This is normal behavior for the battery icon.
From the manual:
Low Battery Warning: (solid and fast flash)
Your scanner will provide two warnings when the batteries are low. The first warning will typically occur approximately 15 to 30 minutes prior to the point where the batteries are completely depleted, and is indicated by a solid battery icon in the top row of the LCD display. The second warning will typically occur approximate five minutes before the batteries are completely depleted, and is indicated by a flashing battery icon (approximately two flashes per second) in the top row of the LCD display, and an audible low battery alert every 30 seconds.
Battery Charge Indicator: (slow flash)
Your scanner provides an indication when the internal battery charge circuit is active. The battery icon briefly flashes approximately once every second when the radio is connected to external power and the internal battery charger circuit is active.