When the e-mark is turned on, lights on the back of the e-mark provide a status indication of charging. A yellow blinking light means, that the the battery is still below 50 %. A green blinking light indicates a battery level between 50 and 99 % and a steady green light indicates a full battery. You can also follow the state of the battery in the App under 'e-mark information'. Note that there is no indication for charging when the e-mark is turned off.