Memory effect, nickel cadmium batteries

The memory effect—the tendency of a battery to adjust its electrical properties to a certain duty cycle to which it has been subjected for an extended period of time—has been a problem with nickel-cadmium batteries in some applications. Pocket, fiber, and plastic-bonded plate cells do not show this tendency. See Sec. 27.7.2 for a description of the memory effect with sintered-plate nickel-cadmium batteries. 

This phenomenon varies with the design and formulation of the electrode and may not be evident with all sealed nickel-cadmium batteries. Modern nickel-cadmium batteries use electrode structures and formation processes that reduce the susceptibility to voltage depression, and most users may never experience low performance due to memory effect. However, the use of the term memory effect persists, since it is often used to explain low battery capacity that is attributable to other problems, such as ineffective charging, overcharge, battery aging, or exposure to high temperatures. 

Nickel-zinc batteries may exhibit only a very mild memory effect that is associated with the nickel electrode. Nickel-cadmium batteries commonly exhibit what is termed memory effect or fading. This is a reversible phenomenon usually caused by repetitive cycling at less than full depth-of-discharge. The observed effect is a depression in the discharge voltage (as much as 120 millivolts) when the battery is discharged below the depth at which it was previously cycled. Nickel-zinc batteries are only slightly affected by a similar phenomenon. 

The electrochemical equivalent of about 480 Ah kg is one of the lowest for all metallic anodes, and the OCV of 1.35 V for the Nicad is not favorable for many applications. Studies of failure mechanisms [26] revealed that the cadmium electrode is responsible for capacity loss and memory effect of the nickel/cadmium battery. Additionally, it is desirable to restrict the use of cadmium for environmental reasons. The consequence is a continuous retreat of this system from many applications, and battery packs for electric tools may eventually be the only remaining use. 

However, because of the toxicity of cadmium and the memory effect of nickel-cadmium batteries, this system was replaced by the MH-Ni battery invented later the total reaction for this battery during the charge and discharge processes is shown in Equation 1.3, whose open-circuit voltage is about 1.42 V. 

The memory effect in nickel-cadmium batteries and cells... 

Memory effect A phenomenon in which a nickel-cadmium battery, operated in successive cycles of identical depth of discharge, temporarily renders the rest of its capacity inaccessible at normal voltage levels. 

Although more expensive, the nickel-cadmium cell is superior to the Leclanche cell in almost all respects, except that the toxicity of cadmium places some restrictions on the disposal of defunct nicad cells. Even the rechargeable Ni/Cd cell has a limited life, due to a memory effect after discharge (i.e., it is not quite fully rechargeable), and consideration must be given to proper disposal or, better, recycling. Peugeot s entry in the ZEV field, the Model 106 electric car, uses 20 liquid-cooled 6 V Ni/Cd cells to deliver 120 V, and the supplier undertakes to recycle the battery at the end of its useful life. 

In response to the need for better batteries, the Uthium-ion battery (LIB) was conceived and developed in Japan by Asahi Kasei Co. and first commercialized by Sony Co. in 1991, followed by A T Battery Co. (a joint company of Toshiba Battery and Asahi Kasei Co.) in 1992. The LIB was accepted immediately because of its high-energy density, good performance, and no memory effect as occurred with nickel-cadmium (Ni-Cd) or nickel-hydride (Ni-MH) batteries. LIBs have been used mainly for portable electronics, especially cellular phones and notebook computers. Recently, the application area has been extended to power tools and battery-assisted electric bicycles. Several companies are working to adapt the lithium-ion system for use in hybrid electric vehicles to replace the Ni-MH. 

Nickel-metal-hydride (NiMH) battery (13.5) Secondary (rechargeable) battery used in many small electronic devices involves the oxidation of hydrogen from a metal hydride and the reduction of nickel oxyhydroxide (NiO(OH)). Compared with nicad batteries, NiMH eliminates the use of toxic cadmium and is less prone to memory effects. 

Nickel metal hydride (Ni-MH) batteries have application in hybrid electric vehicles, portable electronic devices such as cameras, shavers, toothbrushes, etc. They offer a higher energy option than Ni-Cd batteries with the hydrogenabsorbing metal hydride alloy (MH) replacing the cadmium electrode in Ni-Cd cell construction. Like Ni-Cd, the Ni-MH also shows the memory effect on repetitive cycle regimes. The nickel metal hydride battery was introduced commercially in 1989. The technology is based on the development of rare earth alloys with nickel that have the ability to reversibly absorb and desorb hydrogen. The... 

I. Y. Sato, K. Ito, T. Arakawa and K. Kobaya Kawa Possible Causes of the Memory Effect Observed in Nickel-Cadmium Secondary Batteries. J. Electrochemical Society, 143 L225 (October 1996). 

The problem with the modern NiCd battery is not the cychc memory but the effects of crystalline formation. (When memory is mentioned we refer to crystalline formation.) The active materials of a NiCd battery (nickel and cadmium) are present in crystaUine form. When the memory phenomenon occurs, these crystals grow, forming spike or tree-like crystals that cause the NiCd to gradually lose performance. In advanced stages, these crystals may puncture the separator, causing high self-discharge or an electrical short. 

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