A typical cell construction is cylindrical for greatest energy density
Click to go largeHowever prismatic (rectangular) cells are more popular nowadays for packaging reasons
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Memory effect in nickel based rechargeable batteries
Nickel based battery cells have a typical service life of around 800 to 1000 charge cycles but do suffer from memory effect.
When cells are repeatedly discharged partially before recharging, overly large crystals can form within the electrolyte. These large crystals have a much smaller surface area per unit of volume which means the cell's chemical reactions are slowed, causing voltage depression and high resistance. Additionally the sharp edges of the large crystals may cut or force through the separator causing short circuits.
Repeated cycling between full discharge and full charge, 'conditioning' will often break these large crystals down however, allowing further use of the pack. Unfortunately, damage to the separator is irreparable.
These micrographs clearly show the chemical changes associated with memory effect in nickel cadmium battery cells
New CellThe anode of a new NiCd battery, the cadmium hydroxide crystals average 1 micron across, giving a large surface area available to the electrolyte.
Low capacity cellLarge crystals, here seen over 30 micron across, have formed, restricting contact with the electrolyte and the desired flow of electrons. This results in increased resistance, lowering the current the cell is able to pass, and, since the surface area of a few large crystals is much smaller than that of many small crystals, it lowers the capacity, the amount of charge the cell is able to hold. Large crystals may be sharp enough to pierce the insulating separator between the two electrodes and conduct, causing a small short-circuit. This manifests as increased leakage, the cell discharges by itself more rapidly. Unfortunately damage to the separator is irreversible.
Reconditioned cellAfter cycling the large crystals have been successfully broken down, here seen around 4 microns across, allowing the battery to work nearly as well as when new.