Do you know what ambient temperatures is suitable for your Apple device? Apple declares : "Apple Devices are designed to perform well in a wide range ambient temperatures, with 62° to 72° F (16° to 22° C) as the ideal comfort zone." It is very important you don't expose Apple device to environment temperatures higher than 95° F (35° C). More than this temperature, it damages to battery capacity. You can obtain more information about performance of battery at https://www.apple.com/batteries/maximizing-performance