To install the battery, line up the molded-in arrow on the battery housing with the pictorial of an unlocked lock on the mating part on the trimmer. Push the two together and slide the battery assembly down so the arrow lines up with the locked lock symbol molded into the mating part. The battery will self lock into place. It may require some force to install the battery on the trimmer.
To remove the battery pack, depress the two buttons on either side of the neck of the string trimmer, just below where the battery attaches. Keeping the buttons depressed, slide the battery pack up until the arrow on the housing lines up with the unlocked arrow symbol on the mating part. Now the battery is free to be removed.