You might try balancing the airplane so the Center of Gravity is located at the thickest part of the wing. Balance it right side up and near the fuselage. If you have the recommended engine size on the airplane it should balance without adding weight. I know you are limited as to how far you can move the battery but, that is what you want to move first before adding weight.

Also, you might contact support at www.balsausa.com and ask them where the CG is supposed to be.

Regards,
doubledee