“Sigh....I'm trying to say Lithium batteries are perfectly safe, that is all, and to say that they can't be used in a glider is an excuse.”
I am sure you are right, the batteries are perfectly safe, but why is it that you consider the glider types to be using it as an excuse? If you have an aircraft and you fit a part which you are not allowed to fit, have an accident (nothing to do with the part) and then came on the forum complaining that your insurance has not paid out you would be slated. If EASA say you cannot fit Lithium batteries to gliders then you do not fit them.
All you can do is try to convince EASA. A few changes to regulations and you would increase the number of gliders that could fit transponders. Changes to the regs for Micros to allow them to fit transponders (by increased MEW) have been under discussion since 2005. If the changes are approved between 1000 and 2000 micros will be able to fit a transponder and most want to. To the best of my knowledge the CAA are still saying no. If you are offering to help the BGA prove to EASA that the batteries are perfectly safe I am sure your offer would be gratefully accepted.
Rod1