Snipping from another thread (Per):

"And this is exactly why retrofitting an older glider to FES is impossible. With an max weight of non lifting part typically in the range of 230-250kg is it not enough marginal to put in an extra weight of 40-50kg for motor, electronics and battery in the fuselage unless you are a really light weight pilot.
I have asked the FES guys several times if it would be possible to get batteries suitable for wing installation and the answer is no. "

There might be a lot of practical reasons for that. I quess FES is married with Kokam battery (certification/approval) which has it's own form factor unsuitable for wing installation. FES system also requires weight in the rear fuselage as motor is quite heavy. Cutting old wings open and installing batteries in might not be that easy unless you have nice open root rib and empty wing shell to begin with.

GP has announced that they use Sony battery, it would be interesting to know if that consist of box filled with normal-size cells. Schleicher (ASG 32El) and Binder (EB 29DE) seem to use this kind of approach.

One point that has not been discussed is the lifetime of batteries. So far they cost arm and leg to replace, I wonder what kind of experience people have with FES batteries? Only report I have heard is 70% of capacity left after 4 years use, which is not encouraging. FES manual recommends replacing battery when capacity is 30% of maximum, but range of system is useless at that point.