While iPhone 4 owners tend to be a pretty happy lot, perhaps the single largest complaint now that the antenna issues have died down are the iPhone's short battery life.

To alleviate the problem of short battery life, a California based firm called mophie has developed a series of protective cases which double as rechargeable iPhone battery packs.

Today the company announced its second rechargeable external battery case designed for the iPhone 4 and its most powerful called the "juice pack plus". The case, with its hard shell case and rubber grip, adds about 1mm of thickness to the iPhone 4 and provides 2,000 mAh battery capacity, enough to double your iPhone's battery life.

Features include: pass-through USB so you can charge and sync your iPhone 4 to iTunes without having to remove it from the case, a toggle switch that allows for standby and charging mode and an integrated LED status indicator tells you exactly how much “juice” is left and also displays how much time remains during the charging process.

Mophie says the juice pack is rechargeable for 500 full cycles and after 500 cycles, its fully charged capacity will be less than 75% of the capacity of when it’s new.

The juice pack plus for iPhone 4 comes in four colours and is now available from the Mophie website for 99.95 USD. The company also offers the Juice Pack Air with 1500mAh battery capacity for $79.95 USD.

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