I think I use 9v off of the FC, but you can use vbat. Careful though. Although DJI does have better onboard power management than any of the other systems (mads tech went over it last week). Still with any, better to use something from the FC or a separate BEC. 3A
Vbat will be fine for the O3 Air unit. 7.4-26.4V is the input voltage range so 2s to 6s power. Just make sure you have a decent cap to absorb those power spikes 
Oh! I didn’t think 9V was enough as it’s only 1.5A on the GOKU GN 475.
I did just spot this in the manual for the GEP-F722-45A AIO V2. So it would be relatively safe to use a 4S battery and plug the O3 AU without a BEC?
I.e. the 35V cap soldered to the battery pads?
that’ll do it
Has the FC not got a dedicated DJI connection? 
The GOKU GN 745 40A AIO BL_32 V1.2 has a 9V /1.5A pad, which their wiring diagram shows connecting to a Caddx Vista but I thought this wasn’t enough to power the O3 AU (please correct me if I’ve misunderstood). The FAQ on the Flywoo website does say the AIO is compatible with the O3 AU and to refer to the wiring diagram, so I assume they mean to wire up the same as you would a Caddx Vista 
The GEPRC GEP-F722-45A AIO V2 that I sent above has a plug, but it seems it’s just passing through Vbat.
Scratch that, The 9v bec isn’t powerful enough after a quick google search. It will need to be powered direct from the Vbat. With a cap
Will you be using the DJI radio as well? or a different radio?
Just re read the first post you’re planning to use ELRS. So the Air unit wiring will but be the 4 wires, remove the other 2 tou won’t need them.
Red wire to Vbat Black wire to a gound pad or the main ground next to the Vbat. The TX and RX to a spare Uart. the other 2 wires are the receiver connection for the DJI radio but if you’re using ELRS then you won’t need them
Thanks. I think I follow. So final question, as the board doesn’t have any Vbat pads, would I just solder a wire from the AU straight to the same positive pad used for connecting the battery pigtail connector?
yeah, and while you’re there, you might as well solder the ground to the ground pad next to it as well Keep it neater 
Cheers Deano, much appreciated!
Like I said before, though, make sure you’ve got a decent cap to absorb the voltage spikes. I’ve never liked soldering VTXs direct to battery power. Was always scared the voltage spikes would fry it. Loads of people do it, though. I was just paranoid 
Well, not the start I was hoping for my sub-250 build! I’m putting all the parts together to see how everything will fit and the dry weight is 177g. That’s about 7g heavier than I had calculated before buying all the parts and it doesn’t include a battery pad yet.
The TPU VTx antenna mount that comes with the frame seems unnecessarily chunky, so I might try to saw that in half or find a slimmer one to save a gram or two. I might lose another 2-3g once the cables are cut to length as well.
All in, I’ll probably need a 4S 450mAh, maaaaybe a 550mAh to keep the weight in check. Both are smaller batteries than I was hoping to have to use but they are a means to an end
Any other weight-saving tips?
A smaller mah pack doesn’t necessarily mean shorter flight times so I wouldn’t be to disheartened just yet. A good tune can make a more efficient quad.
Also depending on your use case what do you want from the drone? freestyle? or long range cruising?
if the latter maybe consider a Li-ON pack you might be able to get one/make one that is a similar weight to the lipo but will give you considerably more flight time. but only if you’re planning cruising and not freestyle
Evening, Deano. Ah yes, the tuning. That’ll be another first for me but quite looking forward to figuring out the art of it.
I’m planning to use it for freestyle, so it’d be a LiPo.
betaflight 4.4 has presets to aid with tuning, I haven’t tried them myself yet, well I haven’t flown any yet. I’ve applied the SupaFLY 5 inch tune to my latest build but just haven’t flown it yet. Going to start with a minute or so hover to check motor temps before actually giving it full beans lol
there is a 3inch tune as well by the same guy
i
Cut out carbohydrate Intake ?
More fresh vegetables, less wine ?
A 650mAh is still within +/- 3%. Your scales calibrated ?
These are the batteries i use, 720mAh at 72g
That would bring you up to 249g currently. Give or take a gram I’m sure you’d be fine.