According to the leaker and news aggregator account "yeux1122" on the Korean-langauge Naver blog, Apple is testing a battery in the 5,400–5,800 mAh range, which would make it the largest capacity of any current or previous iPhone. For reference, the iPhone 17 Pro Max has a 5,088 mAh battery.

As for how it would compare to rival foldables already on the market, the Google Pixel 10 Pro Fold has a battery size of 5,015 mAh, while the Samsung Galaxy Z Fold 7 has a 4,400 mAh battery, so anything in Apple's alleged testing range would likely offer superior battery life.