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A lithium ion screwdriver is simply a power screwdriver that is powered by a rechargeable lithium ion battery, sometimes abbreviated as a Li-ion battery. Electricity from the power cell is converted into torque, the force used to rotate interchangeable bits with various screwdriver heads. At a minimum, power screwdrivers come with slotted and Phillips head screwdriver bits, but many other bit heads, such as square or hex are also available.
Originally, rechargeable power tools used nickel cadmium batteries, but the lithium ion power cell represents a technological advance upon the older batteries. The Li-ion battery has almost twice the energy density of an equivalent nickel cadmium battery, meaning the power cell in a lithium ion screwdriver is lighter than a nickel cadmium cell with the same amount of energy. The Li-ion cell also has greater power density, which means it is better at providing bursts of power than the nickel cadmium cell, making it appropriate for the stop and start usage of cordless drills and screwdrivers. Finally, the Li-ion cell is more eco-friendly, as the cadmium in used nickel cadmium batteries is classified as hazardous waste.
Li-ion power cells do have a few disadvantages compared to their nickel cadmium counterparts. A Li-ion cell can be recharged less than half as many times as a properly maintained nickel cadmium battery, even though the Li-ion cell is not subject to the "memory effect" that can reduce battery life if a nickel cadmium battery is not drained fully before recharging. The Li-ion cell is also about 40% more expensive to manufacture than an equivalent nickel cadmium battery. Manufacturers have judged these disadvantages minor compared to the weight savings and battery life of the Li-ion cell, however, and the lithium ion screwdriver has steadily multiplied on hardware store shelves.
Overlap exists between cordless screwdrivers and cordless drills, because they perform similar functions by transforming battery power into rotation of a bit. The lithium ion screwdriver has less power than a cordless drill, ranging as low as 3.6 volts, the baseline for li-ion power cells. Tools between 7.2 volts and 9.6 volts are often specifically categorized as drill / drivers, and tools of 10.8 volts or more are intended as drills.
Other distinctions between cordless drills and cordless screwdrivers are general and prone to exceptions. Drills almost always have a pistol grip, while cordless screwdrivers may have a pistol grip, an inline grip, or in many cases twist between grip style. Some cordless drivers have a chuck that can hold different size bits like a cordless drill, but often a lithium ion screwdriver will instead have a sleeveless chuck that only holds a standard size bit. Battery operated drills almost always have an external battery pack that removes from the tool to charge, but rechargeable screwdrivers may have an internal or external power cell. Cordless screwdrivers are usually cheaper than cordless drills and marketed toward the homeowner instead of the contractor or builder, but many exceptions mark this generality as manufacturers have sought to capture a lot of specific niches within the market for power tools.