Remove the lead you have added and remove the battery. Measure 3 1/8 inches back from the leading edge of the wing, mark it, turn the plane over on it's back and try to balance it on that mark by sliding the battery back and forth on the bottom of the fuselage. Once it is balanced mark the spot. Install the battery inside the fuselage over the balance spot you marked. Let us know what you come up with.

Regards,
doubledee