The Efest K4 Slim provides fast USB-C charging for up to four battery cells and is compatible with various battery sizes.

Four-bay charger with USB-C


Compatible with Lithium IMR 3.6V/3.7V Batteries:
10440 | 14500 | 16340 | 17340
18650 | 18490 | 18500 | 18650
26500 | 26650 | 20700 | 21700

Compatible with many sizes of lithium IMR batteries.


The Slim K4 will charge your battery cells a few seconds after installation. The indicator shows blue when in standby mode or when the batteries are fully charged. The indicator will show white when the battery cells are set.

LED Indicator. Shows blue when in standby and when fully charged. White colour shows when batteries are charging.


Auto-detect charging current: 1 x 1.0A / 2 x 1.0A / 3 x 0.5A / 4 x 0.5A
Safe charging mode: CC & CV | Safe and convenient USB charger (5V 2A)
Monitors and charges each slot independently | Low voltage smart activation recovery function
Super lightweight and portable

Lightweight and portable. Safe charging, safe and convenient USB charger. Auto-detect charging current


The piano wire-made spring ensures smooth gliding of each slot when installing and removing batteries. The Slim K4 is designed with a steel cathode, low resistance and hard chromium plating to achieve a more accurate charging voltage.

Piano wire spring allows for smooth gliding. Accurate charging voltage.

USB Port Design

Type-C USB helps to reduce the heat of the port effectively.

Type-C USB port reduce the heat of the port effectively

Six Protections

The built-in intelligent chip comes with six protections.
Short circuit protection | Automatic overcharging protection
Automatic over-discharge protection | Reverse polarity protection
0V activation recovery function | Hardware and software double protection.

The Efest K4 Slim has six protection features


Global insurance technical support with FCC, CE, and RoHS certification

Certifications and global insurance technical support


Outside packaging

Please see our comprehensive guide to battery safety.

Customer Reviews