Standard Charge
Extended constant current charge at the standard 14 hour rate in NiCd and NiMH
cells has little effect on the overall performance or operating life. It is
however good practice to discontinue the charge within 24 hours to minimise
long term effects, particularly when using NiMH cells.
The standard overnight charge period of 14 hours equates to a rate of C/10
(C=Ah capacity of the cell). If accelerated or fast charge is required at
rates of typically C/5 to 2C, the correct management of the charge
characteristic is vital.
Voltage Depression Memory Effect
One effect of continual overcharge and high temperature operation is voltage
depression. The voltage of each cell in the battery pack may be reduced by
up to 150mV reducing useful operating hours significantly. This is a more
probable reason for loss of capacity than the near mythical "memory effect"
which is only apparent after a cell has been subjected to precise repetitive
shallow cycling. Both voltage depression and memory effect can be removed by
a controlled conditioning discharge to a known end point voltage.
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