Apple has released its first ever official battery charging case for the iPhone 6S and iPhone 6, promising to almost double the battery life of your smartphone at a cost of $99.

Launched on the company's online store without any warning, the new battery case is available for Apple's smaller iPhone 6 and iPhone 6S and resembles the Silicone Case the company already sells, but with a large hump on the rear to accommodate the extra battery.

iPhone 6S Smart Battery Case Apple has launched its first official Smart Battery case. Photo: (Apple)

While the exact size of the battery pack in the case has not been revealed, Apple's description of the new iPhone 6S Smart Battery Case says that the combined battery life will give you up to 25 hours of talk time (up from 14 hours) while you will now get 18 hours of Internet user on LTE (up from 10 hours). "With the Smart Battery Case on, the intelligent battery status is displayed on the iPhone Lock screen and in Notification Center, so you know exactly how much charge you have left," Apple's description says.

The case is initially available in grey or white and has a pass-through to allow you to connect your lightning cable for data transfer as well as charging both batteries.  

There have been third-party battery cases available for the iPhone for many years, with the best-known brand Mophie the exclusive third-party brand selling its battery cases in Apple Stores around the world as well as online. The $99 price tag on Apple's official case means it will be competing directly with the Mophie Juice Pack Air that also costs $99.

It is unclear why Apple chose now as the time to launch an official battery case, but with battery life consistently among the biggest areas of complaint for smartphone users, this product is likely to be extremely popular.