Leveling NiCd/NiMH Cells
Not all interconnected to battery cells are exactly identical. Already during production of the single cells little differences occur eg in the capacitance. Some cells have a smaller, others a slightly larger capacitance. In addition, during daily use, not all cells of a battery are equally loaded (eg some become warmer than others due to their location closer to a sorce of heat). The results are different stages-of-charge of the individual cells. These differences, if not compensated, will shorten battery-life and decrease the capability of storing and delivering power. A rule of thumb says that after about five charge-discharge-cycles a load leveling should be performed.
For NiCd / NiMH cells, there are two options for leveling the state-of-charge of the individual cells. The first one is during charge. If the battery charger by means of its minus-delta-V-cut has detected, that the first cell of a stack is fully charged, further charge is performed with a drastically reduced charging current. This small current is substantially harmless to the already fully charged cells for some time but will fully charge the other cells in the stack. The stack is overcharged for some time with a small current.
The second option is to store a NiCd/NiMH battery stack with a high self-discharge-rate, if not in use for a longer time, in a discharged state. The self-discharge-effect will ensure that the not yet depleted cells lose their charge and thus all cells are totally discharged after some time. However, it should be noted, that this method will not work for cells with a very low self-discharge rate . These types of cells are often referred as "ready to use" and available by various manufacturers.
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