Hi!

The system tray icon is (the projection of) a three-dimensional battery. The three-dimensionality makes it more difficult to determine the battery percentage just from looking at the icon, especially if the panel size is not very large. It is necessary to hover the mouse pointer to get an idea. 
My panel size setting is "40 pixels" and I'm using a vertical panel on the left hand side of the screen.

Although perhaps not as visually impressive, in my opinion a two-dimensional battery icon would serve the purpose much better.

Best regards
Torquil Sørensen