Every time a new build of Windows 10 is released - or leaks, as in this case - there are always small changes that show incremental improvements as Microsoft pushes closer to the final feature set for the OS. In this case, we can see a few improvements to Battery Saver.
Battery Saver, as the name implies, will help your mobile device make the most out of your battery. This feature works by limiting the background activity on your device when the mode is activated. You can turn the feature on at anytime but there is also a setting to have it automatically turn on when there is a user-defined percentage left of the battery; e.g. turn on when there is 18% battery life left.
In 9888, we can see that the UI has been updated and more importantly, it's now working properly. In the previous builds, while we could see Battery Saver, it was not functional.
You can now also choose the point at which Battery Saver will activate from the settings page. There is a bit of a UI issue here - why does there need to be a separate panel for settings when there is only one item in the "Battery Saver settings" panel? Microsoft is either planning to add more features here or, if not, hopefully they will bring that function to the Overview page.
This is a small update and considering that this build was not supposed to make its way out of Redmond and that the company is not releasing any new builds this year, this may be the best look we get until the consumer preview arrives.