This is an issue that a few devices suffer from. The easy cure is to use the charger it came with. I have this message on 2x640's, if I don't use the correct charger. As Maurizzio said, clean the contacts and, if possible use the correct charger, or one with the same amp output.
A 2 year old device may be starting to show battery degradation, but before you splash out on a battery, try the suggestions offered.