Advantage: The battery holders are made of PC+ABS high quality material. Small size, easy to carry. Easy and convenient to use. Feature: The battery holder has four compartments or chambers for holding four 3.7V battery. It is easy to solder and conduct electricity! Function: Keeping your standard or rechargeable batteries together and safe. Prompts: Batteries Not Included. You had better use consumer type 21700 cell with button cap to get good electricity conductivity of positive and terminal of box. important：It is suitable for 21700 flat-head cylindrical lithium batteries, with a length of 70mm and a diameter of 21mm. The battery must be installed from the positive pole, then the negative pole! To disassemble the battery, it must be disassembled from the negative pole, and it is forbidden to disassemble from the positive pole of the battery! Otherwise there is a risk of short circuit! Bistook 21700 Battery Holder Box Case for PCB Projects, 2-Pack, 4-Slot Each