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The Nickel Cadmium, NiCd rechargable cells were prone to an effect known as the memory effect.
The NiCd memory effect, affected cells that were repeatedly only partially discharged. The result was that after multiple partial discharges the cell would only discharge to the level it had been repeated discharged to.
The memory effect was obvious in applications where a NiCd had been used in an applications where it acted as a battery backup. Under normal circumstances a mains or other supply was used, but repeatedly the cell was discharged partially when the mains supply was not available, and then it was recharged.
It was reputed that the effect was first noticed in satellites where the Sun was used when it was available, but the NiCd batteries were used when the satellite passed over the dark side of the Earth. Accordingly the batteries were repeatedly partially discharged and then immediately recharged again without experiencing a full discharge. Soon it was discovered that their overall capacity was reduced as they "remembered" the amount by which they were normally discharged.
For most normal applications it appears that the NiCd memory effect is not a major issue, although it helps if the cell is run through a complete cycle occasionally, ensuring that it is completely discharged. If the cells are contained within a larger battery, it is helpful to discharge them separately (if possible) as this will ensure that no individual cells become reverse charged as some cells will contain slightly more charge than others. By performing the occasional complete discharge / charge cycle this may help reduce the NiCd memory effect if it is suspected.