Alkaline storage batteries having a wide application in various portable appliances include nickel-cadmium storage batteries, and nickel-metal hydride storage batteries. The nickel-cadmium (NiMH) battery, in which nickel oxyhydroxide is used as a positive active material, a hydrogen storage alloy is used as a negative active material and potassium hydroxide aqueous solution is used as an electrolyte, has an operating voltage of about 1.2 V. The NiMH battery having a feature of a high energy density has been mainly put into practice for various consumer appliances. Although newer NiCD batteries offer higher energy densities, the demand for more power and longer operation time is outstripping the pace of its progress. There is also an increasing concern about contamination of the environment during disposal of spent NiCD cells due to the toxic nature of its cadmium content. To solve this problem, the use of nickel-metal hydride storage battery without cadmium has been adopted instead of the nickel-cadmium battery with negative electrode containing cadmium. A nickel-metal hydride (NiMH) storage battery is a secondary battery comprising a positive electrode formed from an active material composed mainly of nickel hydroxide and a negative electrode composed mainly of a hydrogen storage alloy, and has been widely used as a power source of portable equipment. Nickel-metal hydride storage batteries have been attracting attention as rechargeable batteries having a high energy density and a long cycle life. Nickel-metal hydride storage batteries are excellent in their energy density and cycle life as compared with the conventional rechargeable batteries such as nickel-cadmium storage batteries. As an environmentally friendly battery exhibiting high energy density, high output and the like, the nickel-metal hydride storage battery has been widely used for the power sources of cordless equipment such as communications equipment and personal computers, and electronic equipment. The nickel-metal hydride storage battery has also been applied to power tools, electric vehicles and the like, each of which requires large current charge and discharge. Application and inclusion of the nickel-metal hydride storage battery has been widened even to electric vehicle (EV) and hybrid electric vehicle (HEV) and further to electric instruments and emergency lights to which the nickel-cadmium storage battery has conventionally been applied widely. A nickel-metal hydride storage battery is designed to have negative electrode capacity larger than positive electrode capacity for the purpose of suppressing degradation through oxidation of the negative electrode active material and increase of the battery internal pressure in overcharge. Positive electrodes of nickel-metal hydride secondary batteries are roughly classified into sintered type and unsintered type. The former are prepared by sintering a nickel powder to obtain a porous nickel sintered substrate having a porosity of about 80%, impregnating the resulting porous substrate with a solution of a nickel salt such as aqueous nickel nitrate solution and then dipping the substrate in an aqueous alkali solution, thereby to produce a nickel hydroxide active material in the porous nickel sintered substrate. The unsintered type positive electrodes are prepared by filling a three-dimensionally continuing spongy porous substrate comprising nickel metal and having a porosity of 95% or higher with nickel hydroxide as an active material. A sintered nickel electrode using a sintered substance of a metal as a conductive substrate has a disadvantage of a small packing amount of an active material, namely, low energy density, because of low porosity of the sintered substance. Therefore, a nonsintered nickel electrode using a foamed metal with high porosity as a conductive substrate for packing a large amount of active material has recently become noticeable.

A nickel-metal hydride (NiMH) storage battery is obtained by laminating a nickel positive electrode containing nickel hydroxide as the active material and a negative electrode comprising a hydrogen storage alloy containing hydrogen as the active material, with an alkali-proof separator interposed therebetween, and impregnating the laminated body thus obtained with an alkaline electrolyte, followed by sealing. The nickel-metal hydride storage batteries requires a sealing technique for withstanding a high pressure, because the casing of these batteries is originally under high pressure brought by hydrogen which is one of the fundamental reactive substances. Hydrogen absorption alloys, capable of absorbing and releasing hydrogen at and near normal temperatures and pressures, are used as the materials of negative electrodes in nickel-metal hydride rechargeable batteries. Nickel-metal hydride storage batteries practically made using hydrogen-absorbing alloys have the characteristics such as low environmental pollution and high energy density. This alloy must be able to store and give up the hydrogen depending on whether the storage battery is being charged or discharged, at a rate which is adequate for normal operating conditions. Multicell nickel-metal hydride batteries may be packaged in a variety of configurations. Individual cells may simply be secured together with the use of end plates and a strap to form a "bundle" of individual cells. Alternatively, the individual cells may be all be housed within a common outer battery case. Rechargeable battery packs are a packaged assembly of a plurality of interconnected cells that are typically joined together to generate a desired overall voltage potential and current for powering a desired device or system. An extremely wide variety of modern electronic devices are powered by battery packs that are configured as multiple-cell packs with particular voltage and current-handling capabilities tailored to the respective device. A large range of sizes and types of battery packs are produced to power these devices. A general method as a method for activating the nickel-metal hydride secondary battery includes charging and discharging the battery with a constant current or constant voltage. The charging is carried out under such a charging condition that the value of the charging current is determined considering the increases in internal pressure and temperature of the battery in the vicinity of a full charge, and the charge amount is set to exceed the battery capacity to prevent the occurrence of the memory effect.