Results: Battery Life
The 2600 mAh battery in HTC's One (M8) and (E8) is larger than last generation's by 300 mAh. That's not a huge difference, particularly if the newer device uses power faster than its predecessor. But to ensure battery life actually does improve, the company adds a power-saving mode, too. This setting throttle back the SoC, lowers brightness, disables the vibrate feature, and slows your data connection. Consequently, the phone uses little power in standby.
Based on our time with the One (M8), HTC appears to have a not-altogether elegant, but still-effective solution for road warriors who can't get their mobile devices to last long enough. At times, this phone's battery seems to last forever.
Basemark OS II
The Basemark OS II battery test scores are derived by repeatedly running the devices until enough data has been collected to determine a drain rate.
The Basemark battery test mirrors the experience we just described, going so far as to appear over-optimistic. Let's move on to the other benchmarks for confirmation.
BatteryXPRT 2014 is a specialized battery testing application for Android devices that provides users with an "expected" Lifetime score, as well as an overall Performance score. The test has two variations: Network-Connected and Airplane Mode.
We had trouble getting the airplane mode test running on a number of devices, but HTC's One (M8) at least performs well in the networked benchmark.
GFXBench's battery test measures battery life and performance stability by logging frame and battery discharge rate as the on-screen T-Rex test runs for 30 consecutive iterations. The results are given in two scores: estimated battery life in minutes, and the number of frames rendered on the slowest test run (to gauge if a device is throttling). Both tests are run at the device's 50-percent brightness level in the free Community Edition, while the paid Corporate Edition can be set to 0 percent, 25 percent, 75 percent, 100 percent, or whatever the device's native slider is set to. We very specifically calibrate all units to 200 nits before doing any battery testing.
The One (M8) does well from a performance standpoint, finishing second to the class-leading iPhone 5s, which renders at a much lower native resolution.
Given strong performance, as indicated by the frame rate measurement, a 140-minute result isn't bad compared to the rest of the field.