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?
The weight of the battery is mentioned here:
Power is provided by 127 kg. of 250-volt lithium-ion polymer batteries from Kokam in South Korea built into the wing.
A slightly different weight is specified here:
... E-Fan has two lithium-polymer battery packs weighing 65 kg each.
I could not find which Kokam battery is being used on the e-fan, but this page has specifications for all of their batteries.
Slightly adjusting your energy estimate (3.6V × 40A × 120 Cells = 17280Wh vs original 19000 Wh), taking an energy density of 125Wh/kg for a high quality battery, it can be estimated:
$$\frac{17280}{125}=138\text{kg}$$
which I think sounds reasonable for a plane with a empty weight of around 500kg.
