Recently my Galaxy SII twice flattened the battery in three hours of standby use, with the battery getting very warm. Using Settings - Battery: showed that McAfee had used 94% of the battery, so obviously it had gone mad somehow or other. Uninstalling McAfee changed the battery life back to three days or so of standby, with a small amount of screen/Internet/phone use too. If you can do without them except when required, switching off Wi-Fi and Mobile Data also extends battery charge life. If you keep the phone in an area of poor or non-existent phone signal, it uses a lot of battery power trying to find a signal.