Mike Schumann wrote:
Why would you spend $2K on a transponder, and not another $50 on a dedicated
battery so you can run it for the duration of your maximum flight?

Because, there may be no place to put it. In my case, I tried for
several weeks to find two smaller batteries (preferred) or a single
larger one that would fit in an available space, without eliminating
much of the remaining luggage space, or resorting to a Sawzall. In the
end, geometry won...