Unit runs, has charged battery but won't turn fast enough to cut grass, spins slowly.

SOURCE: after a few seconds the line stops spinning fast

This model has gotten some bad reviews because of the short battery lifetime. Mine started doing this after ~8 mo of use, so I replaced it with a corded model. Depending on how old it is, you can try (1) taking it back to where you purchased it, (2) calling a black and decker service center to replace the battery on their $, or (3) buying a replacement battery yourself. Sorry, but your battery is bad.

SOURCE: grass hog 18v nst2018 trimmer

After reading all of the reviews on Sears.com, it seems these batteries require charging and discharging 3-5 times before they hold a full charge.

All of the reviews mentioned this; they said it lasted 5-10 mins the first few times, but then they suddenly got 30-50 mins per charge after the 3rd to 5th charging.