Apple has filed two patents for portable device batteries. One is for a "curved battery cell" and the other for a "non-rectangular battery". Both patent applications were filed on Oct. 28, 2011. According to the filing, "the curve is formed to facilitate efficient use of space inside a portable electronic device." Although, it should be noted that Apple's filing for curved battery cells use the same pouch mechanism as other curved batteries already on the market.
While a curved battery could be implemented in a device with a more dramatically new design, the non-rectangular battery is something that fits more into what we've already seen from Apple. Apple loves to use tapered edges on devices to make them lighter and seem thinner, so being able to taper the battery could help fit a bigger battery into a tapered design.
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