Discover if memory is myth or fact, and how to prevent and eliminate it.
During the nickel-cadmium years in the 1970s and 1980s, most battery ills were blamed on “memory.” The term memory was derived from “cyclic memory,” meaning that a nickel-cadmium battery could remember how much energy was drawn on previous discharges and it would deliver the same amount on repeat discharged. If more was demanded, the voltage would abruptly drop as if to protest against pending overtime. Improvements in battery technology have virtually eliminated the phenomenon of cycling memory.
Memory occurs if a NiCd battery is overcharged. The effect can be reversed with a pulse charge or a full discharge cycle. The stages of memory are illustrated in Figure 1. The first enlargement shows the cadmium plate in a normal crystal structure; the middle image demonstrates full-blown crystalline formation and the third reveals restoration with a pulse charge.
New nickel-cadmium cell.The anode (negative electrode) 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.
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 no longer has cyclic memory but suffers from crystalline formation. The active cadmium material is applied on the negative electrode plate and with time crystalline formation occurs that reduces the surface area of the active material, lowering 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.
When introduced in the early 1990s, nickel-metal-hydride (NiMH) was hailed as being memory-free. 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 simplistic explanation of why NiMH is less 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 1–3 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. Restoration is often 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 as NiCd can only tolerate a small amount of cell reversal. (See BU-501: Basics About Discharging). 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 cycles of a battery analyzer (Cadex)
Courtesy of Cadex
Recondition is most effective to rejuvenate batteries that had not been exercised. Battery analyzers (Cadex) automatically apply the recondition cycle if the capacity on a discharge to 1V/cell falls below the user-set target capacity that is normally set to 80 percent. Although low performing batteries can often be fully restored, high self-discharge makes some old-timers unusable.
Most main-ship batteries in large aircraft are NiCd. Resembling an oversized starter battery, these batteries are also affected by memory. As part of a routine service, the flooded NiCd cells are fully discharged and kept at zero volts for 24 hours before recharge. Each cell is then check for the correct voltage and a final capacity verification is taken with a full discharge-charge cycle.
Last updated 2015-04-29
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