I will tell you my own opinion. Life is made of energy, energy we need to keep on living, and energy we loose in order to do this. The average person takes some food, drink or even a good night of sleep. The ones I mentioned before, of course. But for some people filling up the battery can be done through other things.
Some people take a stroll through nature, have a bath in the sea or hear some music.
A psy-vampire is plainly someone who is able to drain energy from other sources than food and drink. Well, that's not a surprise to me. A lot of people recharge their batteries that way. Don't be upset since I am one of them. My batteries get discharged a little bit faster than yours, so I need more energy than food could give me.