal voltage of a cell at 70% SOC. Unloaded, a cell tends to rest at it's nominal voltage. 

Maybe I'll test this on some extra cells I have, since I have the equipment to do so (CBA-II and a CC400 discharger), on headway batteries, and on A123 cells. All are brand new, never been used. Only kept charged, I've had to do it twice in 1.5 years while they were sitting waiting for my project (indicating they self discharged every single time, and I've charged over 250 cells). The amount that I charged was small, but it was there.


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## Jozzer (Mar 29, 2009)

frodus said:


> That's great, except that Voltage is not an indication of the state of charge. You can have a cell at 50% SOC and it measures the same nominal voltage of a cell at 70% SOC. Unloaded, a cell tends to rest at it's nominal voltage.
> 
> Maybe I'll test this on some extra cells I have, since I have the equipment to do so (CBA-II and a CC400 discharger), on headway batteries, and on A123 cells. All are brand new, never been used. Only kept charged, I've had to do it twice in 1.5 years while they were sitting waiting for my project (indicating they self discharged every single time, and I've charged over 250 cells). The amount that I charged was small, but it was there.


 I can confirm that Frodus. I demonstrated for a customer recently 2 different M1 A123 cells recently, we charged them both, then discharged 0.3AH from one and 1.3AH from the other (2.3AH cells) and it was impossible to tell which was which from the voltage (I won £5;-) ).
I'm beginning to wonder if TS cells behave completely differently from every other LiFePo4 cell out there from this discussion!


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## JRP3 (Mar 7, 2008)

Jozzer said:


> I'm beginning to wonder if TS cells behave completely differently from every other LiFePo4 cell out there from this discussion!


I think they, and CALB, and maybe all the large format LiFePO4 cells, must indeed behave differently. Cells resting at 3.30V are at a different SOC than cells at 3.25 or 3.20. That's 118.8V, 117V, and 115.2V for my 36 cell pack, a noticeable difference. True the resolution is not fine in the middle but I think a volt meter accurate out to a thousandth might correlate to actual SOC. Someone needs to take 2 TS or CALBs, discharge one to 50%SOC and another to 40% SOC and take a voltage reading and see if there is a discernible difference.


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## frodus (Apr 12, 2008)

I'll volunteer to test with my equipment (and repeat on other cells) if people would send them to me. I can send them back upon finishing my test. I can post results.

I just don't want to buy a couple TS or CALB cells if I won't use them, and won't know if a 20 or 40Ah cell would act the same.


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## JRP3 (Mar 7, 2008)

Pretty much anyone with a TS or CALB pack and an amp hour counter can do this test if they have some time. I'm away on vacation tomorrow for a week so I can't, but just drive and check the voltage of one specific cell every 10 amp hours, or at least take a few readings in the middle of the SOC. Or do it as you charge, just let the cell rest for a bit before reading, and charge at low current if you can.


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## T1 Terry (Jan 29, 2011)

JRP3 said:


> I think they, and CALB, and maybe all the large format LiFePO4 cells, must indeed behave differently. Cells resting at 3.30V are at a different SOC than cells at 3.25 or 3.20. That's 118.8V, 117V, and 115.2V for my 36 cell pack, a noticeable difference. True the resolution is not fine in the middle but I think a volt meter accurate out to a thousandth might correlate to actual SOC. Someone needs to take 2 TS or CALBs, discharge one to 50%SOC and another to 40% SOC and take a voltage reading and see if there is a discernible difference.


I have a graph from a cell logger where 4 x 90ah Winston LYP cells were used. The test was a 1C discharge of 16ah each a total of 4 times with a 5 or 6 min break between each discharge. You're probably going to laugh but the load was a 12 cup coffee percolator powered via a 1000w inverter, each load test started at 89amps and slowly increased to 93amps, close enough to 1C. The 5 or 6 mins is how long it took me to empty the hot water, cool it down, refill it and start the next stage.
A few interesting things came out of it, the cells don't hold up the 3.2v per cell at a 1C load as the Winston chart says, the 4 cells under load are slightly different voltages but almost identical voltages when at rest. 
So the discharge voltages can be seen at different stages as the cells discharge, right down to 70% DoD.
Now to work out how to attach a picture here, is there a photo library attached to this forum or do I need to store it on some where like Photo bucket?

T1 Terry


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## JRP3 (Mar 7, 2008)

Use the Manage attachment button when you reply, if the image is small enough you can upload it.


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## tomofreno (Mar 3, 2009)

> Now to work out how to attach a picture here, is there a photo library attached to this forum or do I need to store it on some where like Photo bucket?


Are you using Logview to view the files? If so you can export the graph as a picture file and post that by using the attachment function (have to click the "Go Advanced" button here to get that option listed). The file will be stored in the "logs" folder under Logview. Can also save it as a pdf like the one below, or use photo bucket as you said.
I've posted this curve recorded while charging before:
View attachment 12.26.09 cell log data, logview graph.pdf

Obvious the curve is not flat, and voltage is a function of Ah.


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## jddcircuit (Mar 18, 2010)

tomofreno said:


> Are you using Logview to view the files? If so you can export the graph as a picture file and post that by using the attachment function (have to click the "Go Advanced" button here to get that option listed). The file will be stored in the "logs" folder under Logview. Can also save it as a pdf like the one below, or use photo bucket as you said.
> I've posted this curve recorded while charging before:
> View attachment 10518
> 
> Obvious the curve is not flat, and voltage is a function of Ah.


That is a great chart. Thank you

Am I reading your graph correctly that Cell3 was allowed to reach approximately 4.25V and that it reached its knee about 30 minutes before cells 2&4?

It seems to me that if we started posting charge and discharge curves like this one it would be more obvious to everyone how the knees of the charge curves align to each other. If this relative knee alignment does not change after repeated cycling then we could state that the cells do not drift. The rumor busted claim had me a little confused with the data it provided.

I assume that this knee alignment could be analyzed at the top end of the charging or at the low end of the discharging. Knee alignment changes at the ends may also turn out to not be considered drift at all if shift is symmetrical on a pack that is center aligned. In this hypothetical we would be observing capacity shrinkage over time. I am not promoting center, top, or bottom alignment. I am only stating that having good data can help us agree whether SOC alignment is changing or not so if it is we can better understand it.

I don't watch all the videos or read all the blogs so I may be restating some things.

I apologize for not having my own data to share yet. Hopefully soon.

Regards
Jeff


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## JRP3 (Mar 7, 2008)

tomofreno said:


> Obvious the curve is not flat, and voltage is a function of Ah.


Travis is suggesting that a cell's resting voltage does not represent it's SOC, and since your charge curve shows the cells while receiving current it's not showing resting voltage in relation to SOC. We need to see resting voltage in relation to known SOC.

PS.
I sent you a couple of PM's, don't know if you were notified.


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## T1 Terry (Jan 29, 2011)

Did it work?

This is log view chart of 4 X Winston LYP90ah cells under an approx. 1C load, the load varies from 89amps to 94 amps as the voltage drops but it's close the the 90 amps required for a 1C test. As can be seen, the Winston charts claiming that the 1C load holds up at 3.2v is not the case here but all up 65ah were drawn from the cells before the voltage started to fall away. another interesting point is the cells with the low load voltage bounce back with a high resting voltage. The resting voltage still has a 1 amp load, that was to power the inverter.

T1 Terry


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## tomofreno (Mar 3, 2009)

> Am I reading your graph correctly that Cell3 was allowed to reach approximately 4.25V and that it reached its knee about 30 minutes before cells 2&4?


 Yes. I didn't say this was the way to charge cells. These are CALB cells and should only be charged to 3.6V. I caught them a bit late (bms didn't work). They are still working ok. The charger was cutting back current at the end. That is why two curves at the top don't rise as steeply near the end as the other two. They reached the exponential rise part as the charger was cutting back.


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## dtbaker (Jan 5, 2008)

tomofreno said:


> Yes. I didn't say this was the way to charge cells. These are CALB cells and should only be charged to 3.6V. I caught them a bit late (bms didn't work). They are still working ok. The charger was cutting back current at the end. That is why two curves at the top don't rise as steeply near the end as the other two. They reached the exponential rise part as the charger was cutting back.



or is the difference because the cells went overvoltage and are slightly fried?!


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## T1 Terry (Jan 29, 2011)

The testing I'm doing at the moment is for house batteries in a motorhome, these cells are brilliant for this purpose. A few of the things we've found, there is no point in charging the cells above 3.45v, it's just surface voltage after that point and has no amps behind it, as soon as a load is applied the cell voltage drops back to 3.4v. What the 3.45v charging does do though is allow the cells a little bit behind to catch up by 0.5v per recharge so there is a sort of self balance each charge. The pack is being fitted to a motorhome in the next few weeks configured as 2 cells in parallel X 8 sets to give 180ah @ 24v nom. This vehicle is only recharged via solar and an alternator/start battery link when the vehicle is being driven. It has a cell logger fitted so we will be able to watch if the cells do drift apart or self balance each recharge as we believe they will do.

T1 Terry


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## tomofreno (Mar 3, 2009)

> or is the difference because the cells went overvoltage and are slightly fried?!


 Uh no, the two at the top didn't go over voltage.


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## JRP3 (Mar 7, 2008)

This Jack Rickard chart shows the resting voltages along the discharge curve. I don't know how long they were resting after the load was removed but you can see the relationship between voltage and SOC within a 10 ah range, with voltage reading out to one hundredths. I'd assume out to one thousandths would correlate even more accurately.
http://3.bp.blogspot.com/_i_c2BM_uB...AABTg/hpmMRUpxA0w/s1600-h/SkyEnergy1Cload.jpg


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## jddcircuit (Mar 18, 2010)

tomofreno said:


> Yes. I didn't say this was the way to charge cells. These are CALB cells and should only be charged to 3.6V. I caught them a bit late (bms didn't work). They are still working ok. The charger was cutting back current at the end. That is why two curves at the top don't rise as steeply near the end as the other two. They reached the exponential rise part as the charger was cutting back.


I understand. However, I like the fact that you included that region inadvertently. I am working on my own bms and charging algorithm and you showing me the behavior of the charging in that region is helpful to me.

I assumed that the slope of the later rising cells was flatter due to reduced charging current but thanks for clarifying.

I have 50 CALB now and plan to have upto 100 when finished. Just to confirm a couple of things. My plan is to be able to detect the beginning of the knee for each cell and not so much target a fixed voltage but may use a combination of both. When to stop charging is to be determined and I want to use the position of the knee to track my cell drift (if any).

I also like the discharge graph in the other post. It looks to me that maybe the voltage sag under load may be a way to approximate SOC as a secondary means to my current counting. If I characterize each cell during multiple cycles, as I am driving (discharging) I might be able to do a best fit approximation of SOC based on the voltage of each cell under load.

Thanks
Jeff


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## Jozzer (Mar 29, 2009)

So, firstly only 0.05v resolution between 5% and 50% discharge is really not very much (especially not if your trying to determine SOC from overall pack voltage of many cells in series), secondly, we need that same graph for several different cells to see if the voltages are consistant at a very fine percentage at each SOC to be meaningful.


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## JRP3 (Mar 7, 2008)

I'm suggesting that resting voltage out to one one thousandth of a volt would give a direct correlation to SOC. If that is true, and if Jack's 2 year old cells show the same resting voltage of the brand new cells, then this is strong evidence of little to no self discharge.


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## Jozzer (Mar 29, 2009)

Thats why I say we need to see the same results for other cells, it's possible that even 2 cells of the same age don't follow that curve within that close tolerance. (I know this would be the case for any other LifePo4 cell).


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## coryrc (Aug 5, 2008)

gottdi said:


> Yeah! No self discharge. No degradation of the cell sitting for 2 1/2 years either.


THESE ARE NOT THE SAME.

I agree on this: Nearly no degradation of a cell sitting at 50% for 2.5 years. That is no surprise and the same experience everyone has with non-broken LiFePO4.

I do not agree on this: "No self discharge", because it is not true.




gottdi said:


> I don't need to wait either. I suppose you did that test you are outlining me to do, right? If you did then you should have full documentation and should be able to present your results for all to see and learn rather than just say something.


*Please explain to me what you think this test shows:*



Jozzer said:


> They also self discharge at different rates. 10 boxes (80 cells) of 10AH PSI cells were charged in parallel to 3.65v 2 years ago, then put on the shelf for storage. 18 months later they were taken down and checked, recharged to 3.65v and notes were taken of the capacity needed to recharge fully. Maximum was 7AH needed, minimum was 4.5AH.





gottdi said:


> I go with Jack because he does test. Where are your tests? Documentations? Results?


HE DIDN'T TEST SELF-DISCHARGE!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
IF YOU DON'T KNOW THE STARTING CHARGE OF THE BATTERY, YOU CAN'T KNOW HOW MUCH IT HAS SELF-DISCHARGED.

You just don't want to understand what his test showed: low *loss of capacity*. This was already known and is NOT the same as self-discharge.


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## Guest (Jul 25, 2011)

I fully understand. The answer is easy. No degradation of capacity after years just sitting which was an argument over 2 years ago. 

No self discharge either. It has been argued that the self discharge is at least 3% per month. A half charged cell from the factory would show a loss of voltage at this rates in 2 1/2 years. Sorry but 3% is no small beans. It must once again be noted that the argument is for LiFePO4 cells and not any other lithium cells. 


Pete. 

I have cells that have been at 2.4 volts for over a year in my possession and who knows how long before that. No cell voltage change there and the cells I have that are at 1 volt have never changed. Yes, there is still voltage that can be lost. 

Sorry but there is no self discharge on these.


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## frodus (Apr 12, 2008)

You wouldn't know a difference in self discharge because you didn't measure the Ah of the cell before, and compare it to the Ah discharged after. 

There may be minescule differences in voltage over the year of a self discharging a cell. You'd never know the difference because you aren't actually cycle testing the cell. You first need to know how many Ah you can take out of the cell when it's new, and not just go by ~50% from the manufacturer. Then you charge it, let it set for 2 years, then either charge it again and measure Ah in, or discharge it to the same level as before and mark any differences.

None of the tests I've read people doing actually test this. I've got a discharger (several of them), lab supply, data logger and all sorts of stuff. I've witnessed a very small self discharge of A123, headway, K2 batteries. I had a batch of all 3 of them, charged them up until the current in was 0V. let them settle for a few days and then topped them off, and let them set because they weren't being used. I put them on a charger and all of them took current. Compared the Ah discharges before and after, and they differed slightly.

Just measuring a cell voltage doesn't tell you "jack". It may go from 3.311 to 3.310, and you'd never know unless you used a meter good to the thousandths. To you, it may look like the same voltage. But even then, it's so close that the only real way to test how much of the energy inside the cell has actually self discharged, is to charge and discharge a cell and compare it to what it was before.


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## Guest (Jul 26, 2011)

If after 2 1/2 years and you get only a couple thousandths of a volt difference then the argument of self discharge is moot. It has been however stated that they show a 3% or greater self discharge per month and if that were true you'd see a voltage difference after 2 1/2 years of self discharge per month. If however it is shown that a cell will discharge a few thousandths of a volt after two years then there is in effect NO self discharge or at least not enough to be part of any equation. 

As for the 50% SOC from the manufacture, it is a valid SOC since they are the ones that build them and set them. They are shipped at 50% SOC. 

Measuring cell voltage does mean jack.


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## Guest (Jul 26, 2011)

> HE DIDN'T TEST SELF-DISCHARGE


He checked voltage after sitting for 2 1/2 years. If there was any true validity to the issue then there would be some change. Remember 3% per month for 2 1/2 years is not small amount. Enough to see even with a voltage meter measured at the hundredths mark. He also showed no degradation of capacity after that time either so the shelf life rumor is busted too. Yes he did check voltage because all the other cells he pulled out were still in the same voltage zone as all other TS cells when delivered when new. I also again have cells that have been sitting for at least that long at a much lower voltage so are in the very low SOC mode but the voltage has not changed one bit at the hundredth of a volt since I have had them. So a low voltage (SOC) cell still exhibits no self discharge. NONE. 

Insist if you must. 

I know what I know. 

Take a cup full of water and punch a small hole in the bottom and watch the self discharge. Even a tiny hole. Drip drip drip drip drip. Now empty the cup and fill it half full. Guess what, you still get to watch the drip drip drip. Capacity has nothing to do with self discharge. You will eventually see a lower level over time. But many are now saying that it depends upon how full it is and that the closer to empty it is the slower the self discharge will be. Bogus Bull shit.


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## frodus (Apr 12, 2008)

Sure is lots of theory.

it's not volts that discharge, it's energy, and without doing a discharge cycle on them after 2.5 years, and actually measuring Ah in/out, you'll never know. 

I've seen TS, CALB, Hipower, A123 all over the place with their SOC..... maybe it's because they're from different batches and they self discharge....lol. But they're not all 50%. You HAVE to measure what you have to start with, then let them set, then measure what you end with. 

It's kind of like water leaking out of a pressurized water tank. The water pressure might be about the same after a few months, but the water that leaked out of the valve still happened, and the only way to measure what's left in the tank is to either refill it and measure what you put in, or drain it and measure what you take out and compare it to the total volume that it could have.


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## Guest (Jul 26, 2011)

True but volts show energy that did discharge.


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## Guest (Jul 26, 2011)

If this argument was over cells only a month or two old I'd have to agree but over a 2 1/2 year time span there would be a noticeable change in voltage showing a loss in energy in the cell. Much harder to prove over a short span but easy over the long haul. If 2 1/2 years is not good enough then what is. 

We are not doing capacity checks. We are just checking that energy did discharge. Not how much or any thing like that. But just that there is a measurable discharge. I know many cells that discharge enough that I need to recharge after a couple months of sitting to bring them up to normal. Surly these would show something after 2 1/2 years even sitting on the shelf.


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## frodus (Apr 12, 2008)

Just stop with the blanket statement that all lifepo4 does this, or that. 



gottdi said:


> True but volts show energy that did discharge.


No they don't. The volts for ONE CELL in his test showed decreasing voltage, but you know just as well as I do.... anyone can make that data show what they want. No indication on exactly how long he left the cell rest each measurement, or if it was the same. 

The only REAL way to test the energy in a cell is to discharge it, or recharge it and measure both amps and volts. Voltage alone tells you part of the story. We've shown several times to customers (jozzer and I) that a cell with different SOC can rest at the same voltage as another cell.

Now that being said, I don't think the discharge rate for my cells is as high as 3%, but I know I charged them a year ago, and recharged them all again (200 cells from different manufacturers) and all took a charge of varying amounts.


So, from my experience, Lifepo4 cells show a small self discharge.


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## MN Driver (Sep 29, 2009)

< with a _ under it means less than or equal too. > with a _ under it means more than or equal to.

Winston shows <= for self discharge data. Basically saying the max is expected to be less than 3%. It is clear its not that high, and if any is very small. I still don't think we have the data to quantify it at all. Voltage is not a method to reliably obtain SOC data. Also for every battery that I've ever done anything with that does self-discharge, self-discharge is less at a lower SOC and much higher at a higher SOC. For example I have some NiMh cells that will self-discharge over an Ah in a week, but they don't ever self-discharge below a volt per cell even as they sat around all winter. It isn't linear, it's exponential. Granted I'm talking about NiMh but I don't have idle LiFePO4 sitting around not in use to check this but the concept would be the same if they do self-discharge unless there is separator failure with a high resistance short than they might drain to empty in a shorter period.


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## JRP3 (Mar 7, 2008)

In his most recent video Jack Rickard did the exact tests we were talking about, and they came out just as I've been saying they would. He doesn't have the graphs up on his blog yet, but after fully charging a 2.5 year old 90 ah cell it took 45 amp hours, so it was at 50% SOC, at 3.2999 volts, just as his brand new cells are at 50% SOC at 3.3000 volts. He then discharged the cell at 1C, stopping at 10% intervals and waiting 5 minutes, then recording resting voltages, and continuing the discharge cycle. Five minutes is not enough to get to true resting voltage but the relationship is still there. There are specific voltages that directly relate to each 10% difference in SOC if you go out to thousandths of a volt even in the middle of the curve. This is of course exactly what I and others using TS and CALB cells have seen. Even out to hundredths of a volt will show 20% difference in SOC in the middle of the curve, getting more accurate towards the ends of course. His numbers could still hide a small percentage of self discharge and capacity loss but they seem to be effectively pretty close to zero.


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## dtbaker (Jan 5, 2008)

I am coming up on 2000 miles on my Thunderskies. I have a split pack, where the temps have been 'a little' different because the front pack doesn't have a lid right now and the rear does. I hardly see a difference with short drives, but the enclosed pack does warm up maybe 5 degrees more on longer higher speed drives.

I won't get to check balance for a couple weeks, but will post results when I do....


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## coryrc (Aug 5, 2008)

JRP3 said:


> In his most recent video Jack Rickard did the exact tests we were talking about, and they came out just as I've been saying they would. He doesn't have the graphs up on his blog yet, but after fully charging a 2.5 year old 90 ah cell it took 45 amp hours, so it was at 50% SOC, at 3.2999 volts, just as his brand new cells are at 50% SOC at 3.3000 volts.


This is not the test I proposed. The above is meaningless.



JRP3 said:


> He then discharged the cell at 1C, stopping at 10% intervals and waiting 5 minutes, then recording resting voltages, and continuing the discharge cycle. Five minutes is not enough to get to true resting voltage


So why even bring it up? Let me present a better title for you:
SLOPPY, IMPRECISE METHODS "PROVE" MY FAITH



JRP3 said:


> His numbers could still hide a small percentage of self discharge and capacity loss but they seem to be effectively pretty close to zero.


I don't think anyone is debating that Thundersky batteries have low self-discharge, particularly at 50% SoC.


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## JRP3 (Mar 7, 2008)

coryrc said:


> This is not the test I proposed. The above is meaningless.


What test did you want to see?



coryrc said:


> So why even bring it up? Let me present a better title for you:
> SLOPPY, IMPRECISE METHODS "PROVE" MY FAITH


I have no faith, I look at data. All that more rest time would do is give slightly higher resting voltages, not change the relationships between them. I can't fault Jack for not waiting hours between discharges, but feel free to take weeks to do the same test. Use plenty of caps when you post your results since that will make them more meaningful 



coryrc said:


> I don't think anyone is debating that Thundersky batteries have low self-discharge, particularly at 50% SoC.


Oh really?


coryrc said:


> I do not agree on this: "No self discharge", because it is not true.


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## EVfun (Mar 14, 2010)

To be fair, *low* does not equal *no* (even though they rhyme.)

I would be much more interested in determining the self discharge, or if it exists, at 100% SOC and/or 90% SOC. I'm pretty sure the self discharge by the time you are down to 60% SOC is so close to none that it doesn't matter from a storage standpoint.


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## Guest (Aug 5, 2011)

> But you can bet that someone will still bring up something that will absolutely prove him wrong again.


What an amazing prediction. And so fast too! 

Sloppy? What have you provided? Spend a week or more doing the same thing and you will get the same results. 

Multiple things were done by this. One that there was NO self discharge after 2 1/2 years of kicking around in a non controlled environment. From the point of being in the box to full was 45 AH in. He happened to do so at 1C. Could have been .5 C for all that mattered. What mattered is if there was even a 1% self discharge you would see a difference and you would have a difference in capacity due to self drainage. NOPE. NADA. NO SELF DISCHARGE. He showed that there is a correlation between voltage and SOC. He did the 5 minute rest to show more like what you'd see in your car driving it vs doing an hour rest between discharges. It would not matter if you did that at 1 minute or 50 minutes, you'd get the same results. Your looking for correlation of Voltage and SOC which was an argument too. So he killed two birds with one stone. 

I'd almost bet you did not even watch the videos and hear exactly what he had to say. His testing is more than yours and for that matter more than most or anyone else here. He is also the only one showing his results and showing you how paint dries. Pretty amazing. But it is something he already knew. Like he said in the show. He does way more testing that what he shows. He does know what he is talking about. 



> Originally Posted by coryrc
> 
> I don't think anyone is debating that Thundersky batteries have low self-discharge, particularly at 50% SoC.


What, That is precisely what the argument is about. And that there is any self discharge. There is no indication of self discharge. NONE. What's to argue now? 



> Originally Posted by coryrc
> I do not agree on this: "No self discharge", because it is not true.


What? Myth BUSTED. BAM 

Pete


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## dtbaker (Jan 5, 2008)

who cares about self discharge?

what I care about is whether and how much and how fast cells may or may not drift apart when connected and cycled in series (i.e. actual use) to answer whether or not a 'balanced' pack will become unbalanced over time, how much, and how fast.... as well as other factors like configuration, split packs at different temps due to different config or insulation, etc.


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## Guest (Aug 5, 2011)

They DONT. Thats the point. Why do you keep harping and worrying. If your that worried go get your BMS and be done with it.


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## JRP3 (Mar 7, 2008)

dtbaker said:


> who cares about self discharge?
> 
> what I care about is whether and how much and how fast cells may or may not drift apart when connected and cycled in series (i.e. actual use) to answer whether or not a 'balanced' pack will become unbalanced over time, how much, and how fast.... as well as other factors like configuration, split packs at different temps due to different config or insulation, etc.


Some have suggested that cells will go out of balance because of uneven self discharge, so you care about self discharge if it's actually significant


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## dtbaker (Jan 5, 2008)

gottdi said:


> They DONT. Thats the point. Why do you keep harping and worrying. If your that worried go get your BMS and be done with it.


I'm not worried... but AM open to reviewing test data and verification or disproving a theory! If I were worried I'd be checking and measuring more than every thousand miles of use.....


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## dtbaker (Jan 5, 2008)

JRP3 said:


> Some have suggested that cells will go out of balance because of uneven self discharge, so you care about self discharge if it's actually significant



with relatively daily use, I see little opportunity to self-discharge compared to drift due to variable like internal resistance or temp of split packs in cycled use.

so far I have not seen any 'drift' in my pack, but am open to what others have done and seen as long as the test is well described to help understand the results.


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## Guest (Aug 5, 2011)

> ........if it's actually significant


Which it's not. My prediction is dead on correct. As soon as one is busted another myth surfaces. So far all have been debunked. What makes you think these won't. It always happens and it always happens with folks unwilling to do any testing. Mostly it is from those who only care to make a stink. There is no issues with split packs either. The main one doing the testing is driving a few all electric cars with split packs with ZERO, Get that ZERO issues. Tons of testing goes on behind the scenes and you only see a scratch. Jeeze, guys, you should be thankful that he is spending his own money and time on this stuff and feeding you too. 

It is from my experience that many will just dump in the trash what they do not earn themselves. No respect and no ethics. If a person is doing the testing and you keep coming up with junk then why don't you invest in the equipment and do it your self and then post the results LIVE on video for all to see. Show your work. Then you will have earned some respect and admiration among those who do learn from others. Not many here but there are some.


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## coryrc (Aug 5, 2008)

JRP3 said:


> What test did you want to see?





coryrc said:


> No need to wait 5 years. Just do the following:
> 1. Take a few cells of the same type. Charge all to full.
> 2. Measure the full discharge, in amp-hours, of each cell at a low C-rate (to avoid any heating effects). Record.
> 3. Charge each to full again.
> ...





JRP3 said:


> I have no faith, I look at data. All that more rest time would do is give slightly higher resting voltages, not change the relationships between them. I can't fault Jack for not waiting hours between discharges, but feel free to take weeks to do the same test. Use plenty of caps when you post your results since that will make them more meaningful


I don't understand how you can think you are testing self-discharge if you don't allow the battery to self-discharge?



JRP3 said:


> Oh really?


It would have been better worded as:
I do not think anyone thinks that Thundersky batteries have high self-discharge at 50% SoC.


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## Guest (Aug 5, 2011)

> I don't understand how you can think you are testing self-discharge if you don't allow the battery to self-discharge?


What? The cells sat for 2 1/2 years what friggin else do you do to self discharge than to let them sit for 2 1/2 years. At half charge or full or empty. I have a batch of cells that have been sitting for almost 2 years now in the 2 volt or lower range where if there was any self discharge you would know long before 2 1/2 years and guess what. NO CHANGE in voltage. We are doing capacity tests now to see how well an abused cell will accept a full charge. There is NO self discharge. NONE. There is no DRIFT either.


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## coryrc (Aug 5, 2008)

gottdi said:


> What an amazing prediction. And so fast too!
> 
> Sloppy? What have you provided? Spend a week or more doing the same thing and you will get the same results.


To repeat:
1. Headways with high self-discharge:
http://www.diyelectriccar.com/forums/showthread.php?t=59330

2. Jozzer's test showing self-discharge: http://www.diyelectriccar.com/forums/showpost.php?p=251339&postcount=10



gottdi said:


> Multiple things were done by this. One that there was NO self discharge after 2 1/2 years of kicking around in a non controlled environment. From the point of being in the box to full was 45 AH in.


How do you know that there was exactly 45Ah in it? How do you know it didn't start with 47Ah or 49Ah or 45.02Ah? To what precision are his results?

Your claims of "NO self-discharge" are extraordinary considering I've used other LiFePO4 cells that demonstrate self-discharge and that it would violate the laws of thermodynamics.

Extraordinary claims require extraordinary proof. The test I've mentioned will demonstrate self-discharge. Anything less is just wankery.


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## Guest (Aug 5, 2011)

So what is the self discharge? Do you know? NOPE because there is none. Not even when you measure in the thousandths of a volt. There is NONE. So if you don't believe the tests then you go do the tests and show all your work. Unless you do that then your words are pure bull shit.


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## coryrc (Aug 5, 2008)

gottdi said:


> Which it's not. My prediction is dead on correct. As soon as one is busted another myth surfaces. So far all have been debunked. What makes you think these won't. It always happens and it always happens with folks unwilling to do any testing.


Err, exactly what testing have you done?


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## Guest (Aug 5, 2011)

> To what precision are his results?


Why don't you go watch and listen.


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## Guest (Aug 5, 2011)

I don't but I trust those that do and show the results and show the process. Go friggin watch and listen and learn. Stop blowing air you have no clue about.


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## Guest (Aug 5, 2011)

He is the only one showing the process and showing the results. The only one.


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## coryrc (Aug 5, 2008)

gottdi said:


> So what is the self discharge? Do you know? NOPE because there is none. Not even when you measure in the thousandths of a volt. There is NONE. So if you don't believe the tests then you go do the tests and show all your work. Unless you do that then your words are pure bull shit.


Self-discharge isn't measured in volts.
I believe this is why you don't understand.

Self-discharge is a rate dependent on state of charge, temperature, and probably other factors. It is measured in coulombs over time aka current (amperes).


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## coryrc (Aug 5, 2008)

gottdi said:


> He is the only one showing the process and showing the results. The only one.


So because I only documented mine using text, it is not valid because I didn't make a video?

And, since you never did any testing, by your own admission, you shouldn't be trying to talk yourself smart, right?


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## Guest (Aug 5, 2011)

So what does putting cells in parallel actually do?


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## coryrc (Aug 5, 2008)

gottdi said:


> So what does putting cells in parallel actually do?


Umm, you'll have to define "do".


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## Guest (Aug 5, 2011)

coryrc said:


> Self-discharge isn't measured in volts.
> I believe this is why you don't understand.
> 
> Self-discharge is a rate dependent on state of charge, temperature, and probably other factors. It is measured in coulombs over time aka current (amperes).


Do friggin what? What? What? What? What? What? Dependent upon SOC and SOC is shown with voltage. Proven. So if you have self discharge you have lower voltage readings. What a friggin ........... 


This is what Jack showed because of all the chatter about SOC not having any relation to voltage. What bull shit. Voltage is related to SOC. So is capacity. Empty the cell and the voltage is low. Fill it and the voltage is high. Even an elementary school student would see that voltage is related to SOC. Self discharge IS lowering the capacity and if the capacity is lower so will the voltage. Even if at 50%. So if over 2 1/2 years the self discharge is 10% total then if you were at 50% SOC you would then be at 40% SOC and you WOULD see a different voltage and you would know by the voltage what SOC your sitting at. 

Still friggin arguing what has been busted. What a waste of time.


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## Guest (Aug 5, 2011)

What does putting cells into parallel do? What is to explain. I am asking you to answer the question. No defining. The question is dealing with what happens when you put a group of cells into parallel?


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## Guest (Aug 5, 2011)

> So because I only documented mine using text, it is not valid because I didn't make a video?


Any one can type themselves smart.


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## Guest (Aug 5, 2011)

Yes, show your work. Your science teacher requires it from the students. The math teacher requires it from the students. Do the work and show the work that got you there.


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## Guest (Aug 5, 2011)

> Umm, you'll have to define "do".


Answer the question.


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## rochesterricer (Jan 5, 2011)

Forgive my ignorance, but I thought that voltage was a poor indicator of SOC due to the flat, or nearly flat area of the discharge curve. I thought this was one of the reasons you top or bottom balance, because there is a large portion of the middle of a cell's capacity where it will carry a constant or near constant voltage.

I'm certainly nowhere near an expert, so someone feel free to correct me if I'm wrong.


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## dtbaker (Jan 5, 2008)

where are you guys (gottdi and coryrc) going with this? you can have opinions all you want, but unless you have data supporting something you are pretty much just wasting bytes.

if there is, or isn't, self-discharge with cells sitting on a shelf, is much less interesting than if there is 'relative drift' between cells in series (after some intial balance) during cycled use under actual conditions. making a case as to whether active BMS is worth the time, expense, and risk of new failure points.

it may be academically interesting to know if cells statistically deviate a few thousandths of a volt, or 1% of capacity, over time if left on the shelf.... but I think most daily users want to know if a pack will BECOME unbalanced, and how fast, with normal use... to determine if an active BMS is cost effective, or an annual manual inspection and re-balance is sufficient.


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## coryrc (Aug 5, 2008)

gottdi said:


> Do friggin what? What? What? What? What? What? Dependent upon SOC and SOC is shown with voltage. Proven. So if you have self discharge you have lower voltage readings. What a friggin ...........
> 
> 
> This is what Jack showed because of all the chatter about SOC not having any relation to voltage. What bull shit. Voltage is related to SOC. So is capacity. Empty the cell and the voltage is low. Fill it and the voltage is high. Even an elementary school student would see that voltage is related to SOC. Self discharge IS lowering the capacity and if the capacity is lower so will the voltage. Even if at 50%.


First, my experience and data from manufacturers agrees that there is a very small change in voltage over SoC change given zero current draw and a rest period. I have stated this myself a couple months ago: http://www.diyelectriccar.com/forums/showpost.php?p=244689&postcount=21



gottdi said:


> So if over 2 1/2 years the self discharge is 10% total then if you were at 50% SOC you would then be at 40% SOC and you WOULD see a different voltage and you would know by the voltage what SOC your sitting at.


Where you are going wrong is implying that I believe the SoC would fall from 50% to 40% over 18 months on Thundersky batteries. I don't know the degree to which it will drop, only that entropy mandates there be some change. If it fell from 50% to 49.5%, jack's test is not precise enough to locate that difference. I would like to know the real number as well.

I would expect the self-discharge to be much higher at a high SoC. I'd be happy to see any data showing this to not be the case.

In most of my testing, the BMS draw swamps the self-discharge, so I never much cared what it was.

If you stated "Thundersky self-discharge is low enough that I may not have problems" I would agree.


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## coryrc (Aug 5, 2008)

gottdi said:


> What does putting cells into parallel do? What is to explain. I am asking you to answer the question. No defining. The question is dealing with what happens when you put a group of cells into parallel?


It connects their positive terminals and negative terminals together, so current shares based on the relative instantaneous impedance of each cell.


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## Guest (Aug 5, 2011)

There is no imbalance. That is the point of the thread and arguments. You can force an imbalance by tapping off a few cells but beyond that there is nothing. Who has the most cars and most split packs and no thermal management or BMS systems and has been doing so for over two years now. If you have not seen anything in that time then the made-up problem is just that. Made up. It is a myth. So lets say that over the course of 10 years you do have an imbalance of a couple thousandths of a volt. So friggin what. It won't kill your pack or do you any damage. If the problem ever does arise then deal with it then. Don't argue about an issue that has been shown to not exist. It is easy to show this with Lead Acid, Nicad, NiMH, and NiFe cells. Hell you can even show it with dry cells like duracell batteries. But with lithium LiFePO4. No. I'd bet that there are other lithium cells that show the same non issues as the LiFePO4 cells. 

You want to know something that is not there and are trying too hard to prove or want it be so. Why not take what has been proven and shown and go with it. 

I don't want to just see in type. The internet has shown the need to show the work properly so you know what is going on and to explain the process. I know of no one else doing that. I just keep hearing what others are saying but not showing. I doubt they even have a lithium cell to play with. 

I have quite a few of them. Used and some abused. None of my playing around with them support the Bull shit being spread around.

I did make a prediction. It came true as expected and about as quickly too.


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## coryrc (Aug 5, 2008)

gottdi said:


> Yes, show your work. Your science teacher requires it from the students. The math teacher requires it from the students. Do the work and show the work that got you there.


Context of this post: 10 10Ah cells had been charged to 3.65V then put in a box for 2 months. Their resting voltage was right around 3.28V. They took a combined 100A for almost exactly one hour.

http://www.diyelectriccar.com/forums/showpost.php?p=249293&postcount=27


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## coryrc (Aug 5, 2008)

gottdi said:


> There is no imbalance. That is the point of the thread and arguments. You can force an imbalance by tapping off a few cells but beyond that there is nothing. Who has the most cars and most split packs and no thermal management or BMS systems and has been doing so for over two years now. If you have not seen anything in that time then the made-up problem is just that. Made up. It is a myth. So lets say that over the course of 10 years you do have an imbalance of a couple thousandths of a volt. So friggin what.


Ah, we're finally getting somewhere. Wouldn't you rather know exactly how far off it will be? I would. I'd like to know exactly how much, because if the SoC drift over ten years is only 1Ah, I may not need to use a cell-level BMS anymore. I could just use connectors and bottom-balance every couple years ago with a one-use machine that may cost more, but I could share with several others.

I don't have any equipment accurate enough to measure the self-discharge of Thundersky batteries at 50% SoC. I know the Headway batteries are easier because they drop so fast (relatively speaking).

Tonight I am going to start a test with some spare Sky Energy cells. I am going to charge and hold them to 3.55V over the weekend. On Monday, I will disconnect and label them. In a couple months, I will pull them back out and measure how much energy it takes to charge them again.


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## Guest (Aug 5, 2011)

Thank you for the answer. That is a good one. So if the percentage is less than 1/100th of 1% over the two and a half years where you can't see any change measuring thousandths of a volt then the problem does not exist. The argument is self discharge. Not just with TS cells but with any of the LiFePO4 cells. There is no measurable difference in 2 1/2 years and with that there is no issue. If any continues to believe so and believes they need a BMS based on the argument then I guess they can go waste their money. And if you have a pack of cells you are more than likely using it and there is no self discharge issue. Even with actually about 3 years now there is not imbalance of cells in the vehicle to warrant rebalancing the cells in any of Jacks cars. NONE. How long does this debate have to go on. How precise must you measure before your happy with the answer? 

It's called NIT PICKING. Just like go pick all the nits you like but stop blasting those who are actually doing the work. If you don't like how they do the work then you do it how you want it. I see no issues with that. But if your not doing any work then back off those who are. I will be posting a couple nit picking issues tomorrow that deal with top balancing and shunting and the upper limits of charging a cell vs bottom balancing and staying off the top or bottom. NO BMS required. That's no BS either. 

It is show and tell time.


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## Guest (Aug 5, 2011)

coryrc said:


> Context of this post: 10 10Ah cells had been charged to 3.65V then put in a box for 2 months. Their resting voltage was right around 3.28V. They took a combined 100A for almost exactly one hour.
> 
> http://www.diyelectriccar.com/forums/showpost.php?p=249293&postcount=27


But they were in parallel. You need to do this with each cell. Not combined. All you did by paralleling the cells was to make one 100AH cell. Unless they were all at the same SOC then the test according to your thinking is moot. 

One of my 100 AH cells was taken apart only to find two 50 AH packs in parallel. Each pack need to be in perfect or near perfect match for capacity before putting them in a single pack. In the case they then become ONE cell. And are treated as such for the remainder of the life of the cell. If all your little cells were at exactly the same SOC and Capacity when put in parallel the the test would be valid. If not then the test is not valid. If one was out of balance with the others the capacity could be different and the SOC would be different but by combining them you'd only see one voltage and SOC and capacity which would be a balance of the two on your meter. But you would not know the value of each by looking at the meter. 

If one cell was never at the top then it could be perceived that that cell had a self discharge when in fact it did not. 

This is way I asked you about what happens when you put a bunch of cells in parallel. 

Did you capacity balance and SOC balance your single cells first? Not from what I get from your post.


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## Guest (Aug 5, 2011)

> SoC drift over ten years is only 1Ah


Your talking capacity. Not self discharge. Self discharge has to do with SOC over time. 

Is there a drift in capacity over 10 years? Well the cells used in the Nissan Leaf have a documented 10000 miles and over 340 Fast charges on them and the cells are still showing a full 100% capacity. No drift of capacity in 10000 miles. Nissan was expecting to see the pack only holding 80% after 5 years. Well with this information the cells will still have at or near 100% in 5 years and Nissan is expecting their cells to last the full life of the car where no owner will ever need to change the pack. 

No drift or self discharge. The issue is moot and point less and so is a BMS pointless. Monitoring to a degree is not pointless but the BMS systems being touted at this time are pointless for our DIY vehicles with LiFePO4 Cells. Pointless.


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## Guest (Aug 5, 2011)

> Tonight I am going to start a test with some spare Sky Energy cells. I am going to charge and hold them to 3.55V over the weekend. On Monday, I will disconnect and label them. In a couple months, I will pull them back out and measure how much energy it takes to charge them again.


Test if you must but video the events and record them. And measure the voltage over the course of the couple months too. To the thousandths or greater. If you can't don't bother.


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## coryrc (Aug 5, 2008)

gottdi said:


> Did you capacity balance and SOC balance your single cells first? Not from what I get from your post.


I think I did not explain to you properly what my test was supposed to show.
It was supposed to show that the (admittedly low-quality) cells had discharged nearly (but not completely) 100% in two months to a resting voltage of 3.28V.

Initially, all cells were placed in parallel and brought to 3.65V and an average of 2.6mA per cell. Do you believe that any cells were not charged at this time? If so, then how would you define full?


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## coryrc (Aug 5, 2008)

gottdi said:


> Your[sic] talking capacity. Not self discharge. Self discharge has to do with SOC over time.


I was not talking about capacity, I'm sorry I wasn't clear to you. I was talking about the remaining charge of the battery, measured in amp-hours. SoC is remaining charge divided by maximum capacity. Charge is measured in coulombs or amp-hours. I chose to use amp-hours rather than a percent. If you wish, pretend I said 0.5%


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## coryrc (Aug 5, 2008)

gottdi said:


> Test if you must but video the events and record them. And measure the voltage over the course of the couple months too. To the thousandths or greater. If you can't don't bother.


Do you have any spare cells so you can do the test with me as well?

Because I doubt you will believe me unless you see it with your own eyes.


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## Guest (Aug 5, 2011)

I know and read what your post said. Having a cell discharge to near 100% is not showing self discharge but showing a problem. 

When you put in AH your counting capacity. A 100 AH cell will accept 100 AH or more.

Fill the glass. Empty the glass. Capacity is capacity. How much can it hold. Voltage is related to capacity. The fuller the glass the higher the voltage will show. The more you empty it the less voltage it will show. So if you self discharge or discharge under load you still show a change in voltage that is measurable and it does correlate to the SOC. 

Goes hand in hand. Capacity, SOC and Voltage. 

Your cells have a problem

I already have seen so I have no issues like you or a few other hold outs. But I already said I don't have the proper equipment to test to the level need to prove. But wait, it was just done last week. How about that. What more do you need? 

Do the test. If I had the equipment I'd have done it long ago. And on video too.


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## Guest (Aug 5, 2011)

> Initially, all cells were placed in parallel and brought to 3.65V and an average of 2.6mA per cell. Do you believe that any cells were not charged at this time? If so, then how would you define full?


Were they put in parallel directly from the box? I suspect that they had been used and charged and discharged and played with but once you kick them around like that you must bring them back into balance before you can put them in parallel. I have no doubt that the cells charged. I have done that too. But the level of SOC remains the same. The now single cell is not different than any other single cell. All I say is for your test to be valid all your cells need to be in balance and SOC and capacity. Do that first. Then parallel them then charge them. It is the easy way to charge up a bunch of cells fast. For a proper test you should not do them in parallel but on a single basis and you must record and measure each cell then when done you have a grouping of cells and all the information and you can even do an average of a grouping and you have more than one to test at the same time. But if your going to test be precise and log and video the long boring paint drying events. Or at least turn on the camera at regular intervals to log and record. It is long slow boring work.


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## Guest (Aug 5, 2011)

rochesterricer said:


> Forgive my ignorance, but I thought that voltage was a poor indicator of SOC due to the flat, or nearly flat area of the discharge curve. I thought this was one of the reasons you top or bottom balance, because there is a large portion of the middle of a cell's capacity where it will carry a constant or near constant voltage.
> 
> I'm certainly nowhere near an expert, so someone feel free to correct me if I'm wrong.


Your forgiven. It is a good indicator of SOC. Actually an excellent one. But a lousy fuel gauge. It is hard to get an accurate reading during active discharge. You can get an indication but not accurate while actively discharging. So as a fuel tank indicator its crappy. Excellent for static SOC measurements. 

Pete 

Go watch Jacks latest and greatest paint drying video. He can explain better. It is after all his work I am making references too. Some from other sources but he is the only one actively doing testing and showing his testing and showing results. I trust that more than someone arguing a point less point.


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## coryrc (Aug 5, 2008)

gottdi said:


> Were they put in parallel directly from the box? I suspect that they had been used and charged and discharged and played with but once you kick them around like that you must bring them back into balance before you can put them in parallel. I have no doubt that the cells charged. I have done that too. But the level of SOC remains the same.


The cells were taken straight out of the box, measured for voltage, then put on the charger. (They were all ~3.3) Charging parallel cells to 3.65V *is* balancing them all at 100% SoC. That's the whole point.

How can you say the cells are charged but the "level of SOC" stays the same? You can't charge a cell and have its SoC stay the same.

Why do you say "you must bring them back into balance before you can put them in parallel"? What would make you assert such a thing?


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## coryrc (Aug 5, 2008)

gottdi said:


> I already have seen so I have no issues like you or a few other hold outs. But I already said I don't have the proper equipment to test to the level need to prove. But wait, it was just done last week. How about that. What more do you need?


I told you several times what is needed to measure self-discharge.

Self-discharge is in amperes. Not volts. You can measure it by fully charging a battery, waiting some time period, then measuring how many coulombs it takes to fill that battery again. Divide by your time period, and you have average self-discharge.

Voltage is not accurate enough to show small amounts of self-discharge. I'm sorry you don't get it.


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## Guest (Aug 5, 2011)

When I said the SOC stayed the same I was saying that two cells at differing SOC levels put into parallel each will stay pretty much at the same SOC as when they were put in parallel. I said it is required to put then in parallel when they are all the same which you say you did. No problem there. My statement was about as clear as mud. 

Yes I do get it. 

Also I am not saying you believe that the self discharge is a specific rate. I am saying in general about all the bull shit about this subject and that it has been stated that the self discharge is at or above 3% per month. At that rate you WILL see a VOLTAGE CHANGE. When you discharge a cell it looses amperage and that amperage loss is measured by a loss in VOLTAGE. VOLTAGE is our visual. Amperage loss is an active loss. At the level of self discharge I doubt we have a meter that will measure it on the spot like when your driving your vehicle. 

Selfdischarge is an active process in which the chemical reactions are changing resulting in a loss of the charge within the cell. I am not talking about loss of capacity due to damage that will never be recovered. I am just talking about the capacity of the charge that was last put into the cell. 

A cell comes from the factory at 50% SOC. Shown on the last test to be true. 
The Voltage reading from the same cell after 2 1/2 years of kicking around in an uncontrolled environment still showed a voltage that relates to a 50% SOC.
That same cell was then charged to full at a rate of 1C according to the requirements for charging these cells. At which time with good accuracy the cell had 50% or 45 AH put into the cell. The same cell was then discharged at the same rate and showed that the cell held 90 AH which is the rated AH of the cell. This is to the top of the curve and to the bottom. 

If there had been even a 1% loss in the amount of energy within that cell over that 2 1/2 years of time you WOULD see a lower VOLTAGE reading related to the SOC of the cell. 

That relation of voltage to SOC was as critical component in the test. VOLTAGE is how you SEE a change in the SOC. 

But for it seems in your case you are more or less in agreement that the self discharge rate is so low that the argument is then moot. If it is soooooooooooo low that it would take 20 years to even register in VOLTAGE a change then the WHOLE argument is MOOT. 

If that is the case it can be said that there is NO FRIGGING self discharge. The proponents of the BMS system had been using the argument to get people to believe that they needed a BMS system on the car. After each time something was shown to be bull shit they jumped in with some other bull shit. Now it's split pack temperature that will cause a change enough so you NEED a BMS (ooops BS) system to make sure you don't have an imbalance in your pack. 

These are the arguments state that the imbalances are going to be great enough because it is what happens with lead and other types of batteries and if it happens with those batteries it MUST happen with these. 

If you don't think self discharge is an issue or a problem enough to warrant a BMS to be sure you don't have issues then fine but stop arguing the point that these cells have a self discharge. They don't.


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## Guest (Aug 5, 2011)

> Why do you say "you must bring them back into balance before you can put them in parallel"? What would make you assert such a thing?


If the cells are out of balance before you put them in parallel or if one cell has a greater or lesser capacity and if the differences are great enough you can have a problem. They won't magically mend themselves. You can't change capacity. You can however change the SOC which needs to be the same so you balance them if they are not balanced. 

That is all. 


I'd say that when you put your cells to bed to wait you had some sort of discharging across terminals. Maybe there is enough of a current path across the material you stored them in that was touching the terminals that caused a discharge great enough to discharge our cells and when you pulled them out and measured the VOLTAGE to determine the SOC or what was left of the capacity you found that they were nearly empty. So either your cells are damaged internally or the packing material was the cause of the discharge. When you do it again be sure you keep one set of terminals fully isolated from any contact with any thing. Or be absolutely sure that the material is not going to cause a discharge over time. Be sure, don't guess.


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## Jozzer (Mar 29, 2009)

I think your misunderstanding may be that others say "BMS's are needed because of self discharge". That is NOT the reason a BMS is required. That's just the reason why you should check your cells every once in a while to make sure they are not discharging themselves to death in storage..
Seems to me you don't have a full grasp of the argument you are making.

One real reason for a BMS is to make sure that you have full capacity avaialable, another is to make sure cell voltage limits are not breached by accident. Yet another is to monitor temperatures to make sure no cells in your pack are overheating.
Who ever said that a BMS was required to prevent self discharge?


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## Guest (Aug 5, 2011)

It's been one of the arguments from the proponents of the BMS systems stating that there is a self discharge. It came about when our favorite person who likes to watch paint dry said there was no self discharge in LiFePO4 cells. They all went up in arms. I fully understand the notion of what the BMS proponents are bitching about and why they THINK they need a BMS system one the car. I am not unaware of the garbage. It just keep piling up and needing to be taken out. But some just keep going out and dragging it back inside. You can't just say that the temperature differences will cause a problem and that you need a BMS system to make sure there is no problem. You must PROVE there will be a problem Our famous paint watcher has been proving but the BMS proponents don't accept the work. So where is the work from the proponents of the BMS systems proving that there is a problem to be monitored and managed by a BMS system. Sure, you may have temperature differenced but you must prove it will be a problem first. Just because there are temp differences does not mean its a problem. It's not. As for having full capacity available is total bull shit. Your pack is regulated by your lowest capacity cell and you never ever want to or need to go to the max level of capacity. The top part of the charge curve is a waste to bother with. I can show that. The bottom is dangerous. So stay within the safe zone and that has been shown too. Even the manufacturers have changed their tune about the safe top and bottom which utilizes the capacity of the cell. No need for a BS system to keep that in line. Proven and shown and done so for nearly 3 years now but the proponents of the BS systems still argue. 

If you have a dumb charger and dumb controller then I will agree you need at least a system to keep you from over charging your cells and over discharging your cells. I have stated that myself many times in the past. That I have no issue with. My charger does a fine job of controlling the top of the charge. My controller does a fine job of keeping me off the bottom cliff. I utilize the safest zone of my lowest capacity cell in the pack which is not much different than the other cells so I do utilize the capacity and keep in the safe zone. No need to put the batteries at the top most part of the curve. It's a waste of time and gives you maybe about an extra mile of 55 mph driving distance. If you need the extra then put in an extra cell. It will give you more for your money than a BMS. 

Yes, I do understand.


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## MN Driver (Sep 29, 2009)

Not all of this discussion is about whether or not a BMS is required, much of the posters in this thread are actually aiming at finding out how long or how many cycles that someone should go through before doing a manual check of the cells to determine that everything is still in check.

Don't ignore too many facts here, there are two threads referenced in this thread about cell discharge of multiple brands of LiFePO4 where self-dishcarge WAS a factor, by people with actual experience. Stop saying that it doesn't exist. It might not be much or detectable with untouched TS cells but it seems its a factor with cylindrical cells which suggests that it is also possible with prismatics or might take some cycles to invoke it.

The other factors of what happens while cycling to pull cells out of balance slightly over time will cause someone to want to take a look at one time or another.

Don't believe too closely to your battery sermon videomaker. He's killed cells in two vehicles(or was it three, can't remember if he killed any in the Mini) and too many through bad negligence. The battery voltages at the end of discharge in one speedster and the spyder had an imbalance. The Spyder wasn't bad and was caused by a load on those cells but the Speedster lost cells and the lack of someone watching the pack on his NEV cause almost his whole pack to croak. Instead of just blindly watching videos, you might want to listen to what people here have to say who've actually experienced exactly what you are talking about and not skip over the details of Jack and his escapades of cell destruction. Not all of it was through stupid mistakes, some has gone unexplained by him and had the sand raked over.

I don't sell a BMS, I don't say you need a BMS, and nothing about this entire post suggests anything about a BMS. To me the real question is: How long many cycles or time should a pack be used before it gets checked up on. You might have cells that don't dip low but unless you check them under load by one method or another to be sure they are not sagging to their death at lower SOC(10-20% or so) or have a few that have degraded, lost capacity, or balance, are you just going to let them die? There are ways to bolster a weak cell but if you don't do it when it begins to weaken you'll kill the cell through either overdischarge or overheat and you've lost a cells worth of capacity.

I'm planning on keeping my pack longer than Rickard does, he's even said in his videos that he doesn't care how long his pack lasts. He doesn't have his original two packs that he's had in cars still operational in those cars. The oldest right now is the pack in the Mini, his very first pack was replaced. We won't be getting long life data real-world from Jack unless he keeps the cells in his Mini for a long time and continues to drive it. But then again his pack in that car is HUGE and he doesn't drive far so that pack is being treated very gingerly at relatively low discharge rates and his drives are short and he's retired so he probably doesn't drive often. His data to me is a bit sparse to me. Not saying I can't use it but there are others here on this forum with decent usage of their packs and over time plenty of people will pass up Rickard with miles, cycles, and total kwh pushed through their pack. I'm still waiting for someone who builds a long ranger 115 mile car and drives a round trip commute of 80 miles a day(70% DOD). If I found someone doing that, I'd be all ears because that person might spend a ton on a battery but the payoff versus using gasoline would be that much bigger for that person.

My thoughts. Try reading them with some critical thinking and don't throw them to the ground too quickly. When you post something and declare it as absolute truth don't disregard other peoples direct hands-on experience that they post in your thread and tirade against what they had to say completely ignoring what they actually said otherwise all you are doing is watching the guy who videos himself smart while killing batteries and cross-threading the terminals, otherwise all you are doing is typing yourself smart based on what he recorded himself smart about.


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## Guest (Aug 5, 2011)

Nope, sorry but I am not basing my knowledge solely upon his work. As to when to check. The answer is very very simple. When ever you like. Until your tired of testing but even then you will. Will test mine in my Leaf? Hell no. No need. There is no issue. Will I continue to check on my own build. Of course but not because I believe there is self discharge or drift. I do just to show you and others that there is nothing to worry your self over. Like Jack says, now that you have the information what the hell are you going to do with it? If nothing the gathering of information is worthless. For Nissan it is to build better. For us here it is to confirm what has been said all along. 

As for the cylindrical cells. There was a problem in the packing of the cells which caused a discharge of the cell. Not a self discharge on it's own. The reason for the argument of a cell undergoing a self discharge is to prove you can let your car sit for long periods unattended with no ill effects. Unless of course you have components attached that put a drain on the cell and if that is the case it is not a self discharge. If you let your vehicle sit for long periods you must disconnect all active components to keep from draining your pack. But if you do drain your pack and your cells are balanced at the bottom you can just go charge it back up and be on your way happy as a clam. 

There is no damage or should I say loss of capacity to the cells sitting long times even at below 1 volt. 

The speedster lost cells at the end of the drive because it was when he TOP balanced his pack and drove a couple prematurely into the ground because they were not balanced at the bottom. So did I. The very end of the curve you will see varying voltages because no single cell is exact. We bottom balance to a specified voltage and stay above that but when you take the whole pack to the utter bottom with the bottom balanced they all go at the same time with tiny variances but never enough to cause the pack to drive a few into the ground. If they all reach the bottom at nearly the same rate then there is no issue. I have 15 cells that were allowed to discharge due to a faulty Factory BMS. The pack was charged to full and there was a faulty BMS in the pack but that faulty BMS allowed the pack to discharge on it's own to near zero volts. One is even at .01 volt. Been sitting there for a couple years. But they still hold a charge and still hold greater than 100% capacity when charged back up. A few cells are in the .6 volt range but the differences at the bottom are not great. For the purpose of being out of balance enough to cause a battery death the varying voltages were tiny. So once again. NOT and ISSUE but you insist that they are. 

I do not blindly follow. Nor should you. Where is the data that SUPPORTS the use of a BMS on LiFePO4 Cells. Or others for that matter. SHOW the data. Don't just type it. 

I follow Jack because he IS testing and SHOWING. Who else here is doing that. 

I know all about all the arguments about the MUST have A BMS. I am not blind and do not follow the blind. I can site others work if they are showing the work. I site it because my messing with them supports his work shown. It does not support the well maybe or any other speculation. If there is a speculation then test it and if it proves unsupported so what. You just learned something. Every one wants to be an expert. But you can't be an expert without proper testing and proper attitude to your findings. If I think one thing and my tests confirm another them my thinking is wrong. No speculating. 

Take the issue of top balancing and keeping your cells at the top of the charge knee. What doe it do for you? Well it keeps your cells fully charged and if shunted, balanced at the top. But what do you gain? Well, about 1.5 miles extra of driving after which your no longer on the top. If you want extra capacity just get some more cells in your pack. It's cheaper. 

As for cylindrical cells? Balance them too and go have fun. 

I will have more to show later.


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## Guest (Aug 5, 2011)

In the simplest of terms there are either issues requiring a BMS or there are no issues and a BMS is not required. There is no maybe this or that and not speculating. It is either Yes there is or No there is not. All testing shows points to No there is not. Consistently reproducible too. You test and test again and again if need be but it either is or is not. If you THINK it is then you go PROVE it IS or IS NOT. If it turns out its not then you say Not. If you THINK it is not and you PROVE it is then it is. 

What do you have to show today?


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## david85 (Nov 12, 2007)

I can understand that BMS and related spin off topics are controversial around here (or anywhere else in the conversion community), but could we leave the name calling and confrontational content out of the discussion, please?


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## Guest (Aug 5, 2011)

Name calling? Calling bull for what it is needs to be done. If you won't allow it I will gladly leave. The controversy is made up by those who think that these type of cells are going to act the same as the old lead acid technology and if they continue to push that I will continue to push back until it stops or I get booted off the forum for being honest.


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## MN Driver (Sep 29, 2009)

"I do not blindly follow. Nor should you. Where is the data that SUPPORTS the use of a BMS on LiFePO4 Cells. Or others for that matter. SHOW the data. Don't just type it. 

I follow Jack because he IS testing and SHOWING. Who else here is doing that. 

I know all about all the arguments about the MUST have A BMS. I am not blind and do not follow the blind."

...again, you seem to assume everyone is in this discussion is about BMS needs. I specifically discounted the BMS argument and its clear that an electrical device doesn't need to be connected to every cell to handle the needs of a pack. If I actually put anything in a car full of batteries and attached them to every cell terminal it would be a thermistor to monitor temperature and know when to be careful because a cell is getting hot enough to cause it damage. It seems through enough information that that isn't needed. I'll probably have one on two or three cells that I measure out to have the highest internal resistance and those will actually be the ones most likely to end up with any damage due to their heat and I'd be more inclined to monitor those and if they are protected from overheat the rest of the pack should theoretically be fine in terms of heat. Heat is a little off topic but not quite because the hotter the cell the more wasted energy in that cell which is your source of a differential discharge between cells. With well impedence matched cells and lower discharge rates this is less of an issue.

"If you want extra capacity just get some more cells in your pack. It's cheaper."
You could just use the same amount because in the end you want the same capacity. Just handle the batteries properly and you'll get what you need if you've planned your system right. The big problem I have with adding more cells to my pack is the idea that a small, aerodynamic, lightweight body is the ideal platform for a high range EV with a higher relative performance for the same battery, motor, and controller. Smaller, aerodynamic, and lightweight vehicles have less space for batteries, less capability for handling the weight, and also have performance detriments to the high range and high efficiency you were originally after. Not to mention, once you've run out of space for capacity, you are at the maximum range you can get from the battery you chose to put in your car unless you are willing to have batteries in undesirable places which eventually ends up eating more cargo space or passenger space than what you really want for the car to still be practical. There's a balance and I don't really think adding more batteries will really get you farther since there is so little to gain if everything is set up right, which is pretty simple, be sure the cell that is fullest at the end of the charge is always the emptiest at full discharge. If that cell is highest in internal resistance it might take some watching as that one will be the one losing the most energy. If its a different cell and you aren't watching it, that is the one up for the chop but it would be pretty clear if someone just took it down and watched voltages at a low SOC but if the one for the chop isn't the one you are watching as you sink lower that one gets damaged. Similar effect for a cell degrading at a different level than others, which seems to be fairly common in LiCo chemistries and in fact every chemistry I've ever handled before there is always runts that come up, we just haven't had enough time and cycles with LiFePO4 to see it enough to characterize it as the same but LiCo has different internal resistance and capacity degradation levels in a series pack, same with NiMh, NiCd, even alkaline cells from the same pack end up at different levels of remaining charge which made digging through industrial grade piled of Duracell Procells fun.

You are right though, for now and a short time period it doesn't seem to matter but none of us have cells put to use in real life cycling for a long time without intervention to really say anything definitively. Sure, no BMS is doable, that's not what I'm discussing.

What do I have to show today, well, see the whole topic is about what happens over time, none of us have anything to show, possibly for years down the line of not doing anything or periodic monitoring to see if anything issues crop up, a few dud batteries are bound to come up here and there. If I need to push an Ah into a cell or two from year to year to keep it moving along, its better than to just let it die and say "it was a bad cell anyway" even though it was fine if there was a watchful eye from time to time on it. Sure, if we run for 3 years and check it once every 6 months and nothing is happening at all its probably not a bad idea to run another year or two before checking again. I don't plan on running a low SOC often enough though to check to be sure everything is in check so it comes at a specific occassion that I'll want to check it so that way the time when I get caught in a detour on the way back home or a road is closed and I need to take a longer way home I'm confidence where the pack stands if I need to really pull the pack down and still be able to trust it. At the same time at a low SOC would be the time I'd load test the cells to the same value for each cell and monitor voltages at a heavy load because the ability for the cells to perform on discharge is more important to me than their actual capacity because if they can't push current, their capacity is useless and its good to know if you are on your way to that happening so you can do something about it.


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## Guest (Aug 5, 2011)

I am not pointing my finger at you specifically. If your not on the band wagon of the BMS crowd then the arguments are moot for you. Go play and have some fun. If you happen to have some information that point towards your standing then please provide it. If not sit back and relax or better yet go play and have some fun. Oh, I said that already. 

I am processing video at this time. Takes a long time. Easy to tape and long to process.


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## EVfun (Mar 14, 2010)

gottdi said:


> The Voltage reading from the same cell after 2 1/2 years of kicking around in an uncontrolled environment still showed a voltage that relates to a 50% SOC.


I wanted to highlight this because I think it is where the disagreement is at. Right now my 2 untouched TS cells read several thousandths of a volt *higher* than when I received them (last check, +0.002 and +0.003 volt.) Of course the reason is that they are warmer, it is summer now and I got them in early March 2010. My garage temperature varies a little.

To find self discharge, or determine that it doesn't exist, you need to know how many amp hour are in a cell and then find out how many you can get out after 3 months or a year. Voltage will miss all kinds of single digit percentage changes -- I'm pretty sure the SOC on those 2 TS cells of mine isn't getting higher.

So, a person could cycle a cell (say 100 ah) and verify its capacity. You could then discharge it, then recharge it to full (lets say it was 105 amp hours), then put it in storage. At the end of this storage you would discharge it the same way as before and record the amp hours. You would then recharge the same way as before and record the amp hours required. The charging and discharging have to be done the same to get accurate results. 

If you get 103 amp hours out of that cell and when recharging it you can only put in 104 amp hour until full you would have determined that the cell had 1 amp hour of self discharge and 1 amp hour of capacity loss. I'm not sure one of those variables can be *accurately* measured without measuring both. I'm not going to take the time to bother, it isn't that important, but I suspect there is a slight self-discharge at very high states of charge (over 90%.)


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## frodus (Apr 12, 2008)

EVfun said:


> I think it is where the disagreement is at.
> .......
> 
> To find self discharge, or determine that it doesn't exist, you need to know how many amp hour are in a cell and then find out how many you can get out after 3 months or a year. Voltage will miss all kinds of single digit percentage changes --


Exactly! But to add, we don't know for 100% sure that each cell that Jack got was EXACTLY charged to 50% (he's actually just assuming because it was never measured to begin with) nor if those cells are actually over spec (i.e. if they actually store 103Ah), nor if the temperature is exactly the same as this time 2.5 years ago. All these things effect his measurements. 

The only scientifically accurate way to tell, is to charge each cell up, do a complete discharge with accurate equipment and test the Ah out. Then Charge again and let it set for a year (6 months might do). 

I do think that there is some self discharge, but I agree that it might be such a negligable amount and most people won't notice anything, but from my experience of working with cells and charging/discharging packs and letting mine sit for over a year and a half, I have seen some self discharge.


What I can offer is this:
have people pitch in, buy a couple TS and CALB cells (low Ah is fine), I'll charge those, some A123 cells, and some headway cells. They'll all be charged so that the Ah going into the cell is below 50mA and then charge stops. Then let em set and repeat the same experiment again. People know I test cells and build packs for people. I really just want to learn, teach and test. 

My equipment: CBA-II, CC400 discharger, ONYX single cell charger and a Mastech power supply to charge, a DataQ logger as a backup to the CBA-II, and a handful of meters for high DC current and voltage.

So, should we get some 40Ah TS and CALB cells and I'll let them sit in the same area of my dry garage for 6-12 months?

Anyone got spares? I won't own them, they're just donated for the duration of the test.


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## Guest (Aug 5, 2011)

> (he's actually just assuming because it was never measured to begin with)


And your assuming he never did. How quaint.


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## Guest (Aug 5, 2011)

None of my voltages have gone up in the heat.


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## JRP3 (Mar 7, 2008)

Wow this really took off like a fully charged cell at the top of the knee I should point out again that I did mention in my post that Jack's measurements could have hidden a small percentage of self discharge. He put 45 amp hours into the cell and took out 90 amp hours, but if like many of the large format LiFePO4 cells we've gotten it was actually more than a 90 amp hour cell, and it was shipped at 50% SOC, then it actually did drop an amp hour or two over the two and half years. Effectively it's almost nothing, but it may not actually be zero.


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## JRP3 (Mar 7, 2008)

gottdi said:


> When I said the SOC stayed the same I was saying that two cells at differing SOC levels put into parallel each will stay pretty much at the same SOC as when they were put in parallel.


Not sure I agree with this. If two cells are at different SOC that means, as we have been saying, they are at different voltages. Cells put into parallel with different voltages will trade electrons until they are at the same voltage, hence the same SOC. Paralleling cells will put them at the same SOC given enough time.


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## frodus (Apr 12, 2008)

gottdi said:


> And your assuming he never did. How quaint.


I'm not assuming, I'm going by what you and others have said about his test:



> A cell comes from the factory at 50% SOC. Shown on the last test to be true.
> The Voltage reading from the same cell after 2 1/2 years of kicking around in an uncontrolled environment still showed a voltage that relates to a 50% SOC.
> That same cell was then charged to full at a rate of 1C according to the requirements for charging these cells. At which time with good accuracy the cell had 50% or 45 AH put into the cell. The same cell was then discharged at the same rate and showed that the cell held 90 AH which is the rated AH of the cell. This is to the top of the curve and to the bottom.


Where did he run a test on the cell when he first got it to accurately measure the State Of Charge? I saw that mentioned nowhere. 

He took a battery and recorded the start voltage. Then after 2.5 years, he measured the voltage and then charged it and measured how much went in, and it took 45Ah back. 

Can you tell me, how did he measure the SOC 2.5 years ago before he let it sit? 

Without measuring the capacity and the exact state of charge to begin with, 2.5 years ago, there is no way he can state these as fact. Especially when he's trying to PROVE that voltage is an accurate way to measure SOC. He never mentions that he measured it via another proven test before he let it set.


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## JRP3 (Mar 7, 2008)

gottdi said:


> Is there a drift in capacity over 10 years? Well the cells used in the Nissan Leaf have a documented 10000 miles and over 340 Fast charges on them and the cells are still showing a full 100% capacity. No drift of capacity in 10000 miles. Nissan was expecting to see the pack only holding 80% after 5 years. Well with this information the cells will still have at or near 100% in 5 years and Nissan is expecting their cells to last the full life of the car where no owner will ever need to change the pack.
> 
> No drift or self discharge. The issue is moot and point less and so is a BMS pointless. Monitoring to a degree is not pointless but the BMS systems being touted at this time are pointless for our DIY vehicles with LiFePO4 Cells. Pointless.


You do realize that Nissan uses a BMS that actively balances the cells.


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## frodus (Apr 12, 2008)

gottdi said:


> None of my voltages have gone up in the heat.


Interesting that TS acts so differently.... ones I've tested show an increase in voltage after a temperature rise. One time I used a heat plate to heat the cells up (no load connected), and watched the voltage increase.

We did this after seeing A123 at 20C have a huge voltage drop, then as the cell heated, the voltage went back up. Very interesting test.

Here's that test, attached.


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## JRP3 (Mar 7, 2008)

frodus said:


> I'm not assuming, I'm going by what you and others have said about his test:
> 
> 
> 
> ...


You are correct. He is making an assumption that since all cells that he has tested were at 50% SOC from the factory then this one was as well. It's a reasonable assumption but not verified by fact.


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## Guest (Aug 5, 2011)

> You do realize that Nissan uses a BMS that actively balances the cells.


Of course I do. I also know that they are not LiFePO4 cells too. But I do know that they act the same as LiFePO4 or your basic lithium cell. Do you know that Nissan actively collects the battery information from every car?


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## JRP3 (Mar 7, 2008)

gottdi said:


> But I do know that they act the same as LiFePO4 or your basic lithium cell.


Not really, they have somewhat different characteristics, especially compared to a basic lithium cell, which is LiCo.


> Do you know that Nissan actively collects the battery information from every car?


Yes I do.


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## frodus (Apr 12, 2008)

I mean it's interesting that he's doing these tests. I had doubts before about the SOC and voltage (it's kinda shooting a moving target though, and the theory still needs tweaking). Some cells I've seen just don't act the same exact way, but My meter may not be calibrated enough or accurate enough to do this test. 

The part I don't like about some of his tests, is that he doesn't back it up with another test, he does a test and uses the hypothesis to prove that his theory is right.....

If I'd have done that in college, I'd have gotten F's in my classes.

His test may be valid (measuring the end voltage, then charging to measure Ah in), but he didn't do the most important thing..... he doesn't seem to have measured the beginning voltage and the AH actually in the cell. Without that, you can't compare. So we'll never know if he's actually right, or "sorta right" or "completely wrong" because there's no initial benchmark.


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## Guest (Aug 5, 2011)

Do you know that after 10,000 miles and over 320 Fast charges included in the normal charging of the cells in a Leaf that there is no change in battery capacity? Do you know that Nissan expects to see up to a 20% loss in capacity in 5 years? Do you know that because there is no noticeable change in capacity even with hard charging that the pack will actually be at or near 100% in 5 years. Average is expected to be around 10,000 miles per year and if in one year there is no noticeable difference there more than likely won't be in 5 years or 8 years unless by some marvel we don't know about yet the battery will at a specific time in it's life just take a dive and its over. 

Come on guys. NIT friggin PICKING. There are no issues that REQUIRE a BMS. Now if you WANT one by all means go get one. Spend the money. I won't. The BMS that was on my cells was a manufacturer BMS and it failed. It still charged the cells but failed and as a result drained the entire pack to below or nearly below 1 volt. Thankfully they are actually still good and still hold more than 100% of the rated capacity. 

Magic beans.


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## frodus (Apr 12, 2008)

Side note: gottdi, I'm not arguing for or against BMS, I use one for my own reasons. I want to log my charge/discharge eventually, both with and without balancing. 

All I'm doing in this thread is questioning his testing, which seems like it could use some more "before" testing to compare to the "after".

If you've noticed lately, I stay away from arguing about for/against BMS anymore. People have their points pro and con for them. I give people the option, but I do think for people that don't have our experience, they need some sort of monitoring to help them along so they don't accidentally ruin the pack. Hell, I'll say use monitoring, but it's up to the end user to decide if top or bottom balancing is right for you or your cells.


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## JRP3 (Mar 7, 2008)

gottdi said:


> Do you know that after 10,000 miles and over 320 Fast charges included in the normal charging of the cells in a Leaf that there is no change in battery capacity? Do you know that Nissan expects to see up to a 20% loss in capacity in 5 years? Do you know that because there is no noticeable change in capacity even with hard charging that the pack will actually be at or near 100% in 5 years.


You already posted that, and it really doesn't have much to do with this thread. It's an actively balanced pack for one, and for another we already know that for the initial 50 or so cycles of lithium cells, at least LiFePO4, capacity can increase, which could hide a small amount of capacity loss after that. It's still a promising result, but it has little to do with this thread.


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## JRP3 (Mar 7, 2008)

From Jacks' blog:


> My simple demonstration of opening a box with a Thundersky battery in it that we had had on hand since October of 2008, manufactured in August of 2008, and measuring the open circuit cell voltage of 3.300 which precisely matched the 3.300v of the Thundersky 400Ah cells we received the day before. This cause quite a stir. There is an entire little army of pseudo poseur engineers over on the DIYelectricjunk forum immediately sprang into action spraying messages in all directions describing what I did and did not do, what I should of done, and why I had it all confused and totally incorrect.





> IT NEVER OCCURRED TO ME that the DIYenginiers did not even comprehend the relationship between voltage and state of charge. THAT's how lost we are in the conversation here.
> 
> But of course I had taken out cells numerous times over the three years, and measured the same thing, and checked exactly how many amp hours were in them and how much charge they would take and what charging to various voltages does and on and on and on. Who but me would care? How many ions can dance on the head of a pin really IS the question, but it has no easy answers and whatever I measure there is more to measure tomorrow. And all if it is terribly time consuming. It takes hours. I don't even want to watch all that.


http://jackrickard.blogspot.com/2011/08/this-week-we-welcome-mark-emon-of-st.html


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## Coulomb (Apr 22, 2009)

frodus said:


> Interesting that TS acts so differently.... ones I've tested show an increase in voltage after a temperature rise. One time I used a heat plate to heat the cells up (no load connected), and watched the voltage increase.


Ok, so that's with no load. So that's open circuit resting voltage. I'll bet the voltage change was quite modest. However, I'm guessing it was many millivolts, which would be enough to invalidate Jack's argument (which as I understand it is that the zero millivolt difference measured implies a zero difference in state of charge).



> We did this after seeing A123 at 20C have a huge voltage drop, then as the cell heated, the voltage went back up.


This is under heavy load. Most of the voltage difference would be due to a change in internal resistance. At high loads like this, the change in internal resistance can make a difference of hundreds of millivolts of terminal voltage. So that's a different effect, IMHO. I know you said that this was what prompted you to do the other test, and it is the other test that shows that voltage without temperature being held constant is not a reliable indicator of SOC. I just thought it was worth clarifying that; there is enough confusion in this thread as it is


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## JRP3 (Mar 7, 2008)

It depends on how much actual voltage changes and by what temperature differential. It should also be quantifiable and repeatable, so temperature compensation would adjust for any difference, and the original premise would still hold true, that voltage at a fine enough resolution does indeed track SOC. Frankly I'm not sure of any battery that doesn't have voltage directly relating to SOC on some level. Just because it's not the best way to track SOC in daily use doesn't mean it does not exist. In combination with a trip odometer I drove using only voltage for a while before I got my EV Display.


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## Tesseract (Sep 27, 2008)

Good grief... 130 posts all arguing about things that are trivial, if a bit tedious, to test. Part of the problem here seems to be that different people are talking about different things. Even the title of this thread has had little bearing on the discussion; to wit, cell "drift" is the (supposed) variation in Ah capacity and/or terminal voltage of seemingly identical cells when subjected to identical charge/discharge cycles. Self-discharge is, quite simply, the loss of Ah over time, while determining if terminal voltage corresponds to SoC is more or less self-explanatory (it does, btw, but it's almost pointless to try to use it in practice because the voltage drop caused by current through the cell's internal resistance totally swamps it).

Here's my take on how to practically test each of these things, but in each case it is critical to start the test knowing exactly how many Ah are stored in each cell!

1. Cell "drift" - Discharge at least 10 (but preferably more) supposedly identical cells to a consistent voltage at a rate of 1C or less then recharge each with a known number of Ah which is less than the stated capacity of the cell (e.g. - 80Ah into a 100Ah cell) at a rate of 1C or less. Connect all cells in series then cycle them by discharging a portion of that charge (e.g. - 60Ah) then recharging with the exact same amount of Ah. Separate the pack back into single cells every 100 cycles and measure the terminal voltage, then the Ah stored in each cell. Repeat until bored or dead.

2. Self-discharge - As above you need to fully discharge all cells then recharge them with a known amount of Ah. If you want to be extra thorough then charge one group of cells to 50% of their stated capacity, another group to 60%, another to 70%, etc., and see if that has any effect as well. Then you simply store all of the cells in a place where they will all experience the same temperature and humidity and, hopefully, be safe from mice/rats. At predetermined intervals you need to check the terminal voltage and actual Ah capacity of each permutation you wish to test (e.g. - 50% SoC at 1 month; 60% SoC at 1 month; etc.). You may recharge those cells and put them back into storage for another testing interval, of course, just note how many times they have been cycled. 

3. Terminal voltage as an indication of SoC - As usual, you begin by fully discharging all of the cells to be tested then recharging with a known amount of Ah which is less than the stated capacity. However, I believe the only plausible method of performing this test is to discharge the cells with a constant current load at a rate of, say, 0.1C and taking voltage measurements at, e.g., 6 minute intervals. Discharging at a high rate then pausing the discharge for a "few minutes" to allow the terminal voltage to stabilize can't possibly give consistent results, especially if done by a human. By discharging at a constant but low rate you can minimize the effects of taking the voltage measurement a few seconds early or late each interval (or you can eliminate yourself from the mix completely and use, e.g., a LabJack to automate the discharge and voltage measurement process).

All of these tests, but particularly the first two, would be tedious and laborious to perform. They also require a large number of cells to be statistically meaningful. Testing one or two cells using a surplus resistor bank for a load and a cheap Chinese DMM won't really cut it in my opinion. You could certainly infer things about the cells with such Mickey Mouse methods but don't treat them as scientific fact.

Carry on


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## frodus (Apr 12, 2008)

JRP3 said:


> From Jacks' blog:
> http://jackrickard.blogspot.com/2011/08/this-week-we-welcome-mark-emon-of-st.html


Oh, Hi jack! 
*
With respect to your 2.5 year old cell and the self discharge test:*
I saw the last video, and the blog post about the fact that you didn't share stuff you did behind the scenes. THAT is the data I was asking to begin with. It sounds like you compare your results to actual measured Ah results of a cell before and then after resting. If you have those, please share. It will put this to rest and we can all get back to our normally scheduled programming. 

Easy enough to share if you have it, but if you don't, it renders the self-discharge test inaccurate.


*With respect to the SOC -> Voltage argument:*
I've seen your results of the SOC relating to voltage, and it's very interesting to read through. It looks like these cells have a direct relation from voltage and SOC. I'm wondering how different the voltages could be with differing temperatures. Have you seen any effect on the resting time in relation to how much the voltage changes?


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## JRP3 (Mar 7, 2008)

I'll pick some nits


Tesseract said:


> Self-discharge is, quite simply, the loss of Ah capacity over time


I'd say that self discharge is a loss of energy. Capacity remains the same, being the ability to hold a certain amount of energy. Loss of capacity is a different matter than self discharge, though sitting on a shelf does not seem to have an effect on capacity either.


> while determining if terminal voltage corresponds to SoC is more or less self-explanatory (it does, btw, but it's almost pointless to try to use it in practice because the voltage drop caused by current through the cell's internal resistance totally swamps it).


Presumably one could create a program that is current and temperature compensated such that it could correlate voltage to SOC on the fly. Certainly not worth it compared to simple amp hour counting, but if Qer is ever bored....


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## Guest (Aug 7, 2011)

> I'm wondering how different the voltages could be with differing temperatures. Have you seen any effect on the resting time in relation to how much the voltage changes?


Voltages would be different at differing temperatures but it would still follow the same pattern. If you decided to scramble all the cells at different voltages then of course the readings will be skewed. 

Resting time will give you a more accurate voltage reading over time to a point. That point is when it is all stabilized. The test showed more like you'd see while out on the road in a real situation. That is the information we need. On the road. Not some skewed testing in the lab. I am glad that the testing in the lab is more akin to in your car. Not exactly but more like you might see. As for temp differences, I can't see much differences as the whole pack is more or less exposed to the same temps. Of course if you have a bad cell you may experience a gross change like if you top balance a pack and you drive to the end and drive a low cell into oblivion. It will heat up and be totally out of joint with the rest.


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## JRP3 (Mar 7, 2008)

gottdi said:


> . The test showed more like you'd see while out on the road in a real situation. That is the information we need. On the road. Not some skewed testing in the lab. I am glad that the testing in the lab is more akin to in your car. Not exactly but more like you might see.


I can't say I agree with that. The point of lab testing is to remove variables. A longer rest period would have increased the accuracy of the test. I realize it's not practical, but it would have been a better test to show true resting voltages. When checking a cell for amp hours that's been sitting on a shelf and using voltage as a reference the five minute rest voltage that Jack charted is not going to be accurate, especially since we are using thousandths of a volt. Jack says a cell sitting for years is at 3.300 volts at 50%. His tests don't show the same voltage at 50% because the rest times weren't long enough.


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## MN Driver (Sep 29, 2009)

Don't forget that this may be similar amongst cells that have similar internal resistance but a few outliers will make more heat and in the end the resting period will have to be as long as it takes to get the cells at a uniform temperature internally. I'm also not sure how close the resting voltage would be cell to cell at a specific SOC and there will be differences too for if you have different capacities of cells and balance(top or bottom) and 50% of a bottom balanced pack will have different SOCs per cell since the larger capacity cells will be at a lower SOC in a bottom balanced pack because they have more Ah. So trying to check cell voltages for SOC seems to be a sketchy and challenging way to try and verify balance. Even more so if the car is parked in the sun and some are under the hood, some baking in the trunk, and others under the rear seats in the solar oven portion. Too many factors for SOC by voltage to tell you the difference to the resolution you would need to have reliable enough data to try and determine if things are balanced. Sure a cell or two in a controlled environment but in a car and with aged cells after lots of cycling over the years? Getting a rough idea of the total pack SOC? feasible.


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## JRP3 (Mar 7, 2008)

Temperature affects internal resistance which can change voltage when current is flowing but I'm not sure how pronounced the effects are when checking static voltage. I think it's quite useful if we can use voltage to check SOC level for a cell sitting on a shelf, or coming out of a box.


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## MN Driver (Sep 29, 2009)

JRP3 said:


> Temperature affects internal resistance which can change voltage when current is flowing but I'm not sure how pronounced the effects are when checking static voltage. I think it's quite useful if we can use voltage to check SOC level for a cell sitting on a shelf, or coming out of a box.


A higher internal resistance of one cell over others results in increased heating(wasted energy too) which will cause the voltage to be different as the resting voltage varies with temperature. I'm not disagreeing that there was no significant self-discharge of Jack's Thunder Sky cells. Which is great if all prismatic cells behave this way. It leaves thermal energy losses through internal resistance as the primary cause of a cyclic change in a normal cell. Separator failure is the only other one I've seen some prismatic LiFePO4 self-discharge with but every case I've seen caused a non-venting cell failure at zero volts. ...unless there are any external loads per cell that may be connected to differently discharge them(different topic to this one but its already been discussed here).


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## Guest (Aug 7, 2011)

Lots of pickin but before you go doing precision testing you first need to just confirm that a problem exists, then proceed to fine tune how much of a problem exists. Then you can determine if it's a problem that needs precision interaction. So with the self discharge of LiFePO4 cells issue we first only need to show it it exists. If yes, then proceed to how much.


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## JRP3 (Mar 7, 2008)

MN Driver said:


> A higher internal resistance of one cell over others results in increased heating(wasted energy too) which will cause the voltage to be different as the resting voltage varies with temperature.


The question is to what degree? A 10 degree cell difference causing different resting voltage is one thing, if it's even noticeable, but the 1-2 degree cell difference you might see from self heating might not give any meaningful difference in resting cell voltage.


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## JRP3 (Mar 7, 2008)

gottdi said:


> Lots of pickin but before you go doing precision testing you first need to just confirm that a problem exists, then proceed to fine tune how much of a problem exists. Then you can determine if it's a problem that needs precision interaction. So with the self discharge of LiFePO4 cells issue we first only need to show it it exists. If yes, then proceed to how much.


Actually what I'm talking about now is not the self discharge issue, it's the SOC to resting voltage relationship. We could have specific set values for each 10% or so of SOC, which would give us a quick check of a cells SOC with a volt meter. Jack's test does not give us those values. He shows the obvious relationship between voltage and SOC but not the exact values that we can use to compare resting voltage and SOC.


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## MN Driver (Sep 29, 2009)

JRP3 said:


> The question is to what degree? A 10 degree cell difference causing different resting voltage is one thing, if it's even noticeable, but the 1-2 degree cell difference you might see from self heating might not give any meaningful difference in resting cell voltage.


I don't know the extent.



gottdi said:


> Lots of pickin but before you go doing precision testing you first need to just confirm that a problem exists, then proceed to fine tune how much of a problem exists. Then you can determine if it's a problem that needs precision interaction. So with the self discharge of LiFePO4 cells issue we first only need to show it it exists. If yes, then proceed to how much.


The problem with ignoring a possible problem is if the problem does exist and you ignored it long enough for damage to occur, then your problem isn't something you can manage after damage has occurred. If you monitor and see that the problem doesn't exist after monitoring for awhile the interval that you check the possible problem is reduced. I think a once a year check is minimal work and far enough out between checks to where I'd check no matter what I find but my first checks come more often for every new pack I buy, its also worth checking cell connections for heating after a faster higher current drive too from time to time. It depends on the problem you are describing too. I don't have a reason to measure voltage to try and determine SOC, I'll do a discharge test to determine where the cells are and get an accurate result without the guesswork as the unknowns of measuring voltage are too big because there is so little Ah left once the voltage of the pack starts a heavy dip while driving and you aren't going to park for 5 minutes just to check it if you are on your way home on a longer trip from the opposite side of the city that might pull me to a planned 80% DOD or possibly a little deeper if any calculations about the trip are wrong(likely situation for me). Knowing the capability of a cell by a very brief load test to each cell at a lower SOC is important to me too in order to determine if any cells are getting saggier which is a bad sign that I think is important to look for here and there but to me that is a once a year task, likely one I'd do right before the weather gets cold and internal resistance becomes a bigger factor(-10f here after a long cold soak for a weeks worth of driving in the morning where I live and I'm not far from work to the uphill on-ramp with a short space to get up to 55mph either). I'd be testing for more than one purpose.


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## Tesseract (Sep 27, 2008)

JRP3 said:


> Temperature affects internal resistance which can change voltage when current is flowing but I'm not sure how pronounced the effects are when checking static voltage. I think it's quite useful if we can use voltage to check SOC level for a cell sitting on a shelf, or coming out of a box.


I measured the internal resistance of a few TS 200Ah cells about a year ago. It was rather informal and not at all lab precise, but at any rate what I found is that internal resistance doubles when going from 25C to -10C but, most interestingly, OCV also drops with temperature (e.g. - the same cell went from 3.292V at 25C down to 3.221V at -10C).

I have some GBS and Headway cells on hand right now which I could do similar testing on, but not nearly enough spare cells of each to be statistically significant. Still, some data is better than none as long as one doesn't start a religion over it... 

ADDENDUM: Heating from current flow is *substantial*! I measured an internal resistance in the 2 milliohm range for the TS 200Ah cells. At 0.5C/100A that would result in 20W of heat - not bad - but at 5C/1000A that jumps up to 2000W (and 2V or so of drop, so below the "maximum power" voltage). This, btw, is exactly why there is a C limit in the first place.


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## Coulomb (Apr 22, 2009)

Tesseract said:


> ... OCV also drops with temperature (e.g. - the same cell went from 3.292V at 25C down to 3.221V at -10C).


I wonder if this in fact represents, in small part at least, a lessening of SOC (recoverable energy) with lowering of temperature. I think I've heard that range reduces at lower temperature, which might not all be attributable to the higher internal resistance (and hence higher losses, and quicker reaching of low terminal voltages).

I also wonder if these effects are reversible. For example, if you take the above cell back from -10C to 25C, does the open circuit voltage recover all the way to 3.292 V, and is the SOC (recoverable energy) restored as well?

There is a lot to learn about these remarkable cells.


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## Tesseract (Sep 27, 2008)

Coulomb said:


> I wonder if this in fact represents, in small part at least, a lessening of SOC (recoverable energy) with lowering of temperature. I think I've heard that range reduces at lower temperature, which might not all be attributable to the higher internal resistance (and hence higher losses, and quicker reaching of low terminal voltages).


As I hinted above, this was somewhat surprising to me, because the OCV in an electrochemical cell should only be proportional to the relative electronegativity of the cathode and anode materials. Unfortunately, this was just one cell which I measured the OCV before and after tossing it into a freezer. The one saving grace to my methodology is that the cell had not been charged for several weeks. I do need to do this test again, but right now I just have a 4-cell GBS battery and a handful of sketchy Headways to play with, no spare TS or CALB cells at all.



Coulomb said:


> I also wonder if these effects are reversible. For example, if you take the above cell back from -10C to 25C, does the open circuit voltage recover all the way to 3.292 V, and is the SOC (recoverable energy) restored as well?


Yep - the OCV does go back up with temperature. This is such a strange result that I hesitated posting about it, but the data is what it is. Also, the OCV declined by ~70mV over a 35C change in temperature, which just happens to be the same as a silicon PN junction diode, so I can't help but wonder if this is, in fact, a diode-like effect... 

Anyway, right now this is the data from just one cell, so I hesitate to conclude anything from so meager a data set. If I see the same behavior from 10 different cells from 2 or 3 other manufacturers then I'll feel better about calling it a trait of the cell chemistry; until then I'll just call it a strange data point.


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## JRP3 (Mar 7, 2008)

I would assume that warming the cell back up would return the recoverable energy, where would it have gone? I'm not even sure a cold cell has less recoverable energy than a warm one if they were both charged at the same temperature. When cold voltage sags more under load, so to make the same power you have to draw higher current compared to a warmer cell. That means a higher C rate draw than a warm cell, higher C rate = fewer amp hours out of the cell. The other half of the low cold weather range is putting less energy in when charging. I speculate that if you raise the finish voltage in cold weather, but still below the 4.25 electrolyte breakdown voltage, you might be able to compensate for the cold when charging.


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## EVfun (Mar 14, 2010)

Tesseract said:


> Yep - the OCV does go back up with temperature. This is such a strange result that I hesitated posting about it, but the data is what it is. Also, the OCV declined by ~70mV over a 35C change in temperature, which just happens to be the same as a silicon PN junction diode, so I can't help but wonder if this is, in fact, a diode-like effect...


It is good to see someone else has seen that temperature has some effect on resting voltage. Your results are about 8x the shift per degree that I saw, but I didn't measure actual temps -- it was just a seasonal change in an attached but unheated garage.

Have you noticed that the terminal voltage of a rested lead acid battery goes *up* as it gets colder? Of course the internal resistance of them goes up even faster so the voltage under load goes down.


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## Tesseract (Sep 27, 2008)

EVfun said:


> It is good to see someone else has seen that temperature has some effect on resting voltage. Your results are about 8x the shift per degree that I saw...


Just got finished testing a single Headway cell after being in a freezer at -15.5C for 8 hours. The OCV before testing was 3.375V. Immediately after pulling from the freezer it had dropped, every so slightly but a drop nonetheless, to 3.368V (or 7mV). Over the period of 4 hours at room temperature the OCV fully recovered to 3.375V again. I didn't bother to test the internal resistance of this cell because it came from a batch of questionable quality (it was 20 milliohms at 25C, which is about 3-4x higher than normal).

As usual, I wouldn't read too much into these results, but the OCV of this one Headway didn't change nearly as much with temperature as the prior TS cell did... More testing is warranted, especially of Rint vs. temp.


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## MN Driver (Sep 29, 2009)

JRP3 said:


> I would assume that warming the cell back up would return the recoverable energy, where would it have gone? I'm not even sure a cold cell has less recoverable energy than a warm one if they were both charged at the same temperature. When cold voltage sags more under load, so to make the same power you have to draw higher current compared to a warmer cell. That means a higher C rate draw than a warm cell, higher C rate = fewer amp hours out of the cell. The other half of the low cold weather range is putting less energy in when charging. I speculate that if you raise the finish voltage in cold weather, but still below the 4.25 electrolyte breakdown voltage, you might be able to compensate for the cold when charging.


That voltage sag means you'll be pulling more Ah out of the cell to get the same wattage(energy) to the motor so you'd get less range because of less kwh that you can pull from the pack. Getting less Ah out of the cell would be a function similar to the Peukert Effect, I'm curious what the numbers are for Ah loss we would have at very low temperature but voltage sag as described by pm_dawn in another thread leads me to believe that it would take either an extremely well insulated pack that is oversized(over 100 miles or maybe well over) to maintain acceptable performance for a highway car but then again there are so many factors such as the ambient temperature of the area(gets to -20 where I am) but heating the pack gets rid of the issues with getting a full charge in the cold and efficiency and power losses in the cold. It's a bit of a compromise.

Either way though, I'm very interested in more information on how they deal with being under load in the cold, it sounds like Jack might try it but I hope he can start his tests at a temperature some of us northerners deal with -20f but I'd assume by then they aren't able to put enough out and with insulation that temperature should be moot for anyone further south in latitude than Canada(or opposite for the other hemisphere).


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