Background

• NiMH or Nickel-Metal Hydride batteries are a type of rechargeable battery. They are used for a variety of purposes ranging from digital cameras to hybrid vehicles and industrial applications. The popularity of these batteries has grown in recent years as concerns over the environmental impact of toxic components and interest in recycling have increased. Most NiMH batteries can be recharged several hundred times over the course of its life. They are also subject to very little decline in the overall power output provided after they are charged.
  
  

Batteries

• Batteries are self contained units that use chemical reactions to produce electricity. Most batteries have a four-component design. There is a positive electrode, negative electrode, an electrolyte and a separator.
  In a NiMH battery, the positive electrode is typically composed of nickel hydroxide, hence the name Nickel-Metal Hydride. The negative electrode is typically made of a metal hydride material, but the metals used vary. The electrolyte is a chemical solution, often potassium hydroxide, which allows for ion transfer between the positive and negative electrode. The separator is used to physically separate the positive and negative electrodes, but to allow ion transfers.
  
  

Electricity Production

• To produce electricity, the battery must be connected to an external circuit. Placing a battery into a device with the battery aligned to the positive and negative symbols is the standard way of connecting a battery to a circuit. Once the device begins to draw power, electrons move out of the negative electrode, which is accompanied by oxidation of the electrode and the release of hydrogen.
  The electrons will eventually be deposited in the positive electrode which will absorb hydrogen. Ion exchanges through the electrolyte complete the circuit. For standard batteries, when the negative electrode can no longer oxidize and produce electrons, the battery is dead. For NiMH batteries, the particular metals and chemicals involved allows the process to be reversed.
  In essence, a charger can pull electrons out of the positive electrode, causing it to oxidize and expel hydrogen. It can then put the electrons back into the negative electrode, causing it to absorb hydrogen. This allows the battery to be used over and over again.