Vector the Robot is powered by a rechargeable LiPoly battery. These batteries are known to fail sometimes or maybe you want to install a bigger one for longer runtimes.
I have gathered technical information about the built-in battery and multiple alternatives in the wiki.
If you are able to use a screwdriver and do real simple soldering work you should be able to change the battery yourself. If not look for a friend or relative who is able to do it, it really is easy.