It’s said to be a 1715 mAh battery.
Apple doesn’t precisely advertise the battery capacity of its mobile devices. For iPads it users vague terms like “all day battery life” and similar terms are used for devices like the iPhone. It is only when the devices are stripped and their components revealed do we find out precisely what the actual battery capacity is.
The existing flagship, iPhone 6, has a 1810 mAh battery and if a latest leak is to believed the next-generation iPhone’s battery is going to be slightly smaller than its predecessor. The leaked image shows a purported iPhone 6s battery that comes in t 1715 mAh. This begs the question why would Apple reduce the battery capacity on its next iPhone when people always expect new models to have improved battery life?
Apple could have been compelled to go with a slightly smaller battery owing to strict restrictions that it places for dimensions and weight but it’s not like the company can’t make it up elsewhere. Apple can go for performance and system optimizations to mitigate the loss of battery life even though it will opt for a slightly smaller battery.
Given that such leaks are often hit or miss it would be best to take this with a grain of salt for now.