During the nickel-cadmium years of the 1970s and 1980s, battery ills were blamed on “memory.” Today, the word memory is still being used to advertise newer batteries as being “memory-free.” Memory comes 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 discharges. If more was demanded, the voltage would abruptly drop as if to protest against imposed overtime.
Memory occurs when keeping a NiCd battery is overcharged. The effect can be reversed with a pulse charge, but it is more effectively to apply a full discharge cycle. Figure 1 illustrates the anode of a normal NiCd, memory that formed, and a restored anode.
Cell with crystalline formation. Crystals have
Restored cell. After a pulsed charge, the crystals are reduced to 3–5 microns: an almost 100% restoration. Exercise or recondition is needed if the pulse charge alone is not effective.Figure 1: Crystalline formation on nickel-cadmium cell
The modern nickel-cadmium battery no longer has cyclic memory, but it suffers from crystalline formation. The active cadmium material is applied on the negative plate and with time, a crystalline formation develops that reduces the surface area and lowers the 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, but this claim is only partially true. NiMH is 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 simple explanation of why NiMH is less susceptible to memory than NiCd.
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 1 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 6 months or longer, the crystals ingrain themselves and a full restoration with a discharge to 1 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 about 0.4V/cell and lower.
Tests 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.
Recondition is most effective in rejuvenating batteries that have not been exercised. Battery analyzers automatically apply the recondition cycle if the user-set target capacity cannot be reached with a discharge to 1V/cell alone. Although low performing batteries can often be fully restored, high self-discharge makes some old-timers unusable for service.
Most ship batteries in large aircraft are NiCd. Resembling an oversized starter battery in a vehicle, these batteries are serviced by applying a full discharge and keeping each cell at zero volts for 24 hours before recharge. Each cell is then checked for correct voltage, and capacity verification is taken with a full discharge/charge cycle before installing them again in the airplane. Aviation batteries follow strict maintenance schedules.
As battery care-giver, you have choices in how to prolong battery life. Each battery system has unique needs in terms of charging, depth of discharge and loading that should be observed. The following two papers summarize what batteries like and dislike.
[1] Courtesy of Cadex
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