The SM1 Slim Mechanical Backlit Bluetooth Keyboard is equipped with a built in Lithium-Ion Battery which is non-replaceable. 

Input: 5V-0.8A

Battery Capacity: 2500mAh

When the keyboard's battery is running low and voltage is less than 3.5V, the LED light on the Spacebar will flash slowly and the keyboard will enter Power Saving Mode (backlight will shut off) 

When in this current state, we recommend charging the keyboard for 2-3 hours for full charge.