Squeezing operational life out of a shrinking energy capsule

Technologies
Within the context of a portable environment, the two most important parameters of a sourcing technology are power density and energy density, with response time a close third. A battery must therefore be large enough to supply the peak-power demands of a functionally dense application and also large enough to store enough energy to sustain it for extended periods of time. Unfortunately, energy dense technologies like fuel cells and nuclear batteries have low power densities when compared against their Li Ion and ultra capacitor counterparts, as shown in Figure 1 [1]. In other words, given similar volume constraints, the fuel cell cannot source the power a Li Ion can and a Li Ion cannot sustain a low power load as long as a fuel cell can. This difference is especially troubling in space-constrained applications where over-sizing a battery just for the sake of energy or a fuel cell just for power is not an appealing option.

Figure 1. Ragone plot: energy and power densities of various devices

The complementary power-energy characteristics of these sourcing technologies are the driving motivational factors behind hybrid sources, and response time further justifies their increasing demand. Portable devices are notorious for hopping across an array of power modes to enable functions only when needed, extending the operational life of a system by minimizing unnecessary power sinks. Unfortunately, waking up a device from idle modes can often subject the sourcing technology to fast load dumps, requiring a quick response. Unfortunately, energy-dense fuel cells and nuclear batteries are slow to respond. Li Ion batteries are quicker to respond, but not as fast as ultra capacitors, and ultra capacitors not as fast as conventional capacitor technologies, all of which fuels the demand for hybrid devices, which are nothing more than natural extensions of the battery-capacitor hybrids normally used in most, if not all, existing electronic applications.