NiMH

NiMH stands for nickel-metal hydride, a type of rechargeable battery. In a NiMH cell, the positive electrode is nickel oxyhydroxide (NiOOH) and the negative electrode is a hydrogen-absorbing metal alloy. The electrolyte is typically an aqueous potassium hydroxide solution, and the nominal cell voltage is about 1.2 volts. Cells are commonly assembled into sealed cylindrical or prismatic packs for use in devices.

NiMH was developed as an alternative to nickel cadmium (NiCd) batteries to reduce cadmium toxicity and memory effects. It entered widespread commercial use in the 1990s and became the standard for many consumer electronics, power tools, and early hybrids. Improvements in low self-discharge variants helped extend shelf life and practical usability.

In terms of performance, NiMH offers higher energy density than NiCd but generally lower energy density than modern lithium-ion chemistries. They have moderate self-discharge, which is higher than Li-ion but can be reduced with low self-discharge (LSD) versions. NiMH batteries typically provide several hundred to around a thousand charge–discharge cycles, with efficiency and longevity strongly influenced by temperature and charging practices. They are relatively tolerant of overcharging when managed by proper chargers, but deep discharge or overheating can shorten life or cause venting.

Applications include portable electronics, cordless power tools, and various backup or energy-storage roles. In hybrid vehicles, NiMH has been widely used, especially in older models, though lithium-ion chemistries are increasingly common in newer designs. End-of-life NiMH batteries are recyclable, with recovery of nickel and other materials.

Charging and maintenance require appropriate charging equipment, moderate temperatures, and avoidance of deep discharge when possible. Proper disposal and recycling are important due to the nickel content.