Memory: Myth or Fact?

During the nickel-cadmium years in the 1970s and 1980s, most battery ills were blamed on “memory.” Memory is derived from “cyclic memory,” meaning that a nickel-cadmium battery could remember how much energy was drawn on previous discharges and would not deliver more than was demanded before. On a discharge beyond regular duty, the voltage would abruptly drop as if to rebel against pending overtime. Improvements in battery technology have virtually eliminated the phenomenon of cycling memory.

Figure 1 illustrates the stages of crystalline formation that occur on a nickel-cadmium cell if overcharged and not maintained with periodic deep discharges. The first enlargement shows the cadmium plate in a normal crystal structure; the middle image demonstrates full-blown crystalline formation; and the third reveals some form of restoration.

Figure 1: Crystalline formation on nickel-cadmium cell. Crystalline formation occurs over a few months if battery is overcharged and not maintained with periodic deep discharges.

Courtesy of the US Army Electronics Command in Fort Monmouth, NJ

The modern nickel-cadmium battery is no longer affected by cyclic memory but suffers from crystalline formation.The active cadmium material is applied on the negative electrode plate, and with incorrect use a crystalline formation occurs that reduces the surface area of the active material. This lowers battery performance. In advanced stages, the sharp edges of the forming crystals can penetrate the separator, causing high self-discharge that can lead to an electrical short. The term “memory” on the modern NiCd refers to crystalline formation rather than the cycling memory of old.

When nickel-metal-hydride was introduced in the early 1990s, this chemistry was promoted as being memory-free but this claim is only partially true. NiMH is also subject to memory but to a lesser degree than NiCd. While NiMH has only the nickel plate to worry about, NiCd also includes the memory-prone cadmium negative electrode. This is a non-scientific explanation of why nickel-cadmium is more susceptible to memory than nickel-metal-hydride.

Crystalline formation occurs if a nickel-based battery is left in the charger for days or repeatedly recharged without a periodic full discharge. Since most applications fall into this user pattern, NiCd requires a periodic discharge to one volt per cell to prolong service life. A discharge/charge cycle as part of maintenance, known as exercise, should be done every one to three months.Avoid over-exercising as this wears down the battery unnecessarily.

If regular exercise is omitted for six months and longer, the crystals ingrain themselves and a full restoration with a discharge to one volt per cell may no longer be sufficient. However, a restoration is often still possible by applying a secondary discharge called “recondition.” Recondition is a slow discharge that drains the battery to a voltage cut-off point of about 0.4V/cell and lower. Tests done by the US Army indicate that a NiCd cell needs to be discharged to at least 0.6V to effectively break up the more resistant crystalline formations. During this corrective discharge, the current must be kept low to minimize cell reversal and, as discussed earlier, NiCd can tolerate a small amount of cell reversal. Figure 2 illustrates the battery voltage during a discharge to 1V/cell, followed by the secondary discharge to 0.4V/cell.
 

Recondition is most effective with healthy batteries and the remedy is also known to improve new packs. Similar to a medical treatment, however, the service should only be applied when so needed because over-use will stress the battery. Automated battery analyzers (Cadex) only apply the recondition cycle if the user-set target capacity cannot be reached.

Recondition is only effective on working batteries. Best results in recovery are possible when applying a full discharge every 1–3 months. If exercise has been withheld for 6–12 months, the capacity may not recover fully, and if it does the pack might suffer from high self-discharge caused by a marred separator. Older batteries do not restore well and many get worse with recondition. When this happens, the battery is a ripe candidate for retirement.

Results of Battery Maintenance

After the Balkan War in the 1990s, the Dutch Army began servicing its arsenal of nickel-cadmium batteries that had been used for the two-way radios. The technicians in charge wanted to know the remaining capacity and how many batteries could be restored to full service using battery analyzers (Cadex). The army knew that allowing the batteries to sit in the chargers with only two to three hours of use per day during the war was not ideal, and the tests showed that the capacity on some packs had dropped to a low 30 percent. With the recondition function, however, nine out of 10 batteries could be restored to 80 percent and higher. The army uses 80 percent as a threshold for usability. At time of service, the nickel-cadmiumbatteries were two to three years old.

To analyze the effectiveness of battery maintenance further, the US Navy carried out a study to find out how user pattern affects the life of nickel-cadmium batteries. For this, the research team responsible for the program established three battery groups. One group received charge only (no maintenance); another was periodically exercised (discharge to 1V/cell); and a third group received recondition. The 2,600 batteries studied were used for Motorola two-way radios deployed on three US aircraft carriers. Table 3 summarizes the test results, including the cost factor.

Table 3: Replacement rates of nickel-cadmium batteries
Exercise and recondition prolong battery life by three- and ninefold respectively.

GTE Government Systems, the organization that conducted the test, learned that with charge-and-use the annual percentage of battery failure was 45 percent; with exercise the failure rate was reduced to 15 percent; and with recondition only 5 percent failed. The GTE report concludes that a battery analyzer featuring exercise and recondition costing US$2,500 would return the investment in less than one month on battery savings alone.

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