The MiLi Power Spring 4 is an extremely thin battery case designed to double the life of the iPhone 4 & 4S. 

Features:

-Worlds first external battery for iPhone4.

-2000 mAh high capacity enhances iPhone battery life.

- All-inclusive shell design effectively maintaining signal stability.

Specification:

-Battery type: Lithium polymer

-Capacity: 2000 mAh

-Input: 5V/0.5A

-Output: 5V/0.5A

-Self-charging time: 4 hours

-Standby time: 450 hours

-Talk time: up to 7.5 hours on 3G-up to 16.7 on 2G-Internet use: up to 7.5 hours on 3G-up to 13.3 hours on Wi-Fi

-Audio play: up to 45 hours

-Video play: up to 15 hour

Size:

-124x65x18mm

Warranty :

-6 Months Warranty.