The phone has a 3.82 V, 1821 mAh cell that provides up to 6.96 Wh of power, compared to 7.56 Wh of the iPhone 7 or the Samsung Galaxy S8 at 11.55 Wh battery.
However Apple says the increased efficiency of the A11 Bionic chip and Qualcomm Snapdragon MDM9655 x16 LTE modem, WTR5975 Gigabit LTE RF transceiver and PMD9655 PMIC shown in the X-ray above means the battery life is comparable to the previous generation.
The iPhone 8 has also moved to wireless charging using the Qi specification. Both Apple and its power management chip supplier Dialog Semiconductor joined the Qi consortium earlier this year, but the charging management in the e iPhone 8 is supplied by Broadcom's BCM59355. The teardown also includes the coil used for the inductive wireless charging connection.
The details of the iPhone X may well differe signficantly from this version. The X is promising a fast charging mode called Air Power, which Apple is aiming to encourage other manufacturers to adopt. The silicon for that will be a significant element in creating an eco-system of devices.