Building a Better Power-Tool Battery
By Renee Gryzkewicz - June 2005 - Equipment Rental & Tools
In the past 20 years, power tool manufacturers have come a long way in making cordless tools lighter, smaller, more powerful and more durable. Much of their focus has been on the batteries that power these tools, experimenting with new technologies and fine-tuning current ones.
By understanding the advantages and disadvantages of each kind of battery, maintenance and engineering managers can more effectively specify tools that best fit their departments’ needs.
Nickel cadmium (NiCad), nickel metal hydride (NiMH) and lithium ion (Li-Ion) are the three main types of batteries for cordless power tools today. Each one has unique characteristics.
NiCad batteries — currently the most commonly used type of battery — have been around the longest, are the least expensive, and are known for their high durability. Many manufacturers, however, believe newer battery chemistries offer greater potential in improving user productivity.
“NiCad has pretty much reached its peak, and now we’re seeing more and more innovations in battery technology,” says Kevin Fairchild, Hitachi Power Tools’ senior product manager for electric power tools.
Managers should consider a battery’s service life when specifying tools. To determine its service life, users need to look at its run time and life cycle. Run time is the amount of work a tool can perform from one charge. Life cycle is the number of times a user can recharge a battery. As the battery reaches the end of its service life, users often notice a decline in run time.
“End users often say one of their biggest frustrations is needing to buy new battery packs too often,” says Jeff Wilkison, director of product marketing for cordless tools at Bosch Power Tools. This can be particularly true for NiMH batteries, which generally have less life cycles than NiCad batteries.
“One of the disadvantages of nickel metal hydride is that they accept slightly fewer charges than a NiCad battery,” Wilkison says.
Milwaukee Electric Tool’s V28 line of power tools featuring Li-Ion batteries, unveiled last January, provides users with more run time than NiCad or NiMH batteries, Sara says.
“Individuals currently using a 18-volt NiCad battery, should see 2 - 2-1/2 times more work output from a V28,” says John Sara, cordless product manager for Milwaukee Electric Tool.
A battery’s amp-hour rating also helps end users determine its service life, Hi-tachi’s Fairchild says.
“People often just look at look volts when they should really concentrate on the battery’s amp-hour rating,” he says. While the volts relate to the tool’s power, they don’t indicate how long the battery will last. The higher the amp-hour rating, the longer the tool will run, he says.
Eighteen-volt batteries often have different amp hours, Fairchild says.
“In fact, it is very hard to find two 18-volt drills that have the same amp-hour rating,” he says. “The batteries will be the same size and look similar, but users will get more run time from a three-amp-hour battery than a two-amp-hour battery.”
Shrinking in Size
Along with longer run times and service lives, tool users continue to seek smaller, lighter-weight cordless tools that are easier to maneuver and carry. To meet this demand, tool manufacturers are focusing on ways to reduce the size and weight of the batteries that power these tools.
The size of a battery’s cells determines the size of the battery pack. The cells in a NiCad battery are larger than those in a NiMH battery, so an 18-volt NiCad battery is bigger and weighs more than an 18-volt NiMH battery.
“People are okay with the weight of an 18-volt NiCad battery, but anything more than that is really fatiguing,” Sara says, adding that 18-volt NiCad battery packs weigh about 2.4 pounds. Because 24-volt NiCad battery packs weigh more — about 3.3 pounds — they haven’t made a significant impact on the tool market.
The V28 pack weighs 2.3 pounds, Sara says. Thanks to Li-Ion’s chemistry, this line of power tools provides users with additional run time and power without the weight increase, he says.
One characteristic of NiCad batteries that makes them popular is their ability to perform in cold temperatures. According to Sara, NiCad batteries begin to lose their run time at temperatures below 20 degrees.
Of the three battery chemistries, NiMH performs the worst in cold temperatures according to some manufacturers.
“In our testing of NiMH batteries, we found that the cold-weather performance isn’t there,” Sara says. “They lose their run time in freezing temperatures. Because professionals are often working outside or the tools are stored in a cold truck, this is an unacceptable performance trade off.”
Sara says Li-Ion batteries have the best performance in cold weather, maintaining most of their run time down to about 0 degrees.
Although cold temperatures can hamper a battery’s performance, excessive heat can irreversibly damage a battery. While manufacturers discourage users to store tools in hot environments, heat-related problems are more likely to occur while the tool is in use.
“Heat destroys the cells,” Wilkison says. “One of the most important things that power tool companies need to do is understand how to manage the heat that the battery cells generate.
“All of the cells are linked together, so a battery is only as good as its weakest cell. If one cell gets too deeply discharged versus the others, cell damage can occur.”
Says Fairchild, “The less heat the battery produces, the longer it is going to last. That is why many cordless tools feature heavy-duty fans to cool the motor and armature.
“Because nickel metal hydride batteries produce so much heat, we install fans in the chargers so they don’t overheat,” he says. “As they are charging and heating up, they’re actually cooling off.”
One unique feature of Li-Ion battery packs is their protection circuits. These circuits extend battery life by maintaining cell balance when in use and recharging, providing added battery pack life, Sara says. “The electronics make sure all the cells work together as a team and that one isn’t pushed harder than the others.”
The electronics also ensure that the cells receive the same amount of power when recharging.
“The reason lithium-ion batteries have a protection circuit is actually the downfall of lithium-ion,” Wilkison says. NiCad and NiMH power tools do not require a computer circuit to manage the heat. With these types of batteries, design engineers can control heat in other ways, such as through tool design.
“The purpose of the electronic circuitry is to limit the amount of power that goes from the battery to the motor,” he says. “It’s not an added feature but a requirement to be able to use those tools. It protects users from severely damaging the battery and making it inoperable in just one or two uses.”
If tool manufacturers could produce a lithium-ion cordless tool without the protection circuit, they would, he says, because adding it increases manufacturing costs.
According to some tool manufacturers, NiCad batteries suffer from memory effect. This happens when a battery is continually only partially discharged before recharging. The battery “forgets” that it has the capacity to further discharge all the way down.
About 95 percent of NiCad batteries have memory effect problems, Fairchild says.
Other manufacturers disagree, saying that memory effect is no longer an issue regarding power tool batteries.
“One of the biggest myths out there is that everyone thinks that batteries have a memory effect stolen,” Sara says. “This is no longer the case. There is no memory effect for professional grade power tool batteries.” He adds that recharging partially discharged Milwaukee Electric Tool batteries will not damage their capacity.
The professional grade tools that use NiCad batteries make up a small portion of the cordless power tool market, Fairchild says. “About 15 percent of the market is upper-end tools and about half of those tools that use NiCad batteries don’t suffer from memory effect,” he says.
Because of memory effect concerns, end users might try to fully discharge a battery on purpose before recharging it. Wilkison says users should never deeply discharge a battery on purpose, because doing so can severely damage the pack.
“Users should recharge the battery whenever there isn't enough power to do the job,” Sara says. To ensure a full charge for every job, end users should charge batteries after every use, he says. This practice does not damage the battery, no matter its chemistry.
Manufacturers are looking at several different battery technologies, including some that currently power other electronics, such as computers.
Meeting the power demands of cordless tools is one of the biggest challenges for tool engineers and manufacturers. Unlike batteries in a computer, batteries in power tools are subjected to abusive conditions and require high current draws.
“In power tools, there are valleys and huge peaks of power, and users need a lot of power to complete some applications,” Wilkison says. As new battery technologies become more robust and less sensitive, they will start to migrate to power tools.
While new technologies will emerge and set new standards in cordless power tools, manufacturers expect NiCad batteries to stay on the market.
“We will continue to keep NiCad batteries in our product line for many years to come,” Sara says. In specifying power tools, price also remains a top consideration for managers. Compared to newer technologies, NiCad is very economical.
“Not everyone is going to want to spend the money for new technology,” Fairchild says. “Power tools can be a large investment, and I don’t see NiCad batteries dying anytime soon. However, I think eventually people will adopt new technology.”
Just as tool users today view the cordless power tools of 20 years ago as antiquated and rudimentary, they might feel the same way about today’s cutting-edge cordless tools 20 years from now.
While some batteries are more environmentally friendly than others, manufacturers encourage users to recycle all power tool batteries.
— Renee L. Gryzkewicz