You used to be able to set the battery level that triggers Battery Saver to turn on with a high level of granularity in Windows settings, but after recent updates, it can only be set to turn on at "Never, 10%, 20%, 30%, 40%, 50%, Always."
Users may want to customize this more specifically, e.g. setting the level to 45%, or especially if they have an older computer with a weaker battery, something higher like 70%. There does not appear to be any way to do this via Registry.