Several days ago, a friend told me that “The e-cigarette I bought has two built-in unprotected 18650 batteries. The merchant said that they can charge directly without removing the battery. But I don't know if it will be dangerous to charge directly. How do I charge it properly?” Nowadays, many electronic cigarettes, flashlights and camera stabilizers on the market have built-in lithium batteries for power supply, and also provide direct charge. For direct charge and desktop charge, which one do you prefer?