Power Electronics

Li-Ion Cells Build Better Batteries for Power Tools

After becoming the dominant rechargeable chemistry in cell phones, PDAs and a host of other mobile applications, Li-ion batteries are just starting to prove their metal in high-power portable applications such as power tools. Just as in the lower-power applications, high-power Li-ion cells offer higher energy density than the NiCd and NiMH cells they replace. Recently developed Li-ion cells offer two to three times the gravimetric energy density of the nickel-based chemistries — an advantage that can be exploited either to build lighter battery packs or to extend battery run times.

One equipment manufacturer, Milwaukee Electric Tool, collaborated with an established battery manufacturer to develop a 28-V Li-ion battery pack that offers greater power and up to twice the run time of conventional 18-V NiCd models. Despite its higher performance, the 28-V pack weighs slightly less than the 18-V pack. As a result, the company is able to create some of the first cordless versions of professional-grade power tools (see the figure). One example is a cordless bandsaw that is described as “powerful enough for everyday use.”

Along with offering greater power and run time, Li-ion cells offer better performance at temperature extremes. This advantage includes greater cycle life than NiCd batteries under extreme conditions.

The impact of Li-ion batteries on power equipment is so substantial, vendors are using the technology to market their new products. For example, Milwaukee Electric refers to its battery pack as the V28 Lithium-Ion cordless technology and its end products as the V28 line of tools. Cells based on the V28 technology employ manganese-oxide cathodes rather than the cobalt-oxide cathodes commonly used in low-power applications. To create a 28-V pack requires seven of these 4-V, 3-Ah Li-ion cells.

To serve the power tool applications, Li-ion batteries have been modified to operate at high discharge rates. So, while the standard Li-ion batteries specify their capacity at a 1C (or even lower) discharge rate, high-power Li-ion batteries typically specify performance at a 10C or higher rate.

Low-power and high-power Li-ion cells differ with respect to rated capacity and the variation in capacity as a function of discharge rate. In general, the cells developed for high discharge rates sacrifice some of the energy density achievable at lower discharge rates to obtain lower impedance in the cell. For example, in the 18650 cylindrical format, Sanyo now offers a high-power cell with 1500-mAh capacity at 10C, while one of its low-power cells currently offers 2600 mAh at a 0.2C rate in the same package.[1]

Typically, conventional Li-ion cells have limited ability to deliver high power. In contrast, the high-power Li-ion cells offer nearly the same capacity at high and low discharge rates.

Latest Offerings

Sanyo (www.sanyobatteries) continues to develop cells with a high discharge rate/higher power capability. A recent example is Sanyo's UR18650W, a cell that specifies a typical capacity of 1600 mAh and a minimum capacity of 1500 mAh. The cell achieves this minimum capacity at a 10C (15-A) discharge rate (see the table). The cell is constructed with a manganese-oxide cathode to improve safety over what would be obtained with the cobalt-oxide cathode typically used in standard Li-ion cells.

Despite its safer cathode, a combination of risk factors such as the possibility of overcharge and an internal short made it necessary to incorporate overcharge protection into the UR18650W. This circuitry protects against voltage imbalance during normal cell use and abnormal charging conditions. The latter can occur when the battery encounters a broken charger or the wrong charger, or when a battery is charged manually.

The UR18650W follows two power cells previously developed by Sanyo. The UR18650H/V offer 2000 mAh of capacity, but at a lower discharge rate (5C).

Another cell manufacturer, Sony (www.sony.com/energy) describes its VT series of Li-ion cells as the first developed for power tool use.[2] The 18650VT and 26650VT, which were introduced early last year, are cylindrical cells that specify capacities at 10C, 4C and 1C discharge rates (see the table). As these specifications reveal, the capacities of the high-power cells vary only slighty across a range of discharge rates.

For example, the 18650VT specifies an average capacity of 1030 mAh at 10C versus an average capacity of 1080 mAh at a 1C rate. Similarly, the 26650VT specifies a 2400-mAh average capacity at 10C versus a 2500-mAh average capacity at a 0.2C rate.

Although these cells were originally developed using cobalt-oxide cathodes, newer cells incorporate manganese and nickel, which is said to eliminate the need for an overcharge protection circuit.

Although these cells exhibit about half the energy density of the convention Li-ion cells for low-power applications, their performance is still significantly better than the chemistries they are replacing. The VT series cells offer 94 Wh/kg gravimetric energy density versus 47 Wh/kg for NiMH and 35 Wh/kg for NiCd.

Last year, Valence Technology (www.valence.com) targeted power tool applications when it introduced its Power Cell in an 18650 cylindrical format. Like other Li-ion batteries offered by the company, the Power Cell employs the Saphion chemistry, which incorporates phosphate-based cathodes rather than cobalt-oxide. The phosphate-based cathodes offer greater safety and less susceptibility to thermal runaway than cobalt oxide.

The Power Cell offers nearly a 1.1 Ah of capacity at a 10C rate but can deliver higher current pulses.[3] For instance, the cell can supply a 30-A pulse for 30 sec. Valence is working on the next revision of the Saphion power cell, improving the product's cycle life and power density. The company expects the Saphion power cell revision to be in production within six months.

Although Li-ion cells developed for power-tool applications are relatively new, we can expect that their development will follow some of the same trends as their low-power counterparts. With consumers always wanting longer run times for their batteries, cell vendors will likely strive to offer higher energy densities at the higher discharge rates.

Vendors will strive to improve cell safety and lower material costs to enable high-power Li-ion cells to be adopted in more applications. To that end, cell makers are already developing Li-ion cells for even higher discharge rates.

Hybrid electric vehicles (HEVs) represent one of the most promising target applications for such batteries, and several battery suppliers and at least one automaker (Toyota) are developing Li-ion cells to power HEVs. Although most of these batteries may be years away from use in commercial vehicles, Toyota is already using a Li-ion battery in its Vitz, a minivan sold in Japan, to provide power within a stop-start system.

References

  1. “New Concept Lithium Ion Batteries in Sanyo” by Masatoshi Takahashi, Sanyo Electric Co., Power Solutions Group, presented at Portable Power Conference, Sept. 20, 2005.

  2. “R&D Activities & Results for Sony Batteries,” by Kenji Ogisu, R&D Division, Energy Group, Sony, presented at Portable Power Conference, Sept. 20, 2005.

  3. Valence Technology Datasheet, available online at www.valence.com/solutions.asp.

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