FIGURE 3-4 Doyle’s Li ion model results for capacity versus average power, showing difference between continuous and intermittent loads of the same average value. SOURCE: Martin et al., 2003.

tent discharges, the capacity at a continuous discharge of the peak power is a better estimate than the capacity at a continuous discharge of the average power.

The typical power demand of mobile systems usually includes several periods of peak demand interspersed with potentially long periods of very low demand. Figure 3-5 depicts such a measured power demand of a speech recognition system on a mobile platform. Each peak represents processor and disk activity while recognizing a sentence.

HYBRID CONCEPTS

A hybrid power source usually combines a high-energy/ low-power component with a low-energy/high-power component. Examples of hybrid power sources are battery + battery (e.g., Li ion + Zn/air), battery + capacitor (e.g., Li ion + electrochemical capacitor) and fuel cell + capacitor combinations. The rationale for hybrid power sources is to leverage the high-energy component with the high-power component, extending mission life and enhancing power capability while minimizing system weight or volume. Atwater et al. (2000) have demonstrated hybrid power sources with Li ion + Zn/air, Zn/air + electrochemical capacitor (EC), fuel cell + EC, and fuel cell + lead-acid combinations. In the Li ion + Zn/air hybrid, it was shown that the combined mission life (based on a communications equipment load profile) of the hybrid is almost six times longer than that of the individual components. In terms of specific energy, the hybrid had 198 Wh/kg, compared with 126 Wh/kg in the Li ion battery and 177 Wh/kg in the Zn/air battery.

Optimization of a hybrid power source is very complex, and the optimized combination of power sources might enhance only for a certain range of load regimes. Thus, it is highly desirable to develop a model that can predict and analyze the performance of various combinations of power



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