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Discharge Profile
The voltage of a fully charged NiMH battery actually starts at approximately 1.5 volts, but drops sharply to around 1.2 volts, where it remains until a second sharp drop in voltage as the battery reaches the end of its useful storage capacity. During discharge, a NiMH battery must overcome its own internal impedance against the flow of electrical current, known as ohmic impedance. Ohmic polarization produces a voltage drop, which is subtracted from the overall voltage of the battery.
Positive Electrode
The positive electrode, or anode, of a NiMH battery is composed of nickel hydroxide. During charging, the positive releases hydrogen into the electrolyte -- a solution that conducts electricity -- and the hydrogen is absorbed by the negative electrode. The nickel hydroxide in the positive electrode reacts with hydroxide ions in the electrolyte, to produce nickel oxyhydroxide, water and one free negatively charged particle called an electron.
Negative Electrode
The negative electrode, or cathode, of a NiMH battery consists of a metal hydride structure, capable of holding between 1 percent and 7 percent hydrogen by weight. The metal alloy in the negative electrode reacts with water and an electron to produce metal hydride and a hydroxide ion. During discharge, the chemical reactions are the reverse of what occurs during charging. The overall reaction in a NiMH battery can be represented by the equation M + Ni(OH)2 ͛4; MH + NiOOH.H2O.
Electrolyte
The electrolyte in a NiMH battery completes the electrical circuit inside the battery by acting as a medium for the transport of charged atoms, known as ions, from one electrode to the other. The electrolyte does not, however, enter into the chemical reaction at either electrode, so its electrical conductivity remains high throughout the useful capacity of the battery. The electrolyte in NiMH batteries is potassium hydroxide -- an alkali, or base -- which is why NiMH batteries are sometimes referred to as alkaline storage batteries.