From what i learnt in my English classes I think a ballad is a poem or a song that tells as story of something that has already happened and it's usually sad or sentimental.
Power ballad are not exactly ballads but they are songs about love and loss performed by rock instruments.

my list of Bon Jovi song that i think are ballads/power ballads:
Bon Jovi: Love lies, burning for love and come back (i think they are all power ballads)
7800 Fahrenheit: Not sure, most of the songs on this album are about love and loss but i don't know if they are all power ballads (?).
Slippery when wet: Never say goodbye, wanted dead or alive(?)
New Jersey: Living in sin, blood on blood, I'll be there for you
Keep the faith; bed of roses, dry county, i want you
These days:This ain't a love song, these days(?), bitter wine, hearts breaking even (power ballad?).
Crossroads: Always
Crush:Save the world, she's a mystery
Bounce: right side of wrong, Joey, you had me from hello, open all night, misunderstood(?).