The word 'memory' was originally derived from 'cyclic memory'; meaning that a nickel-cadmium battery could remember how much energy was drawn on preceding discharges. On a longer than scheduled discharge, the voltage would rapidly drop and the batterywould lose power. Improvements in battery technology have virtually eliminated this phenomenon. Canon NB-4L Charger for Canon NB-4L Battery.
The problem with nickel-cadmium is not so much the cyclic memory but the effects of crystalline formation. The active cadmium material is present in finely divided crystals. In a good cell, these crystals remain small, obtaining maximum surface area. With memory, Sony NP-BG1 Charger for Sony NP-BG1 Battery, the crystals grow and conceal the active material from the electrolyte. In advanced stages, the sharp edges of the crystals penetrate the separator, causing high self-discharge or electrical short.
When introduced in the early 1990s, nickel-metal-hydride was promoted as being memory-free. Today, we know that this chemistry is also affected but to a lesser degree than nickel-cadmium. The nickel plate, a metal that is shared by both chemistries, is partly to blame. Olympus LI-50B Charger for Olympus LI-50B Battery. While nickel-metal-hydride has only the nickel plate to worry about, nickel-cadmium also includes the memory-prone cadmium plate. This is a non-scientific explanation why nickel-cadmium is affected more than nickel-metal-hydride.
The stages of crystalline formation of a nickel-cadmium cell are illustrated in Figure 1. Canon BP-511 Charger for Canon BP-511 Battery.The enlargements show the cadmium plate in a proper functioning crystal structure, crystalline formation after use (or abuse) and restoration.
How to restore and prolong nickel-based batteries
Crystalline formation is most pronounced if a nickel-based battery is left in the charger for days such as laptop battery, or if repeatedly recharged without a periodic full discharge. Since most applications do not use all energy before recharge, a periodic discharge to 1 volt per cell (known as exercise) is essential to prevent memory.
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