I've been flying a copter with a mix of new and old batteries and I've seen this several times. It looks like the copter loses partial power to one or more motors, almost immediately after takeoff. I'm wondering if the batteries have been improperly stored or otherwise damaged.
I've read a bunch over the years about storing LiPos at 60-80% charge to extend battery life. Has anyone had negative effects of storing them at full charge for long periods? Lets say on the order of 1 month. I have a bunch of second hand batteries that are ~3 years old, 3s1p 30C Zippy Flightmax 5000s, that won't allow me to fly a small quad almost immediately after a full charge to 12.6V. I'm guessing they weren't taken care of properly and I don't want to repeat that with my new batteries.
I have one battery too that I accidentally discharged to 8V a while ago which no longer provides enough discharge capability to fly as well, so I'm wondering if maybe that's what happened to the other ones. The symptom looks the same. Flies great with new batteries though.