sideshow
Posts: 2521
Joined: 11/29/2001
From: Pleasanton,
CA, USA
Status: offline
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*sigh*
Read this....
http://www.repairfaq.org/ELE/F_NiCd_Battery.html#NICDBATTERY_014
Then...read this...
http://www.rcbatteryclinic.com/seminar.htm
Here is an excerpt...
MEMORY OR VOLTAGE DEPRESSION
Contrary to popular belief, the memory effect is not a loss of cell capacity. Memory is a step in the discharge curve of a cell.
See how the voltage of the lower curve is depressed compared with the normal discharge curve that we have seen before? The end result of the step is significant only if a device is designed with too high a cutoff voltage. Most designers take this effect into account and allow for their devices to be run at low enough voltage to avoid this problem. What causes memory? Actually there are two ways to create a step in the voltage profile. One is a precisely repetitive partial discharge followed by a slow full charge. The discharge must be to exactly the same point every discharge in order for this effect to appear. The second and more frequently encountered effect is voltage depression, which is also called memory. This is caused by continuous overcharge at the overnight rate. If a battery is left on slow charge for long periods, the crystals of active material in the plates grow larger. As the crystals grow, the surface area of active material in contact with the electrolyte decreases and this phenomenon manifests itself as a very slight increase in internal resistance, plus a decrease in the open circuit voltage. The voltage step will occur at different times, depending upon how long the overcharge occurs and the temperature of the battery in overcharge. As the overcharge continues, the area of voltage depression will occur earlier in the discharge curve. The area of depression can be removed by one or more discharge/charge cycles, thereby returning the cell's voltage profile to normal. Today’s cell designs have improved to the point where this condition is seldom exhibited.
If after all this (especially that last sentence) you decide that's what you had, then that's what you had.....I guess.
The packs you "revived" had this voltage depression. The packs you couldn't.....were dead (failed).....they didn't have a memory.
Another excerpt:
We generally define failure as the point where the cell fails to yield 80% of its rated capacity. The primary failure mode is the loss of separator integrity that manifests itself in a cell short. This short may be what is termed as a hard short or low resistance or as is typical in onset of failure, the short is of some finite resistance or "soft" short. This soft short causes the battery to "self discharge" in a very short period as well as shunt some of the charging energy during the charge cycle all of which result in what is perceived as a low capacity cell. A functional failure occurs when the cell or battery causes the end-use device to fail to function.
One of the largest (SR), and most respected, battery pack companies guarantees all of it's nicad packs against a "memory". They almost dare you to do it.
If you are able to discharge a pack to exactly the same level every time, or if you are going to continuously overcharge the pack overnight, every night, then worry about it (though it's not really a memory).
Other than that, don't worry about a "memory" and go fly whichever pack makes you comfortable. Right now....I'm trying to make Li give me the warm fuzzy's....almost there! 
Bob
< Message edited by sideshow -- Apr 5 2003 5:41AM >
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Bob Convery
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All Forums >> Radios, Batteries, Clubhouse and more >> Batteries & Chargers >> Is there really NI CD memory? 
Is there really NI CD memory? - 
Where? - 
