Battery life when running a transponder

Thanks for all the answers so far. It is not specifically for me to
fit in my glider, though I am considering it for when the European
system will allow me to, but in connection with a paper I am writing,
so I wanted a rough idea for the benefit of anyone with limited
battery capability. Some UK potential users would be mainly lowish
level thermal soaring, but there would be times when visiting wave
sites and climbing to FL 195 or higher, sometimes for extended
periods.

Also, as more flying things get TCAS, there will be more
interrogations. I thought gliders around Reno and similar high fliers
might have some useful experience.

Chris N. (In UK, but collecting data from anywhere that has it!)