Power for a 4WD Romeo Robot Platform?

Hi,

I just got this kit and it works perfectly when powered using a 9v wall adapter for Arduino:

[url=http://www.dfrobot.com/index.php?route=product/product&path=37_111&product_id=116]http://www.dfrobot.com/index.php?route=product/product&path=37_111&product_id=116[/url]

However, when using 5 x rechargeable AA batteries, the rover moves very slowly and is barely able to turn left or right.

Now, I am thinking to use a 2S Lipo ~2200mah battery instead (7.4v).

I would like to hear opinions before I go forward and fry it...:)

Thanks!