A wide variety of batteries are available to power cordless tools. Batteries range from 2.4 volts to 24 volts. Most screwdrivers and impact wrenches use 9.6- or 12-volt batteries. More powerful cells, such as 18-volt batteries, are typically used to power circular saws and other tools used in the construction industry. Rechargeable power tool batteries are available in either "pod" or "stick" styles.

"The biggest development in cordless technology has been high capacity batteries that can handle high currents," says Dave Selby, business unit manager for cordless products at Milwaukee Electric Tool Corp. (Brookfield, WI). "Robust power packs, such as the 2.4-amp-hour nickel-cadmium battery, have enabled larger, higher powered tools to tackle applications unthinkable just a few years ago.

"The key to future demand will be higher capacity batteries that will allow lighter weight tools," adds Selby. "The higher the battery capacity capability per volume, the smaller the battery can be made and still deliver enough energy."

According to the Freedonia Group Inc. (Cleveland), sales of batteries used in portable power tools are projected to increase 5.7 percent annually through 2005. Only a handful of companies manufacture rechargeable batteries, including Moltech Power Systems Inc. (Gainesville, FL) and Saft America Inc. (Valdosta, GA). Most cordless tool manufacturers use proprietary batteries that are made to their unique specifications. As a result, it is often impossible to interchange batteries and power tools from different manufacturers.

All batteries work according to the same basic principle. Two dissimilar materials serving as an anode and a cathode are linked by a third material that serves as the electrolyte. Many different materials can be used as an electrode and an electrolyte, which results in a wide variety of battery technologies. The choice of chemistries also influences the storage density and voltage output.

Traditionally, cordless power tools have used nickel-cadmium batteries. However, newer technology, such as nickel-metal hydride cells, offer users more powerful alternatives and are becoming more popular.

Nickel-cadmium batteries use cathodes made from nickel and anodes made from cadmium. They can withstand up to 1,000 charge and discharge cycles before deteriorating. Because of their reliable ability to provide large amounts of power on demand, nickel-cadmium batteries are still widely used for power tool applications.

Nickel-cadmium battery packs typically contain individual 1.2-volt cells that are soldered together. For instance, a 12-volt battery pack is comprised of 10 1.2-volt cells arranged side by side.

However, some observers believe nickel-cadmium has reached is capacity ceiling. Many end users are demanding longer run times and better performance. Cadmium also poses an environmental hazard. As a result, nickel-cadmium cells are increasingly being replaced by other types of rechargeable batteries.

Many cordless tool manufacturers are converting to nickel-metal hydride, a technology that has similar characteristics to nickel-cadmium, in that it accepts fast and continuous charging. Cells that use hydride cathodes carry over the nickel anodes from nickel-cadmium cell designs. These batteries have an electrolyte of a diluted solution of potassium hydroxide, which is alkaline in nature.

However, nickel-metal hydride batteries feature a much higher capacity limit. "The energy density in a nickel-metal hydride cell is twice that of a nickel-cadmium cell," says Selby.

The typical nickel-metal hydride cell costs more and has half the service life of the typical nickel-cadmium battery, but it also has 30 percent more capacity, increased power density and less memory effect. The battery doesn't have to be fully discharged before being recharged. Increased energy density offers longer service life between charges, while lack of cadmium, lead or mercury makes it environmentally friendly.

Despite increasing applications, not all cordless power tool manufacturers are convinced of the merits of nickel-metal hydride. "We're still not happy with the overall value for the customer when you factor in shorter cycle life and higher cost," says Selby. "Nickel-metal hydride is coming on and is an option to get longer run time if you can live with the 25 to 35 percent higher cost for packs and 40 percent less cycle life."

Other battery technology may soon be available to cordless tool users, such as lithium-ion, lithium-ion polymer, lithium-iron disulfide and zinc-air. Portable fuel cells are another power option that may be coming soon to a cordless tool near you. But, widespread application of these technologies is currently limited due to exorbitant costs.

The typical lithium-ion cell uses carbon for its anode and lithium cobalt dioxide as the cathode. The electrolyte is usually based on a lithium salt in solution.

Lithium-ion cells have very long cycle lives and their storage capacity does not degrade significantly with repeated recharging. Lithium batteries offer higher storage densities than nickel-metal hydride batteries. Lithium-ion cells also lack the memory effect that traditionally plagues nickel-cadmium cells.

Because lithium-ion cells use a liquid electrolyte, cell designs are limited to the traditional cylindrical battery form. "Lithium-ion is great for constant-drain applications, but it can't provide the peaks of power that power tools need," notes Selby, who remains bullish on the proven benefits of nickel-cadmium batteries.

Lithium-ion polymer appears to be the next major breakthrough in rechargeable battery technology, according to a recent study conducted by Frost & Sullivan Inc. (San Antonio, TX). Lithium-ion polymer batteries use a chemistry similar to lithium-ion batteries. However, this technology uses a gel or laminate that transports the ions back and forth between the electrodes. This enables battery manufacturers to package the cells in a foil forming a pouch, which results in a flat surface that can be as thin as 4 millimeters.

The thickness of lithium-ion polymer batteries has been consistently decreasing--some cells have been produced as thin as 2.5 millimeters. But, there is a physical limitation, and as thickness increases, battery manufacturers are forced to increase the dimensions of the battery pack to retain the same capacity and energy density levels. Valence Technology Inc. (Henderson, NV) holds the patent for lithium-ion polymer technology.