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Normally battery is 3.7v you can use DC-DC Boost Converter to Stepup battery voltage which is 3.7v to normal Arduino voltage i.e. 5v, you can find various DC-DC Boost Converters online. I didnot use any boost circuit because arduino can also work on 3.3V so I used directly 3.7v from battery to power arduino.
Thanks Scott :) This Circuit is very awesome, I did some modifications to the code though, Accidently bought OLED display with SPI instead of I2C :P