A nickel-metal hydride cell battery, abbreviated NiMH, is a common type of rechargeable power tool 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 Bosch power tool 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 buildup 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.
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 Metabo power tool batteriy cool.