"it has a storage density that's right at the low-end of the range seen in lead-acid batteries" is this by volume or by weight? I would guess that the specific

"it has a storage density that's right at the low-end of the range seen in lead-acid batteries" is this by volume or by weight? I would guess that the specific weight if the graphene version would be much lower than lead-acid, and if it is so and "the same" is by volume, this would be much more interesting for many applications.