China's ATL (Ameperx Technology Limited) has kicked off mass production of batteries for the successor to Apple's iPhone XR forecast to be unveiled later this year, according to those close to the matter on June 9. Battery packaging will be done by another Chinese firm, Huapu Technology. The new batteries will have a capacity of 3110mAh, which is more than a 5% improvement from the 2942mAh of the iPhone XR.
While the device still would not have the most battery in mAh but it would have the longest battery life. If the report is correct, this year's iPhone XR will be 64 mAh less than the current iPhone XS Max. The next iPhone XS Max is also expected to gain more battery capacity but it is yet to be seen whether it will have the best battery life of the 2019 iPhones.
Apple is expected to unveil this year's iPhones in September as per the norm. Remember that these are rumors and may not become true this Fall. Take them with a grain of salt as we have to wait for Apple to unveil the iPhones before we know products specifications.