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Enable Wi-Fi while you're at work or home
Wi-Fi and other wireless radios should be disabled when they're not being used, as they use a significant amount of battery life. If you are at work or at home, and you're in a building where cellular signal is weak — such as a thick-walled house or a city apartment — you might find that leaving Wi-Fi turned on and connected to a stable network helps your battery throughout the day.
Anecdotal reports suggest that when your iPhone has good cellular signal, the device's battery will remain at a relatively stable discharge rate. But when you're on an older GPRS, EDGE, or 3G, your iPhone's battery life may decrease far quicker because it is constantly searching and "pinging" for a higher-quality cell site with better signal.
Under the swipe-up Control Center, tap the Wi-Fi button (second in from the left) on. Or, go to Settings > Wi-Fi, and set to On. Also, set Ask to Join Networks to Off to keep that Wi-Fi network connected.