Android: There are many, many apps that can tell you how much battery each app on your phone is using, and how you might save some juice with certain changes. Battery Doctor does those things, but also encourages better overall battery health by well-timed charging.
Battery Doctor uses the best practices for batteries — charging to certain percentages, never letting your charge go below a certain amount, using up fewer cycles — and gives you examples and opportunities to live with them. Instead of just letting you know you're at 64 per cent and need to charge, it will tell you that you should charge for 24 minutes soon, or 40 minutes later on today. Battery Doctor is packed with many tools for explaining and optimising battery life, and its notification icon (which is kind of a widget, too) is a handy shortcut for enabling or disabling features for better battery life. Play around with Battery Doctor a bit, and you'll probably find a few optimizations that give you more peace of mind.
Battery Doctor (Battery Saver) [Google Play]