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Battery Life And Thermal Throttling
Battery life may be the most important performance metric for a mobile device. After all, it doesn't matter how quickly a phone or tablet can load webpages or how many frames per second the GPU can crank through once the battery runs down and the device shuts off. To learn more about how we test this critical facet of mobile computing, please read our battery testing methodology article.
The Nexus 6 runs this real-world workload for 5.5 hours. But despite having the same SoC and the same size battery as the Note 4, it runs out of power thirty minutes sooner. Its 5% performance advantage won't be noticeable and does not make up for the 9% battery life deficit.
Normally, we would turn to the Basemark OS II Battery test to see how CPU performance affects battery life. However, the results for the two Snapdragon 805-based devices look suspect, so we're withholding them pending further investigation.
The Nexus 6 will hold out for a three-hour gaming session, the same as the iPhone 6 Plus and one-half hour longer than the Nexus 5. It would appear from the first chart that the Note 4 offers better battery life while gaming, since it outlasts the Nexus 6 by almost 30 minutes. But looking at the performance chart tells the other half of the story. The Note 4 lasts longer only because thermal throttling forces it to reduce GPU clock rate more than the Nexus 6. The iPhone 6 Plus and Nexus 5 also have fewer pixels to render for this on-screen test, which improves both their performance and battery life.
Looking at the graph above, we see that the performance of the Nexus 6 gradually decreases to about 75% of its original value during the roughly 30-minute T-Rex loop. While not immune to thermal throttling, it does fare better than the Note 4, which loses over half of its performance 8.5 minutes into the test before recovering to about 70% of the original value over the last half.
The Nexus 6's back cover skin temperature reaches 113 °F, certainly warm but not uncomfortable and about average for this group. Looking at the thermal image of the back cover, taken during the GFXBench 3.0 Battery test, shows why the Nexus 6 throttles less than the Note 4. It more evenly spreads the heat load over it's whole chassis, using the larger surface area to better dissipate heat buildup to its surroundings.
Battery life for the Nexus 6 is not bad, at least compared to the Note 4. In PCMark, the Nexus 6 trails by about 9%, which coincidentally, is almost equal to the difference in screen area between the two devices.