Power Electronics

Battery Offers Li-ion Benefits to Power Tools without Compromising Safety

A phosphate-based Lithium-ion (Li-ion) cell now available from Valence Technology offers an alternative to nickel metal hydride and nickel cadmium batteries in power tool applications. Valence’s new Power Cell is an 18650 cylindrical cell that uses the same basic Saphion technology employed in the company’s other Li-ion battery products. However, the Power Cell contains modifications that dramatically boost performance at high discharge rates.

Although at least one other vendor offers an 18650 Li-ion cell for power applications, the Power Cell offers the high level of safety associated with Saphion’s phosphate-based Li-ion chemistry. Unlike conventional Li-ion chemistries employing cobalt-oxide materials in the cell’s cathode, the phosphate-based Li-ion cell is not susceptible to thermal runaway events.

The Power Cell specifies a 1.1-Ah capacity at a 10 C rate (i.e., 10 A). Although the Power Cell performance is characterized at the 10 C rate, it is capable of delivering much higher-current pulses. For instance, the cell can operate at a 25 C discharge rate for 30 sec. At lower discharge rates, cell capacity is only slightly higher. At a 0.5 C rate, cell capacity is 1.25 Ah. At a 1-kHz test frequency, the initial ac impedance of the cell is less than 20 mΩ.

When compared with a conventional Li-ion cell, referred to as an energy cell, the Power Cell is much better able to withstand repeated cycling at high discharge rates. Although a conventional cell cannot withstand a 10-A discharge, its performance at 5 A provides some perspective on its vulnerability in high power applications.

According to Valence, the company’s own Li-ion energy cells can operate reasonably well at a 5-A rate, but their cycle life will be degraded. For the purpose of testing, cycle life is defined as the number of charge and discharge cycles a cell will endure before its capacity is reduced to 70% of its initial capacity.

When tested at a 5-A discharge rate with an 80% depth of discharge, Valence’s energy cells exhibit a cycle life of 300 to 400 cycles. (This test assumes the PTC protection device has been removed from the energy cell.) In contrast, the Power Cell achieves a cycle life of 1000 cycles under similar test conditions (80% depth of discharge and final capacity defined as 70% of initial cell capacity). What’s more, this longer cycle life is achieved at a 10-A discharge rate.

The key difference between the energy cell and the new Power Cell is the lower impedance of the latter. In the Power Cell, a different anode material, different cell structure and different electrolyte composition all contribute to lowering the cell’s impedance. Electrolyte composition—the combination of salt and solvent—is said to be particularly important to cell performance. Although Li-ion cell manufacturers have only a handful of choices for solvents, each vendor has its own particular mix of salt and solvent that it believes optimizes cell performance. So, too, in the case of the Power Cell, the ratio of salt to solvent is critical to minimizing polarization.

The Power Cell is available now with lead times of 12 to 16 weeks. According to the company, cell pricing will be competitive with the existing, best-in-class battery technology.

For more information, visit www.valence.com.

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