As their performance increases and comes ever closer to rivaling that of their corded counterparts, cordless power tools are gaining popularity among electricians. Picking one type over the other involves a trade-off. Battery-operated tools cannot offer complete replacement of corded tools for heavy-duty work or for work of constant and long duration. However, what you may lose in possible sheer muscle power and run time, you may gain in mobility and convenience.

“For some tasks, corded tools are better suited to the job (such as when you are working for a long time in one place). But at other times, particularly when the task site is remote from the power source or there is a punch list of items at multiple locations in a series of rooms, battery power wins hands down,” Todd Langston, Porter Cable public relations manager, said.

Battery technology over the last few years has advanced far enough that cordless tools such as hammers, hammer drills, reciprocating saws, screwdrivers, and drills are worth considering for many electricians’ tool chests. How well and how long cordless tools work between charges depends on the size, voltage, and amp hours (1.2, 2.4, or 3) of a battery, as well as the ambient temperature at which they are used and stored (too hot or too cold conditions can destroy a battery, even one not currently in use). Also, consider the size and weight of the battery pack. (A full sub-C cell battery pack or a 4/5 sub-C cell battery pack is made up of individual 1.2-volt cells connected in a daisy chain series. A 2.4-volt battery has two cells; a 7.2-volt battery has six cells, and so on.) Battery packs that offer greater capacity for power and run time contain more cells that add to the weight of the tool, which could add to job fatigue and, possibly, muscle soreness by the end of a long day.

To minimize weight, select a tool with just enough power and torque to handle the anticipated application; typical voltages for cordless tools are 9.6, 12, 14.4, 18, and 24 volts; 36-volt batteries are also available from at least one manufacturer. A worker screwing outlet boxes onto studs can get enough production from a tool sporting a 12-volt battery, while a worker drilling numerous large holes, requiring a lot of torque, would benefit from a cordless tool with a higher-voltage battery pack.

Because rechargeable batteries can’t survive an infinite number of charges, consider not only the greater cost of a cordless tool compared to a corded tool, but also the periodic replacement cost of the rechargeable batteries. (Remember, depending upon anticipated frequency of use, you might need to buy at least two batteries for any tool: one for action, and one for the charger.) While rechargeable batteries have, traditionally, been nickel cadmium (Ni-Cad), battery choices also include NiMH (nickel metal hydride) batteries.

Regardless of the chemical composition of the battery, the higher the amp hours, the more energy can be stored and the longer the run time between charges. Currently, NiMH batteries are rated at up to 3 amp hours, regardless of the volts, while Ni-Cad batteries are rated up to 2.4 amp hours, regardless of the volts. The most powerful batteries on the market are NiCad batteries rated at 36 volts carrying a 2.4 amp hour rating (these are sold by Hilti).

Which is better to buy for cordless tools—nickel cadmium or nickel metal hydride—is debatable.

“Compared to Ni-Cad batteries, the relatively new NiMH batteries, which offer longer run times per charge, currently cost about 30 to 50 percent more but can survive only about half as many recharging cycles as Ni-Cad batteries (300 to 400 cycles, compared to 600 to 800 cycles),” Dave Selby, Milwaukee Electric Tool business unit manager for cordless products, said. “Furthermore, they are more temperature sensitive and will stop functioning at about the freezing point.

“Ni-Cad batteries offer the best combination of performance as far as capacity, life, and cost, right now, with the best Ni-Cad batteries rated for two years of constant use,” he observed, “but the inherent technology of the NiMH batteries allows them to have the potential for a higher capacity than Ni-Cad batteries, at some point in the future. Ni-Cad technology is near the end of its evolution, while NiMH technology has a way to go, leaving open opportunities for improved performance in the future.”

While some manufacturers, including Porter Cable and DEWALT, concur with Milwaukee and continue to sell Ni-Cad batteries (only) for now, others, while continuing to offer Ni-Cad batteries, are starting to switch over to selling their cordless tools, initially, with NiMH batteries. NiMH batteries, not incidentally, when dead, need not be recycled as Ni-Cad batteries do.

Makita, for example, is focusing all its battery efforts on NiMH batteries. “Ni-Cad battery technology is pretty much tapped out and can’t go much higher than 2.4 amp hours,” Brent Withey, Makita accessory marketing manager, said. “One of the features of the Ni-MH battery is that it has a high starting point already—2.2 A-H.” Using a 2.6 A-H NiMH battery, Makita is marketing cordless tools, such as their 71/2-inch cordless miter saw, that could not run that long on the power of a Ni-Cad battery. Withey said Makita testing indicates the average run time of a NiMH battery is actually 40 percent longer than a Ni-Cad battery, can survive nearly as many recycles as the Ni-Cad batteries, will operate at the freezing temperature, and will cost only 15 to 20 percent more. With NiMH, the recycle time is slightly less than Ni-Cad; however, NiMH provides greater run time, so the total amount of recharging is much less than Ni-Cad.

Makita’s NiMH batteries are, Withey pointed out, interchangeable with Ni-Cad batteries in its current line of power tools. Makita and other manufacturers not yet selling NiMH batteries, now offer chargers that can handle both types of batteries. Milwaukee, for example, recently introduced a charger equipped with a computer chip that recognizes the type of battery plugged in and uses different charging parameters for each. Porter Cable also offers a dual charger.

Hilti suggests that, because the cells within a battery pack are actually manufactured within a tolerance range instead of with theoretical exactness, running a battery pack to the end of its discharge cycle too often can damage some of the cells inside the battery pack and diminish its overall performance.
“The ‘weaker’ cells try to give out as much current as the surrounding stronger cells, which causes the ‘weaker’ cells to heat up more than the other cells, causing a detrimental effect on the cells,” Tim Mautino, Hilti senior product manager of drilling and demolition, explained. “If this cycle is continued, it can have an undesirable impact on the battery pack’s performance.”

Hilti’s chargers, he pointed out, have a built-in balancing feature that safely draws down all the cells of the battery before completely recharging each cell through a slow charge. (This feature, he advised, should be not be used more than once a month.)

Some manufacturers, including Milwaukee, believe that batteries can stay on the charger indefinitely without harm, while others, including Hilti, suggest removing the battery once the charge is complete to prevent the cells from developing unbalanced charges that reduce performance. For best performance for your system, follow instructions that come with it.

Nowadays, there is little risk of overheating a battery during the charging process. Most modern chargers use micro-computer chips to control the rate of charge, and therefore the heat buildup, so that the charge terminates before the battery has a chance to overheat and be damaged.

Makita’s chargers monitor the heat in the battery and regulate the rate of charge, depending on the level of heat given off from the charging battery. Their new DC1439 model is a fast charger as well as an optimum charger; through the built-in CPU unit, the self-adjusting charger detects the most accurate rate of charging the battery by measuring its temperature. It will charge a 1.3 A-H, 9.6-volt battery in 10 minutes, and a 2.6 A-H battery of any voltage in 25 minutes, according to the company.

DeWALT’s chargers offer a three-stage charge that handles both Ni-Cad and NiMH batteries: one-hour fast charging, providing maximum charge of the battery within an hour; one- to three-hour equalization, whereby each cell is fully balanced to maximize battery pack performance and life; and a maintenance charge, whereby a trickle charge maintains the full charge of the battery pack when left on the charger after the equalization stage. The proprietary technology provides cordless tool users with optimized battery life and run time, Mike Wohltmann, DeWALT cordless products product manager, explained. The charger includes a feature that detects the temperature of the battery so the charger won’t charge when the temperature of the battery is outside of the operating range, protecting and helping extend the life of the battery, he added.

Porter Cable has a fully diagnostic charger that senses the temperature of the battery pack and charges the battery only when it is in the optimum recommended temperature range. It uses a pulse technology on a quick charge so as to minimize the heat build-up in the cells. Once the battery is fully charged, it charges continuously at a reduced level and spikes for 15 minutes every hour to maintain the fully charged state.

“The battery can stay on the charger indefinitely,” Mike Whitman, Porter Cable product development manager, said, “and if the battery temperature in not in the optimum range, the battery will not charge, helping protect the investment in batteries.”

Some manufacturers, including DeWALT, Milwaukee, and Makita, also offer an automotive charger that draws power from a vehicle’s cigarette lighter.

The FELDMANs provide Web content for companies and write for magazines, trade associations, building product manufacturers, and other companies on a broad range of topics. They can be reached at wfeldman@att.net or (914) 238-6272.