Nickel discharge characteristics

Figure 8. Ideal discharge characteristic, and discharge characteristic of a nickel/cadmium system.

Figure 20 shows the charge-discharge characteristics of the AA-size nickel-metal hydride battery in comparison with the nickel-cadmium battery produced by Sanyo Electric. Its capacity density is 1.5 to 1.8 higher than that of nickel-cadmium batteries. 

Lithium-nickel oxides form various lithium compounds, lithium hydroxides (LiOH), Li2C03, nickel hydroxide (Ni(OH)2), nickel carbonate (NiC03) and nickel oxide (NiO). Figure 51 shows the discharge characteristics of lithium-nickel oxides synthesized from these compounds. They were heat-treated at 850 °C for 20 h in air. Although the lithium nickel oxides showed a smaller discharge capacity than that of LiCo02, LiOH and Ni(OH)2 were considered to be appropi-ate raw materials. 

Figure 52. Discharge characteristics of some lithium-nickel oxides and LiCoO, (current density 0.25 mA cm2).

Nickel-melal hydride cells can be discharged at the 2 C rate (and in some cases at 4 C) and charged at 1 C. An AA-sized cell with a nominal capacity of over 1 Ah can thus be discharged at over 2 A and with a peak current of over 10 A. The energy density is highly dependent on rate, but for comparable conditions is 25% higher than an equivalent nickel-cadmium cell. Fig. 6.12 shows a comparison of the discharge characteristics of these two systems. 

A major goal of the research on conducting polymers has been the development of a rechargeable plastic battery. Cells based on polypyrrole and lithium electrodes have been developed in which the energy per unit mass and discharge characteristics are comparable to nickel-cadmium cells. Current interest appears to center around stable, processable polymers, such as polythiophene and its derivatives, and polyaniline. 

Size, weight, capacity, and power density are the primary selection considerations for batteries in externally powered prosthetic design applications. The most popular types of rechargeable batteries in use in prosthetics today are nickel-cadmium (NiCd), nickel-metal-hydride (NiMH), and lithium-ion (Li-ion). Li-ion is fast becoming the chemistry of choice because of its high capacity-to-size (weight) ratio and low self-discharge characteristic. 

This sealed-type nickel-cadmium secondary battery is easy to downsize, is excellent in discharging characteristic, and requires no maintenance at all. Therefore, the sealed-type nickel-cadmium secondary battery has spread as the main power supply of portable electrOTiic devices, such as cordless power tools, toys, video cameras, and notebook PCs, and has supported the development thereof. 

Discharge Characteristics. The nickel-iron battery may be discharged at any current rate it will deliver, but the discharge should not be continued beyond the point where the battery nears exhaustion. It is best adapted to low or moderate rates of discharge (1- to 8-h rate). Figure 25.6 shows the discharge curves at different rates of discharge at 25°C. 

FIGURE 26.7 Charge and discharge characteristics of nickel-cadmium batteries at 25°C. (a) Pocket plate battery, high rate, (b) Plastic-bonded plate battery, high rate. 

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