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 Galaxy Note 4 manages to last an impressive 6 hours 49 minutes in PCMark, currently our best test for real-world battery life. It even outlasts the Nexus 6 despite using the same SoC. One reason for this has to do with CPU frequency. In the chart above, the Nexus 6 was still running Android 5.0, where it maintained a higher average CPU frequency, even holding two cores at max frequency (2649MHz) for the Writing test. The Note 4's CPU governor, in comparison, allows the frequency to bounce around, rarely going above 1497MHz.
Having full disk encryption enabled and handled solely in software also hurts performance and battery life on the Nexus 6. The PCMark workloads have several small read and write operations that add up over several hours, particularly in the Writing and Photo Editing tasks. The Android 5.1 update for the Nexus 6 disables the CPU thread migration boost feature, improving battery life to 354 minutes but slightly lowering performance.
At first glance, the Note 4 appears to do well in the GPU/gaming focused GFXBench battery test, lasting longer than the Nexus 6 and iPhone 6 Plus. It's not until we look at the performance chart that we understand why. The Note 4 shows a 40% reduction in performance relative to the Nexus 6 due to thermal throttling. With the GPU frequency scaled back, it uses less power and lasts longer.
Looking at the battery drain and performance graph above from the GFXBench battery test, we can see the Note 4 throttle back to less than 50% about 8.5 minutes into the test before recovering to about 70% of the original value over the last half. Heat dissipation is a design weak point, which is evident in the thermal image of the back cover taken during the same battery test. It's certainly obvious where the SoC is located (the yellow circle is the rear camera and the green rectangle in the upper-right is the battery). The chassis does a poor job of spreading and dissipating the heat generated by the GPU, making the Note 4 more susceptible to thermal throttling.
You should not have an issue getting through the day on a single charge with the Note 4; however, it does have a power saving mode that restricts background data transfers and performance, even an optional grayscale display mode, for stretching battery life a little further. If you're really desperate, the ultra power saving mode limits what apps can be used and switches to a simple black-and-white display mode.
The Note 4 comes with Samsung's Fast Charge feature, which is compatible with Qualcomm’s Quick Charge 2.0, that can charge the battery to 50% in about 30 minutes, according to Samsung.￼