Cordless tools have been a blessing and a curse for Doug Winston. The North Chicago, Ill.-based electrician nearly lost a thumb when the 14.4V reciprocating saw he was using to dismantle some storm windows in a residential demo project slipped, but these days his power tools are actually protecting his hands. Pre-carpal tunnel has made it difficult — and potentially damaging — for him to manually drive screws, so he has come to rely on his cordless drill to finish the job. “Since being diagnosed, I have to be careful about making rotations [with my wrist],” he says. “So now when I'm trimming out receptacles and fixtures, I prefer to use a power drill. And since I'm up and down and moving around a lot, I don't want to have to plug and unplug a corded drill and possibly trip over it, so I use cordless.”

Minimizing wrist fatigue isn't necessarily one of the most common reasons most electricians use cordless tools, but increasing mobility and reducing hassles are. And it was with mixed emotions that the construction industry received battery-powered tools when they first became available in the early '80s. The idea of going cordless was an attractive one, but many doubted it would ever be possible to generate enough power to make them useful in heavy construction. As batteries leapfrogged from 6V to 12V to 24V, though, users discovered there was little these devices couldn't do.

It should come as no surprise that battery and charger technology is at the heart of the cordless revolution. Understanding batteries and chargers can help you select the best tool for your own type of work and protect your cordless tool investment.

Making sense of volts. The most noticeable advance in cordless tools is the ever-increasing voltage rating. The higher the voltage, the more power available at a given moment to perform the task at hand. Higher voltage equals more energy to the motor, which means the ability to drive bigger bits and blades.

Despite what you may hear, bigger isn't always better when it comes to voltage. Jason Goger, product specialist at DeWalt Industrial Tool Co., points out that smaller, lighter packs can sometimes be more comfortable to work with and are easier to use in tight spaces — like inside cabinets — where bigger tools can't fit.

After voltage, a specification to consider is the amp-hour rating, which is a general number that categorizes how much operating capacity a battery has when fully charged. All other factors being equal, a higher amp-hour rating — they range from 1.2 to as high as 3.5 — generally means a cordless tool can run longer or do more work per charge, but it doesn't make the tool more powerful.

Tom Walsh, owner of Chicago-based Active Electric, uses his 18V, 2.4Ah cordless saws and drills about 2 hours a day and can go several days without charging his batteries. Heavier usage requires more frequent charges, though. “If we're not using it all day, I probably only have to charge my batteries once or twice a week,” he says. “But if I use it all day, I'd probably charge it once a day.”

But even within the same voltage and amp-hour ratings, Goger adds that actual performance can vary greatly between tools. “Consider what's inside the tool,” he says, referring to the motor, transmission, and other components that share the work. “A significant percentage of a tool's potential can be lost within the inside mechanisms,” he says. “Details such as high-quality magnets in the motor, metal gears, and carbide chucks ensure that power is transferred efficiently from the battery to the bit or blade. What's inside the battery pack is also important. Compared to professional batteries, the cells within consumer-grade batteries may degrade after as few as 20 recharges. Without actually dismantling the battery, the only way to ensure you're getting the power you paid for is to stick with professional quality tools.

NiCd vs. NiMH. Until recently, nickel-cadmium (NiCd) batteries powered all cordless tools. But some manufacturers have started experimenting with nickel metal hydride batteries (NiMH) as another potential power source. In terms of amp-hours, or run-time, NiMH ratings have a slight advantage, going up to 3Ah. The highest amp-hour rating for NiCds is 2.4Ah, but the potential is there to go higher.

Unfortunately, NiMH batteries currently have disadvantages that stand out on the jobsite. For starters, NiMHs are more temperature sensitive; the batteries may not work in temperatures below 32°F and begin to degrade quickly when exposed to temperatures above 105°F. Comparatively, NiCds can be used in higher temperatures without compromising cell life. Comparison tests also show that NiMH batteries last for only about half the number of recharge cycles as NiCd cells, which means NiMH cordless tool users could end up purchasing replacement batteries more often.

NiMH technology is advancing and may be more viable in the future, but many tool manufacturers continue to use NiCds because they offer the things that matter most to cordless tool users — better performance and longer pack life — at a lower cost than NiMHs. Environmentally conscious buyers may point out that NiMHs are less toxic, but battery recycling boxes at home centers and electronic stores now provide an easy and safe way to dispose of used batteries.

Charge it. Not all chargers are created equal. “Basic” chargers provide a continuous charge for a given period of time regardless of the battery's status. This means that once a battery is fully charged, the charger keeps pouring energy into the pack. This unneeded energy translates into unwanted heat, which reduces the overall number of recharges a battery can take. In rare instances, it can also lead to a meltdown — literally. Winston left a battery in his charger and forgot about it for several days, only to come back and find that it had overheated and melted. “I went to pull it out and realized that it had basically become a part of the charger,” he says. “I was able to separate them eventually, but the charger — and the battery — were done.”

Other basic chargers may have a thermal switch that shuts off the charger at a given temperature to prevent overheating. But recharging a warm battery will fool the charger into shutting off prematurely. In this case, the charger will indicate that a battery is charged even when it's not. The latest generation of “smart” chargers have a built-in microprocessor that diagnoses the battery then delivers the optimum charge.

Smart charging takes place in stages. First, the charger waits for the battery to cool before starting the charging cycle. With some chargers, a fan cools the pack and dissipates heat so the battery can charge faster. Next, the charger performs a “fast charge.” Because the cells may charge at different rates, this stage shuts off to minimize heat build-up as the fastest charging cells reach near-maximum capacity. In the “equalization stage” the charger balances, or tops off, all the cells using a slower charge designed to add power without bringing heat. This process is like filling an ice cube tray when you gradually turn down the faucet as water spreads into each cube opening. Lastly, a “trickle charge” replaces power ordinarily lost during periods of non-use when the battery is stored in the charger.

Debunking battery myths. Even though cordless tools have been around for nearly two decades, misinformation about battery use and capabilities is still rampant throughout the construction industry.

Myth 1. Batteries have a memory effect. For some time, tradespeople have passed along the myth that battery packs must be fully drained before recharging. Goger says that “memory” myth is based on cordless tools in a previous era. “Back in the '80s, it was possible to inadvertently limit battery capacity by methodically using a tool and draining the battery the same way over and over again,” he says. “Battery memory isn't an issue today because technology has improved and tools are used for a greater variety of tasks.”

Back then the cure was to clamp or tape the tool's trigger and completely drain the battery before recharging. In reality, taping a trigger to use up every last electron does more harm than good. “When the battery drains, the weaker cells empty first,” Goger says. “The stronger cells continue to operate, but the drained cells could actually reverse polarity.”

Reversed polarity cells no longer accept a charge. This means a freshly charged 12-cell pack will only have 11 cells working, meaning shorter run time, less power, and faster degradation of the remaining cells.

The best advice for cordless tool users is simply to recharge a battery as soon as they observe a drop in performance, and to keep the battery in the charger until it's needed. Winston is more likely to run his batteries until they're completely discharged, but not necessarily because he's found that it yields better battery performance. “To be honest with you, I have a tendency to let them run all the way down, and that's just because I'm stubborn,” he says. “If it will still sort of drive the screw and I have to manually tighten it up a bit, that's OK.”

Myth 2. Keep batteries in the freezer. In truth, storing those extra batteries for your camera or flashlight in the cold may help keep them fresher than in a drawer. The problem is that some contractors have taken things a step further by bringing a cooler to the jobsite to store spare battery packs for their cordless tools. Goger says keeping cordless battery packs cooler than 50°F is unnecessary and may be harmful. “Chilling the battery can fool the charger into thinking all the cells are cool when the inner cells may still be warm,” he says. “The added risk of getting water in the battery pack far outweighs any marginal benefit.”

While they should be protected from extreme heat, NiCd batteries don't need to be babied. They can work in the same temperatures you can. So when it gets too hot for you to work, just remember to bring your batteries in with you.

Myth 3. Batteries can be rebuilt. Some repair and electronic shops claim they can rebuild battery packs so they'll perform as good as new. The problem is you may not get what you thought you were paying for. “It's impossible to be certain that your local electronic shop is rebuilding the pack to the same specs as the original,” Goger says. For example, not all cells are created equal. “A rebuilt battery may perform good as new at first, but there's no way of knowing how well those cells will accept a charge after a few dozen cycles.”

Some estimates put battery life at about three years per pack, and at that rate, an $80 pack costs less than 40 cents per day. However, Walsh bought his cordless tools two years ago and has already gone through a couple batteries. He's conscientious about caring for them, yet he's discovered they last about a year-and-a-half. Although he's happy with his purchases so far, he's considering a new approach to battery upkeep in the future. “When batteries die, sometimes it's not that much more expensive to just buy another tool,” he says. “The batteries are maybe $85, so you buy a couple of those and it's $170. It might be $230 for another drill, another charger and a couple batteries.”

Winston has had better luck. Six years after buying his reciprocating saw, the original batteries are still working, but he can't quite pinpoint the secret to keeping them alive as long as he has. “Some guys like to drain them all the way down and keep records,” he says. “I just throw them in the charger.”

Hurst-Wajszczuk is a contributing writer based in Denver.