You have to have it infont on the system page, and absolutely nothing infront of that section of screen in the pages you want to display it. This is a pain in the ass especially if you've already spent ages on a design before you realised. I've settled with modifying my home screen and only having the battery icon and time visible on that page.

a further question on this subject, I believe some people have changed the battery icon. How do you do that?