Hybrid battery system for a pocket computer

Summary form only given as follows. Hybrid battery systems that contain both primary and secondary cells may offer compelling advantages for our smallest and most personal electronic devices. While either commodity alkaline cells or a Li-ion pack are used in most such applications today, small air-breathing power sources, such as metal-air cells and fuel cells, promise much higher specific energy and correspondingly longer runtimes. Unfortunately, in scenarios demanding high peak power, the high impedance of most such cells requires using a capacitor or secondary cell for load leveling. We have explored this hybrid approach using our Itsy pocket computer. We started with the largest available commodity Zn-air button cells (#675/PR44) and the smallest available Li-ion and Li-polymer cells. After characterization, we selected a small Li-polymer cell and built two simple hybrid power systems. We evaluated runtime using three benchmarks: sleep-mode, idle-mode, and an MPEG multimedia player. The experiments showed this approach to be practical for systems of up to 50 mW average power, and that the hybrid offers approximately twice the energy of a Li-ion pack of comparable size. We found proper cell-holder design to be critical to realize this potential. With wider availability of large Zn-air cells, higher average power levels are possible, and for sufficiently large cells, the secondary cell could be entirely eliminated. Retaining the secondary cell, however, allows both hot swap battery changes and extending system runtime via opportunistic charging. It is unknown how well this more complex usage model can be understood and exploited by consumers.