I've got a couple DJI builds I'm working on of the winged variety.  I want to power the DJI video feed on it's own battery.  I'm currently using a small 2s 350mah whoop pack and it works great on the bench for both an original air unit and a Caddx Vista air unit but I have no idea how long I can expect the pack to last.  So I'm looking for some guidance on how to determine how long I can expect a XXXmah 2S pack to last when it's just driving the video.  

Ultimately I want to push flight times to 30 minutes or longer so I'm just trying to figure out the math to get an idea how big of a 2S pack I'll need for the video to last the entire flight.  Is it strictly limited by how much power the air unit draws?  Does distance from the camera to the goggles make a difference in terms of current draw?  I'm sure there is probably some kind of formula to use but frankly I'm lost on where to find one or even where to start.  So any help to get me moving the right direction would be great.

Thanks!

I will first suggest you measure what the amp draw is on your Vista Units at the power level you will be flying it.

I tried looking for numbers at Caddx website but I do not see any detail: https://cdn.shopify.cn/s/files/1/0036/39...1584523784

Just as an example a TBS Unify Pro32 HV draws 0.45 A current at 14.8V (3.7V per cell on 4S). On 2S this will be higher in terms of current draw. 

Now assuming a 350 mAh lipo and also assuming the Vista unit draws 0.5 A current (just for calculations sake) at max power:

0.35 Ah / 0.5 A = 0.7 hours (X 60 minutes) = 42 minutes

I would shave 25% off that time estimate because you can't power the Vista below 7.4V that is 3.7V per cell on a 2S lipo.

I am not a DJI user but are you able to see the voltage connected at VTX in your goggles? I am wondering about a way to actually monitor the lipo voltage during flight.