Understanding Cordless-Tool Batteries
Misused batteries die young. Replacements are costly. Here's how to get the longest life from your batteries.
Synopsis: This article explores why higher-voltage cordless tool batteries have shorter lives than lower-voltage batteries, provides a brief overview of current battery technology, and explains how to optimize the life cycle of your batteries so that they last longer.
Three months ago, I carried a boxful of one-year-old 14.4v batteries into my local tool dealer’s service center. The service technician tested the batteries and confirmed what I knew: Those batteries would no longer take a charge. He couldn’t tell me why they lasted for just a year, which only added to the aggravation of buying six new batteries for $60 each.
The batteries in my old 9.6v tools lasted for years, driving thousands of screws. However, I’ve noticed that 12v, 14.4v and 18v batteries degrade faster. After about one year, they don’t deliver their original run time or torque, regardless of the claimed amp-hour rating. (Amp-hour is the industry’s standard measurement of the electricity that a battery can store and release on demand.) Other carpenters had similar complaints.
To learn what was going on, I spoke with representatives from Makita, DeWalt, Porter-Cable, Bosch, Milwaukee and Hitachi, as well as Sanyo and Panasonic, which manufacture most cordless-tool batteries.
The manufacturers confirmed what I suspected: Higher-voltage batteries have shorter lives. Manufacturers estimate battery life along these lines: 1,000 to 1,300 charges for a 9.6v battery; 800 to 1,000 charges for a 12v battery; 650 to 800 charges for 14.4v battery; and 500 to 800 charges for an 18v battery
Battery life isn’t wholly dependent on voltage ratings, though. How a battery is used also affects its life. What follows is a summary of my conversations with the manufacturers.
Heat is a battery’s worst enemy
My crews use today’s high-voltage cordless tools for harder tasks than the light, occasional duty we expected of 9.6v tools. Now, cordless tools run large spade bits, hole saws, auger bits, 6-in. circular saws and even reciprocating saws. High-voltage cordless tools are often our primary tools for tasks that two years ago would have required a corded tool. Unfortunately, cutting and other high-torque applications result in a fast discharge rate. This is bad. Because of the internal resistance of battery cells, fast discharges create more heat. Heat degrades the material within cells that stores and releases electricity.
All cordless-tool batteries, regardless of total voltage, are packages of individual 1.2v battery cells. There are eight cells in a 9.6v battery, 12 in a 14.4v battery and 15 in an 18v battery. Higher-voltage batteries are more vulnerable to heat than 9.6v batteries because with more cells joined together, higher-voltage batteries have less surface area per cell to disperse heat. So not only do we use higher-voltage batteries in ways that create more heat, but also because of these batteries’ construction, they are less able to dissipate that heat than are 9.6v batteries.
New battery technology isn’t necessarily better
Nickel-cadmium (Ni-Cd) batteries are the standard. They powered all cordless tools until about one year ago, when nickel-metal hydride (Ni-Mh) batteries were introduced as an environmentally friendly alternative. Worn-out Ni-Cd batteries are a hazardous waste and must be recycled. Ni-Mh batteries are still free from that mandate.
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