Airbus' own (early) page with technical data about the e-Fan provides some information about the batteries.
It can, for instance, be calculated that they hold approx. 19kWh of energy if you accept a somewhat simplistic calculation by multiplying number of cells (120) with capacity (40A ) and voltage (4V).
What I can't find is information about the weight of these batteries. Is this information available somewhere or can it be reasonably estimated?