https://wccftech.com/2020-iphone-12-will-have-larger-battery-due-to-improved-circuits/

Apple's new iPhone 12 models in 2020 will have improved circuits which will be smaller in size, and will give space for larger batteries. This has been revealed by Korean website The Elec, who have cited industry sources for the news report.

The improved circuits in the report refer to the battery protection module. A battery protection module is used to prevent over-heating and over-charging so that the battery lasts longer and is safe to use. In iPhone 12, Apple will use a custom battery protection module, which would be up to 50% smaller in size. This should free up more space for larger batteries.