21700 flashlights utilize lithium-ion batteries measuring ​​21mm in diameter and 70mm in length​​, offering ​​4,000–5,000mAh capacity​​ and ​​20–30% higher energy density​​ compared to 18650 cells. This translates to extended runtime (e.g., ​​12+ hours on medium modes​​) and compatibility with high-power LEDs​​, which deliver ​​2,000–5,200 lumens​​ for tasks like search-and-rescue or outdoor exploration.