Even the term "massless" is kind of silly. "Structural" should be sufficiently descriptive as most people can infer that if you build the structure out of the

Even the term "massless" is kind of silly. "Structural" should be sufficiently descriptive as most people can infer that if you build the structure out of the material you don't incur any ADDITIONAL weight penalty for the battery (assuming the mass/strength characteristics of the battery are similar to other structural material).