Battery powered transponders
I've installed battery-only powered transponders in non-electric aircraft.
A couple of light-weight/low power examples:
Filser TRT800. Weight 570g. Consumption typical 200A.
Becker ATC6401. Weight 800g. Consumption 370mA.
A typical battery used in Tigermoths and many gliders is the Yuasa 6.5A/Hr sealed lead-acid. When operating both a VHF (Becker AR4201) and a transponder, battery life is normally around 5 hours. A new aviation approved Lithium battery is about to be available which doubles the capacity to 13A/hr in the same size case - it's being targeted directly at the glider community.
If you needed to swap the transponders between different aircraft, both of the above units store the aircraft data in non-volatile memory, either in the aircraft mating connector (TRT800), or in a 'dongle' (ATC6401) which plugs into the transponders (same method as many 406 ELT's use), so you don't need to re-programme or risk sending the incorrect Mode S address when you swap units around.