Efest Slim K4 USB-C Battery Charger can be used to charge electronic cigarette (e-cigarette) external batteries of any size via a Type-C USB charging port. You can charge up to four batteries for vapes in this charger and it keep you safe with its built-in intelligent protection chip that includes smart functions such as:auto-detect chargingmonitors and charges each slot independentlylow voltage smart activation recovery0V activation recovery.It is compatible with Lithium IMR 3.6V/3.7V batteries: 10440, 14500, 16340, 17340, 18350, 18490, 18500, 18650, 26500, 26650, 20700, 21700.The Efest Slim K4 charger will even tell you what it's doing with its LED indicator colour light that will turn blue in standby mode/full charged and white when charging.Please note: a 2A USB port is recommended.