Combine peaches and sugar in a medium bowl. Stir well and let stand for 5 minutes.
Liberally spray a slow cooker with cooking spray.
Stir peaches once more and add to the prepared slow cooker. Sprinkle cake mix evenly over the peaches; don't stir. Distribute butter pieces evenly over cake mix.
Cover and cook on High until cake is golden and bubbly around the edges, about 3½ hours. Turn off slow cooker and let stand 15 minutes before serving.
