Yes, it is your DS's battery. From left to right are the time, C-Gear logo, Ds battery and the wifi signal. It's not totally useful because there are only 2 bars on it...
The battery icon stays there when the C-Gear is off. And if something else is on the bottom screen like the menu or Dowsing Machine, the small bar at the top shows the battery and your connection status.
By the way, I would recommend turning OFF the C-Gear nearly all the time because it drains the battery quite quickly.