BatteryBanners, by Cole Cabral, is an Activator extension that shows your battery percentage in the form of an elegant banner.
The package is simple and straightforward, which results in it not having any options to configure from the settings pane.
Since this is such a lightweight tweak, there really isn't much else to it other than getting creative with Activator. Combining BatteryBanners with the 'Battery Charges' event will also automate the process for you if that's your cup of tea.
Apple only shows a (very annoying) notification when your battery reaches 20%, as opposed to this tweak which can be configured to show other percentages. This tweak is a great little one-off that does one thing and does it well. There is no reason to complain and as the tweak is free on the BigBoss repo, it doesn't hurt to at least try it.