# Depth of Discharge Experiment Planning



## Arthas (Jun 28, 2012)

Roben from CALB China look forward to your test result!


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## Sparweb (May 24, 2013)

I read through thinking "you'd better control the temperature" and then you mentioned it just at the end. Do you have the means to carry out the tests at temperatures around freezing?
Resistor bank for load?


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## jddcircuit (Mar 18, 2010)

Sparweb said:


> I read through thinking "you'd better control the temperature" and then you mentioned it just at the end. Do you have the means to carry out the tests at temperatures around freezing?
> Resistor bank for load?


No resistor load needed. I have four buck boost circuits connected to a common DC bus that can regulate current in and out of each battery bank under test.

My test setup is relatively low cost. Once I have all the automation and data logging fully operational I can easily copy it for other test conditions that include temperature.


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## jddcircuit (Mar 18, 2010)

Arthas said:


> Roben from CALB China look forward to your test result!


Me too. I am especially interested to learn how stressful the knees at the ends of the discharge and charge curves are in regards to cycle life.


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## jddcircuit (Mar 18, 2010)

I have made some progress with my test setup. I only have CALB SE40AHA (blue cells) at the moment which I am using for proof of concept but here is a datalog of a 40amp discharge followed by a 40 amp charge.











I did a constant current discharge until the first cell hit 2.7 volts and then a 40amp charge until the first cell hit 3.7 volts. It took about 53minutes for each leg. I have to work on my Ah counting and calibrate my current sensor but the buck boost concept seems like it will work pretty well.

In order to get to 3C (120amp) charge and discharge I am going to have to get the liquid cooling for the inverter that I am using plumbed up. I am not sure how efficient my buck boost is but there will be some significant heat generated.

At the higher currents I may need to consider some variation in the charge profile but I would like to stick with constant current only if I can.


Regards
Jeff


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## Ziggythewiz (May 16, 2010)

Cool experiment!


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## Siwastaja (Aug 1, 2012)

Good planning. If you get the funding, definitely test 100%->20%->100% and 80%->0%->80% cycling separately to find out which end is more critical.


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## drgrieve (Apr 14, 2011)

Siwastaja said:


> Good planning. If you get the funding, definitely test 100%->20%->100% and 80%->0%->80% cycling separately to find out which end is more critical.


This would be interesting to know.

Also check out John Hardy's experiments, he has covered some of this ground.

I also note that like him, your voltages at the top and bottom cross over each other, as John found.

Which I take to show, that trying to measure SOC using charging / discharging voltage as non-sense. You need to count AH or measure static voltage.

Link for the experiments is here: http://tovey-books.co.uk/testing.php


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## jddcircuit (Mar 18, 2010)

Siwastaja said:


> Good planning. If you get the funding, definitely test 100%->20%->100% and 80%->0%->80% cycling separately to find out which end is more critical.


If staying away from the ends increases cell life than which end or how far from the end will follow.

I am trying to come up with a low cost and configurable method that can be replicated for many experiment setups in parallel.

Thanks
jeff


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## jddcircuit (Mar 18, 2010)

drgrieve said:


> This would be interesting to know.
> 
> Also check out John Hardy's experiments, he has covered some of this ground.
> 
> ...


I would not say that charging/ discharging voltage as it applies to state of charge is nonsense but I might say it is not absolute. Perhaps we just don't have all the variables and a clear understanding of their interaction yet.

Personally I am most interested in the change in voltage per time under a known current to be a good indicator of end of charge. Clearly the slope of the charge voltage profile changes and may be used as a SOC indicator in some regions.

I will run the data through a filter and overlay the slopes to see what we see.

Thanks
Jeff


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## Siwastaja (Aug 1, 2012)

jddcircuit said:


> If staying away from the ends increases cell life


Well, you know, that _will_ definitely happen. It's been studied a lot. It's only about: exactly how much with these specific cells? So make it a quantitative experiment that will show what we already know but in greater detail. People have _mostly_ studied the 100%->20%->100% case in real life, but the opinion that the top end should be avoided too, has started circulating in the DIY scene in a last year or so. So it would be great if this could be either confirmed or dismissed. (With LiCo, it's evident, as high SoC shortens the calendar life considerably.)

If you can only do one test, you will produce more interesting data by doing one that avoids only the top end, as CALB & others have already tested the one avoiding the bottom end and we kind of know the result. In fact, your proposed 80-20 test will probably be the best one as the manufacturer data does 100-20, so you can directly see the difference.

BTW, it's possible that the cycle number increases so much from 80-20 versus 0-100 that it would take practically too long to finish the test. But that would be a very interesting result, too, like: "I'm already at 10000 cycles and I don't want to do this anymore, these cells are too good ". Here, an automated system is a must and it seems you are doing it right. Good luck.

Accelerated calendar life aging test would be interesting, too. Maybe I'll give it a shot. The effect of the temperature on LiCo cells has been studied, and we know that LiFePO4 is much better, but no one has exact numbers. But we could apply the temperature deterioration curve of LiCo (like: what temperature gives 10x reduction in life) and increase the temperature of LiFePO4 by the same factor and then measure how long it takes before the capacity drops to 80% at that temperature. This would require pretty good insulation unless we want to waste a colossal amount of energy in keeping the cell warm for maybe a few years.


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## jddcircuit (Mar 18, 2010)

I get your point about avoiding the top vs avoiding the bottom comparison as it pertains to top balancing BMS being so popular.

I am also anticipating some ambiguity in what defines these profile ends.

I am considering using a Volts/Sample threshold to indicate 100% and 0%. Here is a graph showing a running calculation of the voltage profile slope overlaid with the measured data.











A threshold at +/-.0005 V/sample would work pretty good points to terminate I think. In this data set I am sampling cell voltage every 2 seconds.

I may have to do some testing to see how many Ahrs exist beyond these points using a Constant Voltage charge phase.


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## octagondd (Jan 27, 2010)

Siwastaja said:


> If you can only do one test, you will produce more interesting data by doing one that avoids only the top end, as CALB & others have already tested the one avoiding the bottom end and we kind of know the result. In fact, your proposed 80-20 test will probably be the best one as the manufacturer data does 100-20, so you can directly see the difference.


I don't believe CALB tested 100%-20%. The data Jack Rickard received from the Sky Energy (CALB) representative he was speaking with shows their tests were from full charge to 3.6V and full discharge to 2.0V at 0.3C and had a linear decline which showed it would likely reach 2000+ cycles until the cell only had 80% of its original capacity. This is a common misunderstanding in the entire community and needs to be dispelled because these batteries are even better than we give them credit for. If the tests were at 0.3C and went from 100% - 20%, and we assume it degrades the cell more the lower the SOC, I would be very concerned since my vehicle will be running at 1C and greater, but since the tests were for a full charge and discharge at 0.3C and show 2000+ cycles until 80% original capacity, I feel better that my 1C-2C driving will not kill my pack as quickly.

Here is the link:

http://blog.evtv.me/2010/06/life-in-lifepo4-cycle-life-and-attenuation-data/

If you don't believe me or Jack, just think for a moment how you would do a test to 80%DOD. You wouldn't actually be doing a test to 80%DOD, because the number of AHs in a cell is always a moving target. If you had a 100AH cell and discharged 80 AHs, that would work on the first cycle, but every cycle after that you would be going to a slightly higher DOD% until you reached 100% with only 80AHs available in the cell. These battery tests are done to a HVC and an LVC. This is good news for us.

Here is a test I would like to see since I think it would represent our circumstances better. Most of us probably drive at 1C or thereabouts, but acceleration is much greater, so a test with a constant discharge at 2C would give us a reasonable baseline for cell life.

CALB CA Series
Charge to 3.5V at 0.2C (approx. 5 hours)
rest 15min.
Discharge to 3.0V at 2C (approx. 25 min)
rest 15min.
repeat

This would be about a 6 hour cycle and would get you 125+ per month.

It would be even cooler if a cycling program could take someone's actual drive data and use that on the discharge cycle.


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## Siwastaja (Aug 1, 2012)

Thanks for pointing this out. You are probably right, as the testing method would indeed be different than what their discharge curves show. OTOH, they don't go to 0% as they stop at once when reaching the cutoff voltage, without doing a CV phase. But at currents as low as 0.3C, this is very near to 0% SoC. Definitely not 20% SoC.

Many datasheets show that the cycle ratings are with "80% DOD" and "70% DOD", but they may not mean what they write. This is typical to all kind of datasheets and not limited to batteries.

Instead, they probably mean 80% and 70% of the original capacity left as you say.

This is also a reason why you could expect seeing a much higher number of cycles, such as 10000 cycles.

I guess the calendar life or small manufacturing defects that slowly "eat up" the cells, either with or without cycling, will be the deciding factor in the real battery life, instead of the actual good cell cycling ratings. Most of the cycles are far from 100%.


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## jddcircuit (Mar 18, 2010)

I did a new bottom balance and then ran a 1C charge discharge cycle.

I have been using a constant current discharge of 1C (40amps) and terminating when any of the 6 cells hits 2.7 volts on the discharge.

I wanted to see how many Amp Hours were below this constant current cutoff point in order to establish approximate % SOC.

This experiment showed me 1.75 Ah ( 4.3%) exists below this point. I simply restarted the discharge at a 5 amp rate and terminated at 2.7 volts the second time.










Regards
Jeff


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## PStechPaul (May 1, 2012)

The graph seems to show a 0.03V rise at the 40A discharge rate, which corresponds to an ESR of 0.03/40 = 750 uOhms. This could be expressed as an impedance ratio of 0.9%. For total energy of 40*3.05 = 122 Wh, the power lost due to ISR is 1.2 Wh which is an efficiency of 99%. This may not be truly accurate because you need to integrate the power delivered over the range of 3.05 to 2.7V, or approximately 40*2.85 = 114 Wh. So you might have as much as 122-114-1.2 = 6.8 Wh left. This is 6.8/2.7 = 2.5 Ah which agrees pretty closely to your 1.75 Ah.

It may be useful to chart the energy of charging and discharging and use that for a more accurate assessment of energy efficiency. I know there is some argument over energy storage being based on current rather than power, over time, but I think it is more accurate to use power and its integral over time, which is energy, rather than amperes over time, which is coulombs of charge. Just as the energy in a capacitor is given as 0.5*C*V^2, energy in a battery may have a voltage component, and its effective capacitance may vary with the amount of charge put into it.


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## dougingraham (Jul 26, 2011)

PStechPaul said:


> It may be useful to chart the energy of charging and discharging and use that for a more accurate assessment of energy efficiency. I know there is some argument over energy storage being based on current rather than power, over time, but I think it is more accurate to use power and its integral over time, which is energy, rather than amperes over time, which is coulombs of charge. Just as the energy in a capacitor is given as 0.5*C*V^2, energy in a battery may have a voltage component, and its effective capacitance may vary with the amount of charge put into it.


My own tests at 1/4C charge rate give about a 99.5% charge efficiency with 100AH cells based on AH put back in. The WH figure will always be less than this and vary a lot depending on charge and discharge currents due to sag during discharge and boost (? Not sure what to call the voltage increase over the resting voltage) during charge.

So while coulomb counting will get you a reasonable SOC in both directions, keeping track of watt hours will get you efficiency at the outlet. What you are interested in determines which is more useful to you. I think AH is good enough although 1 AH when the pack is full will take you farther than 1AH when the pack is near empty, WH remaining is going to be an estimate at best because of sag. SOC based on AH will be a really good number that counts down to zero. WH and range estimates based on the WH estimates will be guesses that can be terribly wrong if you change your driving habits when the numbers get low. An example of this could be summed up with the phrase, "That Camero is toast!"

You ought to be able to take a cell of a known capacity and pull 20% out of it leaving it at 80% SOC then remove 60% more and know that you are at 20% SOC. If you put back in a little more than 80% (tunable amount) you should be able to keep the SOC in that 80%-20% SOC range. Keeping the temp the same and terminating the charge at the same voltage with the same current should give you pretty consistent results. GM must be doing something like this with the Chevy Volt as they keep the SOC away from both ends in an attempt to make the battery last. In the lab this should be easy. In a car you would probably need some temperature compensation.


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## GizmoEV (Nov 28, 2009)

PStechPaul said:


> I know there is some argument over energy storage being based on current rather than power, over time, but I think it is more accurate to use power and its integral over time, which is energy, rather than amperes over time, which is coulombs of charge.


The argument isn't about energy storage at all. Energy is based on current AND voltage. That hasn't been in question. The argument is with respect to SOC. SOC means exactly that, State of Charge. It is independent of voltage which is why it is the best thing to use for SOC. The problem comes when people think that Ah is energy, which it isn't. If you want an accurate SOC value do not use energy. If you want energy efficiency the you must use energy not Ah. It all depends on what you want, as dougingraham said.


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## Siwastaja (Aug 1, 2012)

dougingraham said:


> My own tests at 1/4C charge rate give about a 99.5% charge efficiency with 100AH cells based on AH put back in.


Sounds a bit low to me. I'd expect about 100%. But this is all about what you use as a discharge and charge stop condition -- when you call it "empty" or "full".

I remember Wikipedia citing a study that showed a non-100% coulombic efficiency for li-ion. When I read the paper, they charged the cells up to 4.5V IIRC (LiCo), which clearly is a damaging overcharge and explains the reduced charge efficiency. Just like lead acid and NiCd/NiMH, li-ion continues absorbing current after it's full, quickly reducing the charge efficiency, but unlike LA and NiXX, this phase is not needed for li-ion and it can be damaging. 

When we are dealing with numbers that are very close to 100%, every small measurement error accumulates. Also, it has to be noted that there always is small capacity drift with li-ion cells. The capacity can actually go up a little bit for a few dozen cycles, after which it starts to go down. But there always is some "noise" in the capacity, too.

So, I guess the measurement process will have an error margin of at least 1 percentage point when you measure the coulombic efficiency. 

The real-world experience shows that at least the coulombic efficiency of the cells are very well matched as the series packs even without a BMS do not seem to drift. The bottom balanced cells do not even get to the same upper voltages while charging, which would be the source for drift caused by difference in coulombic efficiencies if there was one. So, if the coulombic efficiency of these cells is less than 100%, it's always constant. I still believe that it's practically 100% (or something like 99.9%) as long as the cells are not abused. I think your number supports this quite well as the error margin most certainly includes 100%.


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## jddcircuit (Mar 18, 2010)

Here is one my charge profiles that I would like to propose:

I am considering a fast charge/discharge cycle (1C to 3C) that will be counting Ah as part of the data logging but will not necessarily be using Ah to terminate the cycle. I don't really care about efficiency. I mainly care about how fast can you charge them without killing them prematurely.

Everyone talks about avoiding the ends. I am just trying to determine how much to avoid them and also to maximize my charge/discharge rates for some high power applications.

My theory is that if I avoid the EndZone regions at the top and bottom of the profile where the IR starts to rise sharply then perhaps I am not stressing the cell and may not be accelerating the aging with the high rate of current.

I understand I could use a fixed amount of Ahs but that won't necessarily conform to the aging cell and reduced capacity over time. I could use voltage but in that case I am too deep into the zone. 

I want to stay away from these Endzones but at the same time go right up to them in as short a time as possible. I also want this new cycle termination method to shrink in duration as the cell ages and loses capacity.

I think by calculating a running slope (Volts/sec) of profile I can stop right at the beginning of the end zone. (See graph)

In this data log, if I were using the new End of Discharge threshold technique it appears that I would still have a 4.4 + 1.75 Ah residual left in the pack. (4.4 +1.75)/40 = 15%. This may be the 15% off the bottom buffer that I am looking for to extend cell life.

I will do the same for the high end of charge and start data logging the Ahrs all way in between.

Please bear with me. I promise my graphs will get less cluttered as I get my thinking straight.










Regards
Jeff


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## GizmoEV (Nov 28, 2009)

That will definitely be an interesting test. Potentially quite informing. Apparent capacity will be a combination of actual capacity (however that is defined) and IR. As the actual capacity and/or the IR changes so will the apparent capacity.


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## jddcircuit (Mar 18, 2010)

A couple more graphs. I am in the process of respinning my cell voltage monitoring circuit and waiting on some funding for some CALB CA cells so I haven't done much cycling to date.

I am still working on my voltage profile slope calculation software that will terminate the charge so in the meantime I am still using 3.7 and 2.7 volt cutoffs and using excel for the calculations and graphing.

I wanted to show the difference between a 1C and .5C cycle. I cannot explain the glitch in cell #5 in the 20amp cycle. It might not be real since my cell monitoring is still under development.

I don't know what a 2C or 3C cycle will look like yet. My test setup gets pretty warm at 1C so liquid cooling is being added.

These CALB SE series cells have been sitting on the shelf for over a year and never used as far as I know. Perhaps they are still experiencing some early formative charge behavior. (speculation at best)

I thought is was interesting how much tighter the End of Charge is grouped with the lower current draw. I bottom balanced the cells individually using a 1 ohm resistor so they line up very tight at even lower currents.


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