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What Are the Dangers of Overcharging NiMH Batteries?

Updated February 21, 2017

A nickel-metal hydride cell battery, abbreviated NiMH, is a common type of rechargeable battery found in consumer electronics. NiMH batteries have two to three times the charge capacity of nickel-cadmium batteries (or NiCd) and are inexpensive compared to lithium-ion cell batteries, making them a popular and cost-effective choice for consumer products that require a rechargeable battery. However, care should be taken when charging them, as overcharging can damage the battery cells or create potential hazards.

Depleted Battery Capacity

After a NiMH battery cell begins to overcharge, the increased energy feeding into the battery will reverse the voltage polarity of the electrodes inside the battery and cause a slight decrease in the battery's voltage. Over time, this effect of overcharging can become more and more noticeable until the battery no longer has enough voltage to power a device.

Possible Explosion or Cell Rupture

Overcharging older NiMH batteries can also cause hydrogen to build up inside the cells. When that happens, increased pressure can cause the battery cells to rupture, rendering the battery unusable. In some cases, the build-up of hydrogen can also pose the risk of explosion. Newer NiMH batteries are designed with vents in the cells and will emit small amounts of hydrogen to prevent this from happening.

Excessive Heat

Overcharged NiMH batteries can heat up very quickly, rising to temperatures that could damage either the battery or the electronics in the charging unit. To counteract this danger, some quick chargers are designed with a fan to keep the batteries cool.

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About the Author

Rob Callahan lives in Minneapolis, where he covers style, culture and the arts for Vita.MN and "l'├ętoile Magazine." His work has earned awards in the fields of journalism, social media and the arts. Callahan graduated from Saint Cloud State University in 2001 with a Bachelor's degree in philosophy.