Lithium-ion batteries are rated at 3.7 volts. That doesn't meant that's what they always put out though. When the battery is fully charged, it will read approximately 4.2 volts, and "fully discharged" will be at approximately 3.2 volts. If a battery goes below 3.2 volts, it can possibly damage the battery. That's why a high quality mod like a ProVari will cut off at 3.2 volts.