The J5019 Lithium Battery Charger & DC-DC Boost Module is an all-in-one charging and voltage step-up solution for 3.7V/4.2V lithium batteries (like 18650s). Featuring a built-in micro-USB charging port and an adjustable potentiometer, it allows you to charge your battery while simultaneously boosting the output up to 27V. This ultra-compact module is ideal for DIY power banks, small electronics projects, and custom mini-UPS systems for WiFi routers.