Most cell phones do not have battery power in reserve. However, certain brands of cell phone, such as Nokia, have an option to turn on a half rate codec mode on the phone. This reduces the sound quality of the phone but allows it to last an additional 30 percent longer. Users can also extend battery life by turning off Wi-Fi, locking their phone sooner and dimming its screen.
On Nokia phones, the half rate codec can be activated by pressing the asterisk, pound sign, and then the numbers four, two, seven and zero. An urban legend surfaced that claimed users could activate this by typing in asterisk, pound sign, and the numbers three, three, seven and zero. This actually has the opposite effect and activates the full rate codec, enhancing sound quality but reducing battery life.
Turning off the Wi-Fi when it is not being used is another way to extend the phone's battery power. The Wi-Fi consumes energy because the phone consistently seeks a signal, even in areas where no Wi-Fi is available.
Most smart phones have a variety of options for when the phone locks and turns off. Users can increase the battery lifetime by turning the phone to a lower setting so that it turns off more quickly after it has been used.
Finally, most cell phones allow users to customize how bright the display is, and dimming the display as much as possible can help extend the phone's battery life.