The iPhone 3GS is powered by an internal 3.7 V 1220 mAh rechargeable lithium - ion polymer battery .
The iPhone 3GS is is designed to retain up to 80 % of its original capacity after 400 full charge and discharge cycles .
The iPhone 3GS is powered by an internal 3.7 V 1220 mAh rechargeable lithium - ion polymer battery and is designed to retain up to 80 % of its original capacity after 400 full charge and discharge cycles .