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How do I turn on the Battery Saver function on my Samsung Focus®?
Your phone has a battery saver feature that allows your phone to conserve power when your battery drains down to critical levels. Your phone will ask you when the battery power drains to critical levels if you want to turn the feature on.
You can set your phone up to turn the feature on automatically when the battery power drains to critical levels. Flick left to the Apps list from the Start screen and tap Settings > battery saver. Check the box for Always turn on Battery Saver when battery is low. You also have the option to Turn on Battery Saver now, until next charge, if you want to extend your battery's power longer between charges.