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.
Type "*3370#" into your phone. This is the code that activates the reserve battery after prompting the cell to restart.
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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.