So I had a couple minutes and gave it a shot.. not a perfectly controlled test, but a good ballpark idea of how much juice is used by gimbal and FPV.
I inserted a freshly charged battery into my Phantom running a 250mw fathshark Tx, powering my Tarot gimbal, as well as sending juice to charge my GoPro. I waited perhaps 15-20 seconds to give my gimbal time to boot up and then I set the timer to 2 minutes.
- and then I picked up the Phantom and pretended to fly it around my living room in turbulent conditions (rolling it back and forth and front and back to use both motors to stabilize.)
then removed the battery when the timer ran out.
I tested my battery prior to flying on a stand alone tester.
in the two minutes it went from 4.18 on each cell to 4.13 a cell.
when I then stuck it back on my charger and hit it with a balance charge, total amount to recharge was = 70 mah!
I get about 6-7 minutes of flying out of my 2200mah battery, requiring about 1800mah to recharge..
so being very generous, the gimbal and FPV are eating up 245mah per flight. (13.6%, a bit higher than I thought it would be).
14% of 7 minutes comes to .95 minute., so that's how much more flight time I'd get if I turned off the gimbals and stuff.
So to actually benefit from using an external battery, I'd need one that weight considerably less than 60g, (because every added g = 1 second of lost flight. a 60g battery would only break even in flight time.
tho yes, there appears to be a few 350mah batteries that weigh 32-39g. So theoretically, I could fly an extra 21-28 seconds using one of those batteries. (mounted with tape or velcro, as any additional hardware would add more weight as well).
But then that would also require me to buy enough of those batteries to use for each and every flight , and would have to be installed prior to every flight. (where I'd want maximum flight time). and charged before flights as well.
Kind've a hassle for such a small benefit.
a much better investment would be to merely put the money towards higher Mah main batteries, like the Zippy 2800.