Introduction

As electronic products become more sophisticated functions, more compact sizes and lighter weights, the sources of power that operate these products have been required to deliver increasingly higher levels of energy. To meet this requirement, nickel-metal hydride batteries have been developed and manufactured with nickel hydroxide for the positive electrode and hydrogen-absorbing alloys, capable of absorbing and releasing hydrogen at high-density levels, for the negative electrode.

Because Ni-MH batteries have about twice the energy density of NiCd batteries and a similar operating voltage as that of Ni-Cd batteries, they have become a mainstay in the next generation
of rechargeable batteries.

Construction
Nickel-metal hydride batteries consist of a positive plate containing nickel hydroxide as its principal active material, a negative plate mainly composed of hydrogen-absorbing alloys, a separator made of fine fibers, an alkaline electrolyte, a metal case and a sealing plate provided with a self-resealing safety vent.

Their basic structure is identical to that of NiCd batteries. With cylindrical nickel-metal hydride batteries, the positive and negative plates are separated by the separator, wound into a coil, inserted into the case, and sealed by the sealing plate through an electrically insulated gasket.

Features

• No memory effect - NiMH's do not develop a 'memory' like NiCad rechargeable batteries. Memory effect is when a rechargeable battery is recharged before its is fully discharged. Over time, the battery will require charging much sooner than before. An example of this can be seen with NiCD equipped cordless phones. If you always put it back in the cradle after a short time of use, it will not last for long periods of time even though it seems to be fully charged!
• Rechargeable 500-1,000 times - Alkaline batteries can not be recharged but NiMH's can be recharged many, many times.
• Cost effective - NiMH's are inexpensive with a street price of around € 2.50 for a AAA cell. This becomes very inexpensive when you calculate in how many times you can recharge the battery.
• Available in standard and special sizes - NiMH batteries became popular with laptops and digital cameras. But they are available in other sizes than the standard cells (AA, AAA, C). For industrial applications many different shapes and forms are available.