DU Battery Saver & Widgets for Android is a handy battery management app that includes 3 useful widgets and a status notification bar. It will help you get some very useful information like how much longer to expect your battery to last, and how long it will take to complete charging.
There's a lot of options here, so quite a few different ways that you could use the app. There are modes to select from and create, which have different settings switched on or off (e.g. WiFi, mobile data, calls, messages). You could then switch between these with one of the widgets. But I prefer to adjust individual settings as I need to. There's also a power toggle bar available, which pops up with most of the important settings to toggle when you tap for extra options. It's functional, but it isn't the best set of power toggles I've seen, and could do with some customization options.
One of the widgets has a little leaf button, which means 'optimize'. This will shut down background applications that the app thinks you aren't using. It will remind you when your phone has finished a "healthy charge", to suggest you unplug it.
There's some advertising for other apps, as well as in-app purchases, built into DU Battery Saver 7 Widgets (under the "Toolbox" tab). I would like to see more customization options. You can change your 'battery skin' to any photo, which is cool, but I'd like to be able to switch off the notification status bar, and have time remaining display on my battery instead of the percentage charged. But there are a lot of useful tools here, and for a free battery manager, it is a simple way to squeeze a bit of extra life.