It is very very windy today, 20-25mph gusting to 30mph+. These two flights were mostly hovering and out maybe 1,500 ft and back several times, no real distance flying.
I opened the P3A up and ran a + and - cable (XT60) to the mainboard leads. Batteries are mounted underneath the VPS module (turned off).
Single de-shelled DJi OEM battery (4s LiHV 4480mah)
100% battery to 50% - 17minutes 15 seconds. The craft handled just fine without issue, as expected
2x new de-shelled PowerExtra aftermarket P3 batteries (2x 4s in Parallel 8960mah)
100% battery to 51% - 20 minutes 29 seconds. The craft handled poorly but controllable if you pay close attention. The wind was an absolute bear. I kept the P3 pointed directly into the wind and never put her nose with the wind. Heavier batteries like that along with the crazy wind and I believe I would risk the craft backflipping when it came to a braking stop.
Here are a few observations I made that I didn't realize when dealing with that heavier offset rear weight and such high wind.
1) Arming the motors in a strong wind (pointed directly into) can cause the P3 to rock backwards when the motors pulse right after arming. My P3 did not hit but after this happened once, even without an external battery, I paid attention and was quick on the forward stick even when it was on the ground to catch it from tilting back and having the rear props hit the ground.
2) Braking sensitivity in the GO app matters in heavy wind especially with external batteries. I told myself with the larger battery on there I would adjust the braking all the way down to avoid sudden braking stops and a backflip. Well in the wind even in GPS mode this affects the P3's ability to hold GPS position. The P3 will drift with the wind if this is too low and wind is too high. Once I realized this I watched it drift at the lowest setting then hold steady at the highest setting as I dialed it up in flight realtime. This was news to me and a bit shocking. I had the P3 out there a ways and looked down for a minute to check data. I looked up and the thing was drifting with the wind, I couldn't believe it. No it was not in ATTI mode it was still in GPS. Glad I know this now.
3) Heavy weight in high winds means the P3 is harder to control even in GPS mode. When moving it will drift and jerk trying to maintain level. It can be surprising. My advice is to stay in GPS mode and make short moves then hands off the sticks to let the P3 stop and GPS hold (braking of course adjusted as noted above). Making longer movements means it's effectively out of GPS lock and is susceptible to drift and the natural jerking back and forth in the wind as the craft tries to stay somewhat level.
In hindsight today was probably not a good day to fly period much less with external battery loads. I'll have to go out again sometime this week or next weekend and hope the wind is under 10mph instead of 20-30mph+. It looks like the 8960mah external battery performed not as good as expected, only ~4 minutes more of flight time. But I'm not sure I'd call this a valid test. It was so windy I was mostly hovering with that battery as opposed to flying out 1500ft and back and in circles with the smaller battery. I would hope the larger battery would get 7 minutes more than the smaller battery under ideal conditions.