Inside the Mi Max is an absolutely enormous 4,850 mAh (18.67 Wh at 3.85V) lithium-ion battery, which cannot be removed. A large battery is needed to power the 6.44-inch display, and the increase in battery capacity matches the increase in display size over a typical smartphone: the battery is 62% larger than a 3,000 mAh cell in a 5-inch handset, while the display is 66% larger.
While the increase in battery capacity matches the increase in display size, the massive size of this battery will deliver better-than-average battery life as the power consumption of other components doesn’t increase with a larger display. In other words, the SoC will consume just as much power in the Mi Max as in five-inch phones, and there’s more battery to go around in this handset.
For this reason, it didn’t surprise me that the battery life I experienced while using this phone was excellent. You should expect to get at least one full day of usage out of the Mi Max, if not more.