The best designs use a supercapacitor instead of a rechargeable battery, since these have a longer working life than a battery. This, along with the long-life light emitting diode which does not burn out like an incandescent bulb, give the flashlight a long lifetime, making it a useful emergency light. A disadvantage of many current models is that the supercapacitor cannot store much energy in comparison to a lithium-ion cell, limiting the operating time per charge. In most designs, vigorously shaking the light for about 30 seconds may provide up to 5 minutes of light, though the advertised time omits the reduced output of the LED after 2 or 3 minutes. Shaking the unit for 10 to 15 seconds every 2 or 3 minutes as necessary permits the device to be used continuously. It is often viewed as a toy, or an emergency backup for other flashlights.