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Self Discharge
The rate at which a battery loses charge when not connected to a device is known as its self-discharge rate. NiMH batteries have a much higher self-discharge rate than other competing battery technologies, typically between 2 percent and 3 percent per day. What this means, in practical terms, is that any NiMH battery stored for more than a month or so needs to be recharged before use.
Environmental Factors
Moisture in the air and the housing of an NiMH battery provide a path for negatively charged particles (electrons) to travel to the negative electrode (cathode) causing it to become chemically reduced. When the cathode is fully reduced, the chemical reactions in the battery cease and the battery is discharged. Recharging a NiMH hydride battery involves applying an external voltage across its electrodes to reverse the chemical process and regenerate the cathode. Cold temperatures exacerbate the self-discharge of NiMH batteries, causing them to perform very poorly, so some systems that use NiMH batteries employ climate control systems to limit self-discharge.
Cycle Life
The cycle life of a battery is the number of times the battery can be charged and discharged under normal conditions before it can no longer hold a useful amount of charge. Rechargeable batteries typically have a cycle life of between 100 and 1,000 recharge cycles, depending upon their composition. NiMH batteries typically have a cycle life of between 300 and 400 cycles, but overcharging the battery can reduce this number.
Functional Properties
The functional properties of a good NiMH battery are high capacity, rapid charging and discharging and long lifetime. Although largely superseded by lithium ion (Li-Ion) and lithium polymer (Li-Po) rechargeable batteries -- which not only have a much higher energy density, but also a much lower self-discharge rate -- NiMH batteries nevertheless provide a high-energy alternative to older nickel-cadmium (NiCd) battery technology. NiMH batteries typically last 40 percent longer than NiCd batteries of the same size, making them useful for portable devices such as laptop computers, cellular phones and camcorders.