For the simple fact that it doesn't charge past 78%, you should take it back. Let the store deal with it.
Actually, this might be an intended feature. My laptop will allow you to restrict the amount the battery charges to approx 80% in order to extend the overall lifetime of the battery. It's something you can turn on or off in the BIOS. It's worth checking the settings in there (usually you press delete or F2 as soon as the power comes on).