With Samsung cell phones--particularly new, high-powered models such as the Galaxy S--battery usage can be a serious problem, as battery power does seem to be struggling to keep up with unit power. The idea of a "reserve battery" has been something of a myth since earlier models of Nokia's range of cell phones; however, it is possible to tap into excess battery life with smart usage and even have a literal reserve battery in case of emergencies.
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Type "*3370#" into your phone. This is the code that activates the reserve battery after prompting the cell to restart.
Refrain from pressing the "Send" button. As soon as you type in the digits, your phone will restart; it is not like making a call.
Allow the phone to restart; when it does so, you will find that the cell has 50 percent more battery life than it had before the restart process.