What is the weight of the batteries on the e-Fan?
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.