Memory: myth or fact?

The word 'memory' was originally derived from 'cyclic memory'; meaning that a nickel-cadmium battery could remember how much energy was drawn on preceding discharges. On a longer than scheduled discharge, the voltage would rapidly drop and the battery would lose power. Improvements in battery technology have virtually eliminated this phenomenon.

The problem with nickel-cadmium is not so much the cyclic memory but the effects of crystalline formation. The active cadmium material is present in finely divided crystals. In a good cell, these crystals remain small, obtaining maximum surface area. With memory, the crystals grow and conceal the active material from the electrolyte. In advanced stages, the sharp edges of the crystals penetrate the separator, causing high self-discharge or electrical short.

When introduced in the early 1990s, nickel-metal-hydride was promoted as being memory-free. Today, we know that this chemistry is also affected but to a lesser degree than nickel-cadmium. The nickel plate, a metal that is shared by both chemistries, is partly to blame. While nickel-metal-hydride has only the nickel plate to worry about, nickel-cadmium also includes the memory-prone cadmium plate. This is a non-scientific explanation why nickel-cadmium is affected more than nickel-metal-hydride.

The stages of crystalline formation of a nickel-cadmium cell are illustrated in Figure 1. The enlargements show the cadmium plate in a proper functioning crystal structure, crystalline formation after use (or abuse) and restoration.

	New nickel-cadmium cell. The anode is in fresh condition. Hexagonal cadmium hydroxide crystals are about 1 micron in cross section, exposing large surface area to the electrolyte for maximum performance.
	Cell with crystalline formation. Crystals have grown to 50 to 100 microns in cross section, concealing large portions of the active material from the electrolyte. Jagged edges and sharp corners may pierce the separator, leading to increased self-discharge or electrical short.
	Restored cell. After pulsed charge, the crystals are reduced to 3 to 5 microns, an almost 100% restoration. Exercise or recondition are needed if the pulse charge alone is not effective.

Exercise and Recondition - Research has shown that the crystals ingrain themselves if no exercise is applied to nickel-cadmium for three months or more. A full restoration with exercise becomes more difficult the longer service is withheld. In advanced cases 'recondition' is required.

Recondition is a slow, secondary discharge applied below the 1 volt/cell threshold. During this process, the current must be kept low to minimize cell reversal. Nickel-cadmium can tolerate a small amount of cell reversal but caution must be applied to stay within the allowable current limit.

Tests performed by the US Army have shown that a nickel-cadmium cell needs to be discharged to at least 0.6V to effectively break up the more resistant crystalline formation. Figure 2 illustrates the battery voltage during a discharge to 1V/cell, followed by the secondary discharge to 0.4V/cell.

Figure 2: Exercise and recondition features of a Cadex battery analyzer.If a nickel-cadmium battery has not been exercised for three months or longer, recondition is required to restore capacity. Recondition is a slow, deep discharge to 0.4V/cell. If service is denied for 6 to 12 months, recondition becomes ineffective.

Figure 3 illustrates the effects of exercise and recondition. Four nickel-cadmium batteries afflicted with various degrees of memory are serviced. The batteries are first fully charged, then discharged to 1V/cell. The resulting capacities are plotted on a capacity 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 green line represents 'exercise', and the blue line 'recondition'. The exercise and recondition cycles are applied manually at the discretion of the research technician.

Figure 3: Effects of exercise and recondition.Four batteries afflicted with memory are serviced. Battery 'A' improved capacity on exercise alone; batteries 'B' and 'C' required recondition. The new battery improved further with recondition.

Battery 'A' responded well to exercise alone and no recondition was required. This battery may have been in service for only a few months or has received periodic exercise cycles. Batteries 'B' and 'C' required recondition to restore performance. Without recondition, these two batteries would have been discarded.

After service, the restored batteries were returned to full use. When examined after six months of field service, no noticeable degradation in the performance was visible. The regained capacity was permanent but periodic service will be needed to maintain the performance.

Applying the recondition cycle on a new battery (top line on chart) resulted in a slight capacity gain. This increase is not fully understood, other than to assume that the battery improved by additional forming. Another explanation is early presence of memory. Since new batteries are stored with some charge, the self-discharge that occurs during storage produces some crystalline formation. Exercising and reconditioning reverse this effect.

Recondition has its limitations. If no exercise had been applied for 6 to 12 months, permanent damage may have been inflicted. The capacity may not recover or the pack may suffer from high self-discharge caused by a marred separator. Older batteries may get worse with recondition. These packs can be compared to an old man to whom strenuous activity is harmful. Such batteries must be replaced.

Typically 50%-70% of discarded nickel-cadmium batteries can be restored when using the exercise and recondition methods of a Cadex battery analyzer or equivalent. The recovery rate of nickel-metal-hydride is about 40%. This lower yield is, in part, due to the battery's low cycle count.

Field results on exercise and recondition

After the Balkan War, the Dutch Army examined how many field batteries could be restored with a battery analyzer (Cadex). The army was aware that the packs were used under less than ideal conditions. They had been sitting in the chargers with only 2-3 hours use per day.

The capacity on some packs had dropped from 100% to 30%. With the analyzer's recondition function, 9 of 10 batteries were restored to 80% and higher. The nickel-cadmium batteries were 2-3 years old.

The importance of exercising and reconditioning is emphasized by another study carried out for the US Navy by GTE Government Systems. To determine the percentage of batteries needing replacement in the first year of use, one group of batteries received charge only (no maintenance), another group was periodically exercised and a third group received recondition. The batteries studied were used for two-way radios on US aircraft carriers.

With charge only (charge-and-use), the annual percentage of battery failure was 45% (Figure 4). With exercise, the failure rate was reduced to 15%. By far the best results were achieved with recondition. The failure rate dropped to 5%.

The GTE report concluded that a battery analyzer featuring exercise and recondition functions costing $2,500US would return its investment in less than one month on battery savings alone.

Simple Guidelines
· Do not leave a nickel-based battery in a charger for more than a few days, even on trickle charge.
· Exercise nickel-cadmium every 1 to 2 months and nickel-metal-hydride every 3 months. Running the battery down in the equipment may do this also.
· Do not discharge the battery before each recharge. This puts undue stress on the battery.
· Avoid getting the battery too hot during charge. The temperature should only rise for a short moment at full charge, then cool off.

About the Author
Isidor Buchmann is the founder and CEO of Cadex Electronics Inc., in Vancouver BC. Mr. Buchmann has a background in radio communications and has studied the behavior of rechargeable batteries in practical, everyday applications for two decades. Award winning author of many articles and books on batteries, Mr. Buchmann has delivered technical papers around the world.
Cadex Electronics is a manufacturer of advanced battery chargers, battery analyzers and PC software. For product information please visit www.cadex.com.