True Memory Effect
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 provision 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. True memory cannot exist if any one of the following conditions holds:
- Batteries achieve full overcharge.
- Discharge is not exactly the same each cycle, within plus or minus 3%
- Discharge is to less than 1.0 volt per cell.
True memory effect is specific to sintered-plate nickel-cadmium cells, and is exceedingly difficult to reproduce, especially in lower ampere-hour cells. In one particular test program—especially designed to induce memory—no effect was found after more than 700 precisely-controlled charge/discharge cycles. In the program, spirally-wound one-ampere-hour cells were used. In a follow-up program, 20-ampere-hour aerospace-type cells were used on a similar test regime. Memory effects showed up after a few hundred cycles.
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