Fully charged LI-ION batteries are 4.2 volts. Nominal voltage is usually listed on various batteries at 3.6-3.7v, but when it comes off the charger, it's 4.2v. It drops with use, and you should stop using it before it drains below it's safe-discharge limit. Each battery has it's own set of specs, so you can google how low your particular battery can go before charging. I usually recharge mine when they get to about 3.2v. You'll experience a cooler vape as the voltage drops, so with some experience, you'll learn when to change out batteries based on the vape you're getting. Until then, change out early.