In purchased models, the battery must be of a certain weight value in order to get into the back of the model (25% of the wing width). Take plasticine and make a battery (approximately the size), install it on the plane and throw it on the planning. By reducing or adding the weight of plasticine, achieve the most excellent model planning. The weight of this plasticine is the weight of your battery, If the battery can shift in its compartment, achieve planning by shifting to the nose or tail...Knowing the maximum weight of the battery, you can choose the perfect battery.