What makes the fuel tank critical is that it's the only thing that changes weight during flight.

Although CG is often achieved by moving the battery pack around, once it's achieved it doesn't matter how much the battery weighs, except that it affects vertical performance, increases wing loading, and greatly increases G loads. Keep in mind that at 10 G's 5 ounces becomes 50 ounces (more than 3 lbs!), thus a lot more stress on the airframe.