The Joyetech eGo-T Upgrade is a 650mah battery that has two power-output modes. The first mode has a consistent output voltage of 3.3V, which is indicated by a white light on the power button. The second mode starts at 4.2V when the battery is fully charged and tapers off to 3.3V as the battery loses power. The second mode is indicated by a yellow light on the power button. To change between modes, turn the battery off by clicking the power button 5 times. As the light is flashing, press and hold the power button until the button flashes (should take 8 seconds). This is a great battery for those individuals looking for a compact eGo-style battery, but still desire a significant voltage output.