On the original Pebble and Pebble Steel running Pebble OS versions prior to 3.0, I don't believe there's a clear-cut way to display battery charge as a percentage natively. However, several watchfaces and apps can fill the gap, thus meeting your need. As an example, the Implode watchface displays battery percentage as part of the face, giving you a way to easily glance to see how much battery life remains. Hope this helps!