Arcane batteries are bluish-green materia. Unlike most materia, they glow when charged, and become dim once their charge is expelled. Also unlike most materia, they come in multiple sizes: tiny, small, medium, large, etc. Medium materia are two inches long and one inch wide. Sizes larger are double, and sizes smaller are half the size.
Arcane batteries contain a charge of magical power correlating to their size. Tiny materia have one charge, small materia have two charges, medium materia have four charges, and so on.