5 Things You Need to Know About Nickel-Metal-Hybrid Batteries

What Are Nickel-Metal-Hydride Batteries?

A nickel-metal hydride (NiMH) battery is a type of rechargeable battery that uses a hydrogen-absorbing alloy for the negative electrode rather than cadmium. It can offer A up to three times the energy capacity of an equivalent size nickel-cadmium (NiCd) battery.

How These Batteries Were Developed

Stanford Ovshinsky of Energy Conversion Devices invented the science that resulted in the NiMH battery, while Dr. Masahiko Oshitani from Yuasa Company is credited with developing positive electrodes that appeared in the first consumer NiMH batteries. The company also is credited with creating the first high-energy paste electrode technology. It was the connection between high-energy electrode and high-energy hydride alloys for the negative electrode (found by Philips Laboratories and French CNRS labs) that led to the implementation of today's environmentally friendly NiMH battery.

Nickel-Metal-Hydride Batteries Have Improved

Nickel-metal-hydride batteries are used in all-electric plug-in vehicles like the Toyota RAV4 EV. However, at one time NiMH batteries were considered to have a relatively high self-discharge rate that decreased their shelf life. In 2005, a low self-discharge NiMH battery was introduced that maintained 70% to 85% of its capacity after one year when stored at 68-degrees Fahrenheit.

How Nickel-Metal-Hydride Batteries Impact the Environment

Because they lack the presence of toxic cadmium and because nickel is recycled, NiMH batteries are considered relatively "green." The majority of nickel is recycled and is easy to remove from batteries. However, to some degree the process of mining for the metals that form the negative electrode does have a detrimental impact on the environment.

Caution During Discharging

Because cells are never exactly the same and one will discharge before another, it's important to make sure cells in a series of connected battery packs do not discharge entirely. Otherwise, they can reverse their polarity. Reverse charging can occur when one battery completely discharges before the others, causing the battery to go below zero voltage.

Jennifer Olvera, a graduate of DePaul University in Chicago, has been writing professionally for over a decade. In addition to covering the "green scene" for publications such as Where Magazine and Crain's, she has become one of the preeminent dining, food, and entertainment writers in Chicago. She has regularly been published in Chicago Magazine, Chicago Sun-Times, Chicago Tribune, Zagat, Citysearch.com, and Gayot.com.