This is shown on the Fishman user guides at the top of our electronics page. https://zagerguitar.com/electronic-problems/ In brief, on the 900 and 80 Series guitars the control panel folds open after pulling on the clasp that holds the control box closed. On the 50 Series, 20 Series, Parlor and Travel Series guitars the battery is located on the lower outside box of your guitar. Pull the clasp and open the door and the battery will fall out. Fishman recommends Energizer 9 volt batteries as they're slightly taller and make a better connection. We've had customers who thought their systems weren't working who tried Energizer 9 volt batteries and their systems worked perfectly again.