Memory: Myth or Fact?

A common difficulty with portable equipment is the gradual decline in battery performance after the first year of service. Tampering with any Li-ion charger is not recommended. Although fully charged, the battery eventually regresses to a point where a NiCd may hold less than half of its original capacity, resulting in unexpected downtime. Downtime almost always occurs at critical moments. Under normal conditions, the battery holds enough power until recharged. During heavy activities and longer than expected duties, a marginal battery cannot provide the extra power needed and the equipment fails. Rechargeable batteries are known to cause more concern, grief and frustration than any other component of a portable device. Given its relatively short life span, especially when not properly maintained, the battery is often the most expensive and frequently the least reliable part of a portable device.

Memory: Myth or Fact? In many ways, a rechargeable battery exhibits human-like characteristics: it needs good nutrition, it prefers moderate room temperature and, in case of the NiCd battery, requires regular exercise to prevent the phenomenon called "memory". The word memory was originally derived from cyclic memory, meaning that a NiCd battery can remember how much discharge was required on previous discharges. Improvements in battery technology have virtually eliminated this phenomenon.

Tests performed at a Black & Decker lab, for example, showed that the effects of cyclic memory were so small that they could only be detected with sensitive instruments. After the same battery was discharged for different lengths of time, the cyclic memory phenomenon could no longer be detected.

The problem with the modern NiCd battery is not so much the cyclic memory but the effects of crystalline formation. In most cases, however, there is a combination of the two phenomenon (from now on when memory is mentioned we refer to crystalline formation.) The active materials of a NiCd battery (nickel and cadmium) are present in finely divided crystals. In a good cell, these crystals remain small, obtaining maximum surface area. When the memory phenomenon occurs, the crystals grow and drastically reduce the surface area. The result is a voltage depression which leads to a loss of performance. Some of the capacity may still be present but cannot be retrieved because of the battery’s low voltage table. In advanced stages, the sharp edges of the crystals grow through the separator, causing high self-discharge or an electrical short. Another form of memory that occurs on some cells is the formation of an inter-metallic compound of nickel and cadmium which ties up some of the needed cadmium and creates extra resistance in the cell. Reconditioning by deep discharge helps to break up this compound and reverses the capacity loss.

Regular Exercise- The effects of crystalline formation are most pronounced if a NiCd battery is left in the charger for days, or is repeatedly recharged without a periodic full discharge. Since most applications do not use up all energy before recharge, a periodic discharge to one volt per cell (known as exercise) is essential for the NiCd to prevent the buildup of crystalline formation on the cell plates. All NiCd batteries in regular use and on standby mode (sitting in a charger for operational readiness) should be exercised once per month. Between these monthly exercise cycles, no further attention is needed and the battery can be used with any desired user pattern without the concern of memory.

The NiMH is also affected by memory but to a lesser degree — it only needs exercise once every three months. Because of its shorter cycle life, it is not recommended to over-exercise the NiMH. It is neither necessary, nor advisable, to discharge a NiCd before each charge because excessive cycling puts extra strain on the battery. An analogy can be drawn with a hand towel which, by washing after each use, would wear out its fabric too quickly. It is, however, necessary to clean the towel on a periodic basis. If no exercise is applied for several months, the crystals ingrain themselves, making them more difficult to break up. In such a case, exercise is no longer effective in restoring a battery and reconditioning is required. Recondition is a slow, deep discharge that removes the remaining battery energy by draining the cells to a voltage threshold below one volt per cell. Tests performed by the US Army Lab have shown that a NiCd cell needs to be discharged to at least 0.6 volts to effectively break up the more resistant crystalline formation. During recondition, the current must be carefully controlled to prevent cell reversal. The figure below illustrates the battery voltage during a normal discharge to one volt, followed by the secondary discharge (recondition). Discharge/Recondition voltage curve of a NiCd battery on a Cadex battery analyzer. Recondition consists of a discharge to 1 volt per cell at a 1C load current, followed by a secondary discharge at a much reduced current. The various stages of crystalline formation of a NiCd battery are illustrated in the 3 images below. The enlargements show normal crystal structure of a new cell, crystalline formation after use (or abuse) and restoration. New NiCd cell. The anode is in fresh condition (capacity of 8.1Ah). Hexagonal cadmium hydroxide crystals are about 1micron in cross section, exposing large surface area to the liquid electrolyte for maximum performance. Cell with crystalline formation. Crystals have grown to an enormous 50 to 100 microns in cross section, concealing large portions of the active material from the electrolyte (capacity of 6.5Ah). Jagged edges and sharp corners may pierce the separator, which can lead to increased self-discharge or electrical short. Restored cell. After pulse charge, the crystals are reduced to 3 to 5 microns, an almost 100% restoration (capacity of 8.0A). (Exercise or recondition are needed if the pulse charge alone is not effective.) Crystalline formation on NiCd cell. Courtesy of the US Army Electronics Command in Fort Monmouth, NJ, USA.

The chart below illustrates the effects of exercise and recondition. Four batteries afflicted with various degree of memory are serviced. The batteries are first fully charged, followed by a discharge to one volt per cell. The resulting capacities are plotted on a scale of 0 to 120% in the first column. Additional discharge/charge cycles are applied and the battery capacities are plotted in the subsequent columns. The dotted line represents exercise, (discharge to one volt per cell) and the solid line recondition (secondary discharge at reduced current to 0.4 per cell). Exercise and recondition cycles are applied manually at the discretion of the research technician. Effects of Exercise and Recondition Battery "A" responded well to exercise alone and no recondition was required. This result is typical of a battery that had been in service for only a few months or had received periodic exercise cycles. Batteries "B" & "C", on the other hand, required recondition (solid line) to restore its performance. Without the recondition method, these two batteries would have been discarded. The restored batteries returned to full service and when examined after six months of field use still showed good capacity readings. Applying the recondition cycle on a new battery (top line on chart) resulted in a slight capacity increase. Since new batteries are stored with some charge, the self-discharge that occurs during storage contributes to a certain amount of memory buildup. Exercising and reconditioning remove the memory and prepare a new battery for full performance. The importance of exercise and reconditioning to NiCd batteries is emphasized further by a study carried out by GTE Government Systems in Virginia, USA, for the US Navy. To determine the percentage of batteries needing replacement within the first year of use, one group of batteries received charge only, another group was exercised and a third group received recondition. The batteries studied were used for portable radios on the aircraft carriers USS Eisenhower with 1500 batteries and USS George Washington with 600 batteries, and the destroyer USS Ponce with 500 batteries. Maintenance Method Annual Percentage of Batteries Requiring Replacement Charge only (charge-and-use) 45% Exercise only (discharge to 1V/cell) 15% Reconditioning (secondary deep discharge) 5% Annual percentage of battery requiring replacement on the USS Eisenhower, USS George Washington and USS Ponce as a function of battery maintenance With charge only (charge-and-use), the annual percentage of battery failure on the USS Eisenhower was 45% (see table above). When applying exercise, the failure rate was reduced to 15%. By far the best results were achieved with recondition. Its failure rate dropped to 5%. Consistent results were also attained from the USS George Washington and the USS Ponce. The GTE government system report concluded that a battery analyzer featuring exercise and recondition functions at a cost of US $2,500 would pay for itself in less than one month on battery savings alone. No mention was made on the benefits of increased system reliability, an issue that is of equal or greater importance, especially when the safety of human lives is at stake.

SPECIALS | Home Page | Contact Us | About Batteries | Returns/Warranty | Corporate Profile