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Memory Effect 2

General Electric's technical note regarding memory, by Bob Myers
(posted without permission from GE)

These notes ALSO apply to Ni-MH batteries.

Among the many users of batteries in both the industrial and consumer sectors, the idea of a memory phenomenon in nickel-cadmium batteries has been widely misused and understood. The term 'memory' has become a catch-all 'buzzword' that is used to describe a raft of application problems, being most often confused with simple voltage depression.

To the well informed, however, 'memory' is a term applied to a specific phenomenon encountered very infrequently in field applications. Specifically, the term 'memory' came from an aerospace nickel-cadmium application in which the cells were repeatedly discharged to 25% of available capacity (plus or minus 1%) by exacting computer control, then recharged to 100% capacity WITHOUT OVERCHARGE. This long term, repetitive cycle regime, with no provisions for overcharge, resulted in a loss of capacity beyond the 25% discharge point. Hence the birth of a "memory" phenomenon, whereby nickel-cadmium batteries purportedly lose capacity if repeatedly discharged to a specific level of capacity.

The 'memory' phenomenon observed in this original aerospace application was eliminated by simply reprogramming the computer to allow for overcharging. In fact, 'memory' is always a completely reversible condition; even in those rare cases where 'memory' cannot be avoided, it can easily be erased. Unfortunately, the idea of memory-related loss of capacity has been with us since. Realistically, however, 'memory' CANNOT exist if ANY ONE of the following conditions holds:

1.Batteries achieve full overcharge.

2.Discharge is not exactly the same each cycle - plus or minus 2-3%

3.Discharge is to less than 1.0 volt per cell.

Remember, the existence of any ONE of these conditions eliminates the possibility of 'memory'. GE has not verified true 'memory' in any field application with the single exception of the satellite application noted above. Lack of empirical evidence notwithstanding, 'memory' is still blamed regularly for poor battery performance that is caused by a number of simple, correctable application problems."

1.Cutoff voltage too high - basically, since Ni-Cads have such a flat voltage vs. discharge characteristic, using voltage sensing to determine when the battery is nearly empty can be tricky; an improper setting coupled with a slight voltage depression can cause many products to call a battery "dead" even when nearly the full capacity remains useable.

2.High temperature conditions - Ni-Cads suffer under high-temp conditions; such environments reduce both the charge that will be accepted by the cells when charging, and the voltage across the battery when charged (and the latter, of course, ties back into the above problem).

3.Voltage depression due to long-term overcharge - Self-explanatory. Ni-Cads can drop 0.1-0.15 V/cell if exposed to a long-term (i.e., a period of months) overcharge. Such an overcharge is not unheard-of in consumer gear, especially. if the user gets in the habit of leaving the unit in a charger of simplistic design

4.Miscellaneous

Operation below 0 deg. C
High discharge rates (above 5C) in a battery not specifically designed for such use
Inadequate charging time or a defective charger
One or more defective or worn-out cells (Ni-Cads DO have a finite life; they won't keep charging and discharging FOREVER no matter how well we baby them.)