Lithium-ion impact wrenches

Agriculture.com Staff 03/21/2007 @ 11:30am

Few companies can claim they revolutionized an industry. Milwaukee Electric is one of those few. In January 2005, they launched a line of cordless tools powered by lithium-ion batteries - the same technology used to power cellular phones and laptop computers.

Since then nearly every cordless tool company has responded by introducing its own lithium-ion, or Li-Ion, tool line. A quick count on the Internet reveals 62 Li-Ion tools on the market. And predictions are that all cordless tools will run on Li-Ion batteries, as opposed to nickel-cadmium (NiCd) and nickel metal hydride (NiMH), by the end of this decade.

Li-Ion's advantages over older battery technologies are numerous, which explains its instant popularity. Batteries made from the lithium metal base have a higher energy density and, thus, a greater ability to store electricity per ounce of battery weight. Li-Ion's gravimetric energy density ranges from 100 to 135. NiCd density ranges from 45 to 80, and NiMH's density is 60 to 120.

Then, too, Li-Ion cells operate at higher voltages than other rechargeables. Typically Li-Ion batteries discharge at 3.6 volts vs. 1.2 volts for NiCd or NiMH. This means a Li-Ion tool can operate with a single battery cell as opposed to multiple-celled NiCd or NiMH batteries. This accounts for Li-Ion's weight advantage. For example, Milwaukee's 28-volt Li-Ion battery delivers up to twice the runtime and 40% to 50% more power than an 18-volt NiCd Milwaukee battery of the same weight.

Another Li-Ion advantage is that they maintain their power output throughout their entire discharge cycle. Plus, Li-Ion batteries have a lower self-discharge rate. NiCd or NiMH batteries can lose from 1% to 5% of their charge per day even if the battery is not in the tool. Li-Ion batteries will keep most of their charge months in storage.

The major drawback to Li-Ion tools is their cost, which is around 30% to 40% higher than a comparable NiCd tool. This difference is reflected in the fact that Li-Ion batteries are more complex to manufacture. Li-Ion cells require special circuitry that limits peak voltage during charging or prevents voltage from dropping too low on discharge. These technical requirements mean you can't use a NiCd or NiMH charger for a Li-Ion battery. The industry is, however, working on Li-Ion chargers that are "backward-compatible" and thus can charge NiCd or NiMH cells. That will help absorb some of the sticker shock of purchasing a Li-Ion tool.

Then, too, the cost of Li-Ion tools is expected to drop as more are manufactured. And in that regard, the proliferation of Li-Ion-powered products is amazing. Consider that no Li-Ion impact wrenches were available two years ago. Five models are now on the market. Figure on that number to double by year's end. Other firms selling Li-Ion tools (but not yet impact wrenches) include Bosch, Hilti, Metabo, Panasonic, Ridgid, and Sears Craftsman.

Impact wrenches were chosen for this comparison because they are the one cordless tool that farmers have found to be nearly indispensable for their operation. For example, Darrell Geisler got a Milwaukee NiCd cordless impact three years ago. He has since purchased an additional cordless impact wrench so he can have one for his pickup and one in his tractor. "Having one in the field makes adjusting a planter, for example, so much easier and faster," says the Bondurant, Iowa, farmer.

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