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Red Scholefield
10-23-2005, 11:14 AM
Lots of argument on lithium polymer service life, and performance.

There does exist an industry standard. It seems that many of our suppliers are not aware of it, making claims for performance parameters undefined.

IEC 61960 - 2 - Ed 1: Secondary cells and batteries containing alkaline or other non-acidic electrolytes defines the standards for test and rating of such batteries. Part 2: Secondary lithium batteries covers our Li Po cells specifies the charging and discharging for rated capacity and states in Par 1.3.12 that the reference test current is to be 0.2C. Section 4.5 specifies the endurance in cycles and the procedure to be used to define it. Cycle life is defined as the point in that test at which the cell delivers 60% of the rated capacity (as measured at 0.2C) Interestingly, the "fast rate" discharge specified is at 1 C! Internal AC resistance is specified as being measured at 1KHz. Some of our current work indicates that there are better ways of determining internal resistance. All the manufacturers seem to be using the AC impedance. (AC impedance gives a "better" number than the ANSI method of Delta V/Delta I which is more realistic in our electric flight environment).

The method specified in Par 4.6.2 "Measurement of the internal d.c. resistance" seems more appropriate. It states " The battery shall be discharged at a constant current of value Isub c =0.2C for 10s. Current level is increased to 1C immediately for 10 Sec. The voltage is measured at the cell terminal at the end of the 10 Sec periods.

DC resistance then is the difference in the two current levels divided by the difference in the two measured voltage levels.

For reasons lost in time, the manufacturers seem to be using 80% capacity as the end of life. If most Li Po flyers are as frugal as we are, I doubt that a pack might be discarded until that 60 % is reached.

However, test work does indicate that cell decline goes almost asymptotic when the 80% level is reached. That may be why manufacturers are using that figure. There is good reason: The Failure Mode Effect Analysis for a failing cell is that salts begin to form at pressure points or in areas of heat concentration as the mechanism of failure. Think about it for a moment: As a salt crystal forms, it is a very rough sphere. The surface area is Pi X R cubed. Not quite exponential but close. In other words the crystal that starts forming at 80% capacity grows as the cubed so the cell isn't long for this world. Cell unbalance and safety are affected by this decline. It is smart to keep track of flight time achieved and discard a pack when 80% of the original time is reached. The process of salt formation is greatly accelerated by cell operating temperature. This is the reason some of the lightweight cells pressed into use in high current applications suffer accelerated loss of capacity after as low as 50-60 cycles.

The other accelerator is the presence of "hangar rash" on the cells due to continuous handling and ding marks that cause salt crystals to start forming at pressure points. This is the primary reason for using IMPAD around packs to protect against impact. In other words, avoid the temptation to scrape a thumbnail over that envelope to see if it springs back!

ragbag
10-23-2005, 09:35 PM
The last paragraph mentions dragging a thumb nail over the pack.

Would writing on the cell with anything other than a felt tip marker be detrimental?
Enlighten us please.

By George

Red Scholefield
10-23-2005, 09:47 PM
The last paragraph mentions dragging a thumb nail over the pack.

Would writing on the cell with anything other than a felt tip marker be detrimental?
Enlighten us please.

By George

According to my sources any pressure placed on the thinn envelop of the pack can has the potential for damaging the cell in some way. I would make my markings on a sticky label and use that in the future.

Fred Marks
10-23-2005, 11:38 PM
This info is equally applicable to nickel cells as well. Dr Keith Shaw informs me that, over the years he has used a policy of examining all cells in apack and replacces any cell that shows a dent or ding mark because any indentation ina case means that the underlying matrix has been compressed and that leads to thermal and electrical stress.

Fred

Matt Kirsch
10-24-2005, 12:36 AM
If you go back to elementary science class where you made a crude battery out of a potato and two nails (one copper, one iron IIRC), you can think about it this way:

Denting the surface of the battery is like mashing that potato. There's a good chance that the two nails could get shoved together, causing a short circuit. Not such a big deal with a potato, but a single LiPoly cell is made up of many such "potatos" that are much more potent.

qban_flyer
10-24-2005, 02:38 AM
If you go back to elementary science class where you made a crude battery out of a potato and two nails (one copper, one iron IIRC), you can think about it this way:

Denting the surface of the battery is like mashing that potato. There's a good chance that the two nails could get shoved together, causing a short circuit. Not such a big deal with a potato, but a single LiPoly cell is made up of many such "potatos" that are much more potent.

That's the way I like it, simple and down to earth explanations I can understand in plain, unadulterated layman's language.

Gosh! It's been well over 45 years since I heard that analogy applied to a subject matter such as this one.