In recent years however, because of their large capacity, double layer capacitors have become viable alternatives to batteries. In these applications the double layer capacitor is kept charged from the primary power supply, and functions as a backup battery when the primary supply is removed. Battery-based systems such as pagers, telephones, and personal digital assistants require widely varying power levels at different times. The peak power may be needed only a fraction of the total operating time, but the power requirements of many of these devices are often driven by peak power requirements rather than the total energy. Energy and Power Capacity .
Batteries are electrochemical devices.
That is, when a battery is charged, the electrical energy drives a chemical process, essentially forming new compounds that have higher internal energy. When the battery is discharged, the processes are reversed producing the original compounds and electrical energy. The chemical reaction inside a battery is intrinsically slow, and is can take several hours to charge. The rate at which one can extract energy is also limited. For example, a 100 mA-hour battery may be able to deliver 10 mA for 10 hours, and 100 mA for 1 hour, but can't deliver 1 A for 0.1 hour.