On the back of the battery for 250w, 36v Luna and Orion models, there are 5 Lights that can be illuminated by pressing the black button. This will indicate the charge level.

A fully charged battery will show 4 Green lights:


A battery with a depleted charge will show one Red light:


The display panel on the handle bars has a 5 bar battery level indicator.  This is probably the most convenient way to monitor the charge level of your battery.  The image below shows a fully charged battery.