SilverFoxCPF
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Re:Battery testing questions... - 2007/09/02 17:45
Hello Guy,
Thanks for the updated articles. Summer is a busy time. I hope your summer was productive.
I noticed several inaccuracies in your articles and hope you will correct them as time allows. I won’t go into all of them, but there are a few that you should consider addressing before the others.
You have mentioned several times that the fixed load testing compares favorably with constant current testing.
For example, you state:
“RBR Fixed Load Test vs. Constant Current Test
Here are a few examples of the test results from Rechargeable Battery Review's Fixed Load Test and the Constant Current Tests performed on the LaCrosse BC900 and the Maha MH-C9000 Wizard1.
The majority of the tests results from the LaCrosse BC900 and the Maha MH-C9000 Wizard1 fall within 5% of RBRs Fixed Load Test. It may happen that one of the LaCrosse BC900 or the Maha MH-C9000 Wizard1 might vary more than 5% of RBRs Fixed Load Test.
When both the LaCrosse BC900 and the Maha MH-C9000 Wizard1 fall beyond more than 5% of RBRs Fixed Load Test, we re-run our tests to verify our results.”
Are you aware that neither the BC-900 nor the C-9000 utilize constant current discharging?
Moving on…
The word “nominal” is used throughout the battery industry with little confusion, however when you encounter it in reference to capacity, you raise a flag stating that there is a lot of confusion as to what nominal capacity means.
Nominal capacity, as listed in a data sheet, is a named value only. Just like nominal voltage. The nominal voltage of an Alkaline cell is listed at 1.5 volts. The actual cell voltage starts out at around 1.62 volts and runs all the way down to around 0.6 – 1.1 volts depending on the application. The nominal voltage of NiMh cells is 1.2 volts. However, these cells come off the charger at near 1.5 volts and operate down to 0.8 – 1.0 volts, once again depending on the application. The actual voltage is dependent upon the construction of the cell and the load applied to it. With Li-Ion cells, the nominal voltage is 3.2, 3.6, or 3.7 volts depending on the chemistry used within the cell. A 3.7 volt nominal Li-Ion cell will start off at 4.2 volts and drop down to somewhere around 2.75 – 3.0 volts during use.
Nominal means in name only. There is no test data to support a nominal value. It is just called that. Fortunately the data sheets provide minimum and typical values as benchmarks to compare to.
Let’s look at the GP 2700 example you give:
“GP AA 2700 mAh NiMH Battery Data Sheet Example
The GP 2700 AA NiMH Data Sheet can be found here. GP270AAHC-r0.pdf
Its nominal and minimum are both 2500mAh and the typical is 2600. What does nominal mean in this data sheet? So, when this datasheet says typical, what does that mean? Is it 2600 +/- 5%, 2600 +/- 10%, 2600 +/- 2%? This is an example of being vague.
The graph and the table says 0.2C is 500 mAh. Therefore 1C = 0.2C x 5 = 2500 mA and that means this is a 2500 mAh rechargeable battery. When is it ever a 2700 mAh battery?”
While this may seem a little confusing, let’s take a closer look. GP is offering a typical value of 2600 mAh and a minimum value of 2500 mAh. The difference between these two numbers is around 4%. Perhaps the typical value is the average value. This may put the capacity at 2600 plus or minus 4%. Adding 4% to the typical 2600 mAh gives you the 2700 mAh capacity.
Now I will admit that GP uses maximum obtainable capacity for their labeling, but I think you have to give them “points” for tailoring the data sheet around the minimum capacity values. If they were trying to “hype” their product, I would think they would provide data based on the maximum values. Instead they responsibly base their data sheets on the minimum values. You will also note that this same trend is used for their other high capacity cells such as the 2600 mAh and 2500 mAh cells.
If we take a moment to study the Sanyo 2700 mAh sheet we find that Sanyo doesn’t use the term “nominal,” but only uses typical and minimum. Their typical value is the maximum value. Reviewing the discharge graphs, it appears that they are graphing an average value rather than the minimum value that GP uses.
Overall, it appears that the Sanyo 2700 mAh cell and the GP 2700 mAh are very similar, however the GP cell is rated for a higher discharge rate according to the specification sheet. I happen to know that the Sanyo cells perform very similar to what is shown for the GP cells, but I had to do my own testing to come up with this because Sanyo does not include this data.
Your argument that the data sheets are confusing is unfounded, but we will get to that later. You do have a point that most people buy on impulse and the “bigger is better” mentality seems to accompany those types of purchases. The data is available to make an informed decision, but it takes time to find and review it. Battery testing reviews are very welcome to those that do not have the time or desire to do the research. Also, it should be pointed out that the companies that re-labeled cells usually don’t offer data sheets for review.
Moving on…
I find your comments on the disclaimer used by GP very interesting. GP states:
“The information (subject to change without prior notice) contained in this document is for reference only and should not be used as a basis for product guarantee or warranty. For applications other than those described here, please consult your nearest GP Sales and Marketing Office or Distributors.”
However, you seem to be offering your own disclaimers concerning your data:
“Rechargeable Battery Review’s Basic Assumptions
Any Rechargeable Battery:
· Will not hold the same charge from cycle to cycle;
· Will not deliver the same capacity from cycle to cycle;
· No two charge or discharge cycles are exactly equal;
· All these differences are expected to be small;
· Should deliver close to its rated capacity whether it is used in a device with Fixed Load characteristics at 0.05C or in a device with Constant Current characteristics at 0.2C;
· The term close means a variation of less than +/- 5%.
Rechargeable Batteries from the Same Manufacturer:
· Example: two or more AA 2500 mAh from the Sanyo ( blister pack);
· No two batteries will have the same capacity or behaviour;
· No two charge or discharge cycles are equal, even in the same rechargeable battery;
· All these differences are expected to be small;
· Should deliver close to its rated capacity whether it is used in a device with Fixed Load characteristics at 0.05C or in a device with Constant Current characteristics at 0.2C;
· The term close means a variation of less than +/- 5%.
Batteries from the Different Manufacturers:
· Example: an AA 2500 mAh from the Sanyo and Powerex
· No two batteries will have the same capacity or behaviour;
· No two charge or discharge cycles will be equal;
· May perform differently at various discharge rates or in different devices;
· Should deliver close to their rated capacity whether it is used in a device with Fixed Load characteristics at 0.05C or in a device with Constant Current characteristics at 0.2C;
· The term close means a variation of less than +/- 5%.”
GP states that the data should be used as a reference and should not be used as the basis for a guarantee. You state that you are working with a limited number of samples and that the results vary from cell to cell and test to test. One might infer from this that you don’t offer your test results as a basis for a guarantee either.
To me, it sounds like you both are stating about the same thing…
Are you aware of how the battery manufacturing process works?
Battery manufacturers produce tens of thousands of cells each day. Samples are tested from each batch of cells according to THE industry standard and the cells are binned according to capacity. Cells that are shorted, or are open circuit, or are physically damaged are destroyed and recycled if possible.
Now it is time for labeling. All primary battery manufacturers label their cells according to the IEC test results from the samples taken from the production run. Since they are all using the same standard testing, it is possible to compare their product with another manufacturers.
Getting back to labeling, have you noticed that there are more brands of batteries than there are manufacturers? Since companies like Kodak and Tenergy do not produce batteries, they are free to specify the bin of capacity they want from the manufacture, then they can put whatever label they want on the cell. This is where independent testing becomes very valuable. Usually the data sheets are not available from companies that re-label cells. Standardized testing can sometimes be compared to the manufacturers data sheets to determine who manufactured the cells.
Moving on…
You seem to be having difficulty understanding the manufacturers data sheets, so let’s go through the GP 2700 mAh data sheet to see what it tells us.
The data sheet starts off with the chemistry of the cell, the dimensions, and the recommended application. The GP 2700 mAh cells are recommended for use in applications that draw between 250 mA and 7500 mA. If your application falls within that range, this is a good cell to use.
The data sheet goes on to specify the capacity of the cell. In this case it gives a minimum value and a typical value. It then goes on to specify how the capacity was obtained. In this case the cell was discharged at 500 mA down to an ending voltage of 1.0 volts. The temperature during the discharge was 20 C.
OK, now we know what rate to use for discharging to get the capacity of the cell, and what the minimum capacity is expected to be. Now, how do we charge the cell? The next line covers charging. The cell is charged at 250 mA for 16 hours at a room temperature of 20 C.
Now, what if you want to charge at a different rate? The next line gives the recommended charging rate for this cell as 1250 mA – 2500 mA, but in order to use these charging rates you need to have proper charge termination control. GP goes on to list the controls that should be used to terminate the charge.
You will notice that the recommended charging rate does not include the range from 251 mA – 1249 mA. There are reasons for this, but we won’t go into that right now.
The data sheet goes on to specify the expected service life, the ability of the cell to handle continuous overcharge, the weight, internal resistance, maximum charging voltage, and the temperature ranges you can use the cell in.
A charging graph is provided to illustrate what to expect from fast charging. You will note that when charging at 250 mA the maximum voltage should be 1.5 volts, but when you fast charge, the voltage ends up a little higher.
So far, everything is straight forward and very clear. You have clear statements as to the recommended current draws for the cell, the recommended charging methods, and the size and weight of the cell.
Now we have to do some interpretation…
You posed some questions:
“Here are some more interesting questions to consider:
· Does the 0.2C Constant Current test reflect how you would use a rechargeable battery?
· Do you use your batteries such that it is drained in exactly 5 hours?
· Do you think that the manufacturer designed the rechargeable battery to perform equally well at other rates?
· Do you EXPECT the rechargeable battery to perform equally and reasonably well at other rates?
· Do you use rechargeable batteries in low power devices like MP3 players, remotes, toys or other gadgets where they could last more than 10 hours ( a discharge rate less than 0.1C ).
· Do you use your rechargeable batteries in non-constant high drain devices like digital cameras, high powered flashlights or high powered UHF, VHF or GMRS radios?”
GP realized that these questions would surface, so they provide the expected performance of the cell at different discharge rates. They show the results for low and high rate discharges. They also reference the method of charging they used to obtain this data. If your application does not fall on one of the 6 discharge rates given, you will have to make an educated guess as to the expected capacity you can expect from this cell.
The data sheet falls short and does not give any data for pulsed loads. If your application involves pulsed loading, you probably need to contact GP to see if they have any data on that type of loading.
As you can see the data sheet does not cover every situation, but it does give a very clear representation of what to expect under the loadings listed.
In summary,
Since you don’t have the equipment to do constant current discharging, you should reflect that in your articles. You can get reasonably close results by utilizing the C-9000 in Break-In mode. The Break-In results are slightly optimistic, but close to that obtained through constant current discharging.
Don’t get hung up on the word “nominal.” It is just a name and has little, if any, meaning in the real world. The words like typical and minimum are the ones that should be focused on. It is unfortunate that marketing often focuses on the nominal capacity. However, it is refreshing to observe that along with higher nominal capacities, these cells also have higher minimum capacities. Perhaps it works out in the end, but it is wise to warn the buyer that when considering a purchase based on nominal capacity, it is best to consult the data sheet to determine how the nominal capacity compares to the actual capacity.
I hope that by going step by step through a data sheet you will have a better understanding of what is represented there. The data sheets allow comparisons of the various types of cells without having to do the testing yourself. For example, the GP 2400 mAh high capacity cell seems to be very similar to the GP 2500 mAh consumer cell.
You have brought up the question as to whether the manufacturers hype their data sheets to increase their market share of products sold. I think that presenting the data based on average or minimum values is very responsible. The data sheets present data. It is up to the marketing department to produce a brochure to hype the product.
The problem arises when you don’t have access to the data sheet for a cell. In those cases, if a series of standardized tests are done, you can then compare “apples to apples.”
Are there rogue manufacturers that don’t adhere to the industrial standards? Yes. Are there slip ups in QC? Yes. Are there errors in re-labeling cells? Yes. While the companies that re-label cells almost always exaggerate the capacity of the cell, keep in mind that the battery manufacturers are held to a higher standard. Unfortunately, these higher standards do not apply to the eBay seller that offers 3500 mAh AA cells of some off brand.
As you have indicated in your articles, there are other uncontrollable issues that can also arise to throw off the test results. When you purchase a set of cells, you have no way of knowing the history of the storage of the cells, and often there may not be anyway of knowing how old the cells are. I am often amazed that as complex as battery chemistry is and given that cells often get abused, it is wonderful that they work at all.
Tom
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