Android 5.1 vs Android 6: Doze Battery Life
With Android 6.0 Marshmallow users will notice better battery life on all devices. Even old Nexus 5 owners should see an improvement. This is to the OS as a whole, but also a brand new feature called "Doze".
Doze will let the device and all apps doze off or sleep when not in use, potentially doubling battery life. Doze will recognize when a device is not in use, and put apps in a low-power state (lower than before) and stop apps from waking up a device and those powerful 8-core processors, notifications from coming through and more, and has shown to double the Nexus 9 battery life, and promise around 30% better standby battery life for all devices. This will be huge for those that don't use their phones all day long. In Android N its even more aggressive and puts all apps that aren't in the forefront (and in the recent apps menu) to sleep to save battery, but still allows them to function.
Doze won't stop important notifications like Gmail alerts, Facebook messages and texts from coming through, and users will even be able to manage what apps can doze off, and even remove certain apps from the doze list. The more apps that can take a nap, the better your battery life will be.
Apple has done this for years with iOS, and Android does it as well, but now it will be more aggressive in the hopes of improving battery life without hurting us, the end user.