For tabs, you’ve got to jump through a few hoops. You can’t put the badge on the generic scope in the tabs example. It has to go on a separate scope for each tab or on a higher level “app” controller.
See this for an explanation: Redraw tab badges on variable change?
Here is a working sample : http://codepen.io/calendee/pen/knyLj?editors=101
Go to the home tab. There’s an input there. You can change the value and the Home badge will update.