It would be easy enough to run say a GTX328/330 from a laptop-style LIPO battery.
The other issue is certification of course. Does the CAA/JAA/ICAO allow a solution where, say, you have a plumbed-in antenna but possibly a removable transponder and definitely a removable, rechargeable battery? Because that would seem like the obvious solution in a glider or other non-electric airplane.
I can see a few things where this might be a problem:
- Source of static pressure needs to be calibrated and you need to extend your current static pressure plumbing to fit a quick-connect to which to connect the transponder. Simply taking cabin air as static pressure won't always work. Although I don't know the required accuracy of the FL encoding in the transponder signal. It might be so that the variation between cabin pressure and static pressure is less than the accuracy required for the transponder signal.
- Quick connectors in various places may corrode, particularly to the plumbed-in antenna, leading to degradation of the signal. And what I understand is that devices like Zaon makes, derive part of their solution from the power output from the transponder so even a minor signal loss might lead to solutions that are way off.
- It would make sense to have an indication of battery charge somewhere, and a warning when battery voltage drops below a certain critical threshold. The current breed of transponders, intended for aircraft with an electric circuit, don't incorporate these.
- The transponder itself might end up in the wrong airplane - think a gliding school with dozens of planes and dozens of transponders to recharge every day. Something is bound to go wrong, leading to wrong flight IDs and ICAO IDs to be transmitted.