[mattofak]

The uAvionix EchoESX [1] claims 4W continuous and with antenna probably adds 400g (0.8lb.)

WindBorne claims "12+ days typical flight, with demonstrated capability for 75+ day missions." So 1150Wh minimum (80Ah at 4S, which is probably like 16lb.) But you're up in the atmosphere and probably need to heat that battery so... more. But we're already at 18lb additional weight... Maybe you could offset with solar panels...

But, given that the entire balloon and payload weighs 2.5lb we're already way off the edge of feasibility for an active ads-b out.

Maybe there's something that would only listen and then respond when it heard something and that would reduce the power draw. But we're needing something 2 orders of magnitude less massive.

[1] https://uavionix.com/general-aviation/echoesx/