The unit itself is obsolete.
However....
Open the unit up (six cross-head screws). Inside you'll find a 12V 5.0AH rechargeable battery, like the kind used in many home alarm systems.
You can get a replacement at most battery stores (they'll even pre-charge it for you). Somewhere around $20, I guess.

On the Box for mine "Black and Decker Grass Hog 12" Auto Feed Trimmer/Edger" it has an area that says the following;

Replacement Battery Part #243215
Call 800-258-6003

Goodluck
Mike W.