The Question:
What is cadmium migration and how do I prevent it?
Red Scholefield replies:
It has been accepted as dogma that today’s Ni-Cd batteries are not harmed by continuous overcharge as long as the temperature in overcharge is maintained at or near room temperature. Those knowledgeable about batteries however, are beginning to espouse a somewhat different story. To maximize battery life it has been recommended that the overcharge period or float charge of the battery take on an entirely different character than continuous constant current overcharge that has been the “acceptable” method. This “alternate” technique pulse charges the battery on a relatively long duty cycle. A battery is maintained at a full state of charge once it is completely charged and in overcharge (as is evidenced by a finite temperature rise or after an input of 160% of rated capacity), by a pulse of charge current in the C/10to C rate range and sufficient to replace energy lost by self discharge. This pulse of charge current is only applied to the cell for 1 hour out of 24 in the case of C/10 rate or in the 5 to 6 minute range each 24 hours for a C rate pulse. The theory and countless “casual” laboratory observations support this pulse technique over the “less expensive” constant current overcharge presently popular in the consumer product charging regimes. There is overwhelming evidence that constant current, even at very low “sustaining” rates, contributes to “cadmium migration” that forms a conductive bridge through the separator medium from the negative to the positive plate.
There is overwhelming evidence that constant current, even at very low “sustaining” rates, contributes to “cadmium migration” that forms a conductive bridge through the separator medium from the negative to the positive plate.
The rapidity of the reaction forming this cadmium bridge is a function of the current density and time. There is evidence to suggest that the conditions existing at a C/10 to C/20 overcharge rate contribute to the speed of the cadmium migration more than higher overcharge rates. The higher temperatures produced in higher overcharge rates cannot be ignored as playing a role in seemingly slowing the cadmium migration while at the same reducing battery life by increasing separator degradation. Where the optimum balance point of these opposing actions falls is left as yet one more challenge to our testing programs. It appears however, reasonable to assume at this point in time, that pulse charges at the C rate that will result in a “net” charge input equivalent to a continuous C/10 are much less conducive to cadmium migration than a continuous C/10 overcharge rate. While the beneficial impact of this pulse overcharge has not been quantified to any degree, the observations of battery testing personnel and more sophisticated battery users cannot be ignored. The creation of an economical means for pulse overcharge will be the challenge faced by the engineer wishing to design a product for optimum service life.
Questions and answers from Red Scholefield, Battery Engineer. Reprinted with Mr. Scholefield’s permission.
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