This is not my holy grail device. The proprietary battery must be inside the device to charge...negating most of the benefit of having two batteries..
Not true. You have to buy an external charger, but the batteries can easily charge outside the device. (But yes, with the accessories that ship with the 2, it's not possible.)
I'm not sure I understand this 'full charge' thing you keep saying either. All my batteries get fully charged, then they get used until depleted. Are you/Is FF (sorry, not holding you responsible for any of this) saying that draining the battery -over the course of a day- is enough for the battery meter to lose calibration? This isn't anything i've seen on any other device i've ever owned..
I can't speak to what you've experienced. What I've experienced, across two devices and four batts, goes like this:
1. A fully charged battery, when used up within say a day or so (or maybe 12 hours) always reads correctly in both meters.
2. If I use that battery say 1/4 or 1/2, then set the FF down for a day, and pick it up tomorrow, it'll read full on both meters.
3. If I forget that I've used half of it, and believe the meter, it'll suddenly go to dead much faster than I thought, because I was mis-led by the meters.
4. If I charge both batteries and use neither of them, and set them down for three days, and then put one in the FF, it'll read accurately for the duration...unless I set it down for a day mid-charge. Then the next day it'll read falsely charged again (maybe...this doesn't happen all the time).
I don't think the batteries are having charge issues. Just the meter is having "reading" issues when you start a session on a less-than-fully-charged battery that's been sitting in the device for say a day or so.
FF is aware of this issue, but I don't know how much they've reproduced it in terms of my exact description. I have reproduced the above scenario a number of times. Again, a new firmware update may fix this in the near future. But I guess we'll see.
Basically think of it that apps job is to record battery life but fly isn't communicating correctly with app so not working. Should be simple fix of fixing bug in app. Won't need to send in fly to fix this problem.
FWIW this meter issue should be the same on the device itself and the app meter, since the app gets the data from the FF directly. (At least that's been my experience...a false "full signal" occurs on both the device, four flashes, and the app, a full battery icon.)