IPad Air 2 Cellular battery problems - fixed?

Question

please excuse me if this is a little long, I've been through quite a bit................
my friends girlfriend was selling a brand new iPad Air 2 16GB Wi-Fi & Cellular on all networks.
I was in two minds whether I was going to buy it, I did own an Apple iPad Mini Wi-Fi that I used pretty much all the time.....
I went ahead and bought it, more than 50% off Apple's prices, I was happy.
I used it straight away with my nano sim but noticed there were a few problems...
1: the battery would drain very quick.
2: every so often I got "popping noises".
3: the battery would jump down 2/3%.
it was still under warranty so I decided to make an appointment with the genius bar after speaking to advisor's through the Apple support.
I went to my local Apple store and the employee said to rule out hardware problems, he'd replace the whole device.
I was happy, I thought that was the end....
NO. it was just the start....
it was on iOS 8.1.2 when I got the replacement, I immediately updated to the latest which was 8.2 - yes; 8.3 is out now.
I was still having issues, there was no popping noises but the battery was also terrible.
after hours of reading on the Internet and through the Apple communities; I finally figured it out!
at first, I thought it was my wireless messing up so I changed the router - still had the same problem.
I read online about battery calibration, I didn't really know how it worked until I read more.
as of right now, it seems to be much better after I done the calibration on iOS 8.3....
it's been on for 3 hours and 40 minutes and it's now at 88%.
(when I say on, the screen is on and LIT - Wi-Fi is on and auto lock NEVER)
I have seen a few others having problems with their battery too.
what I did was let it drain completely. waited 30 minutes after it shut down and switched it back on, it said 2% but drain and turned off straight away!
I left it and went to bed knowing it was completely flat. I woke up, tried to turn it on - yep, was flat. I pressed and held the home button and power button, I got a flashing battery. awesome...(just to make sure!)
charged it all night, woke up and it was at 100%! left it for a few hours at 100% and then tried my testing again.
seems to be great now and holding it's charge very good!
3 hours 50 minutes and at 88%.
if my math would be correct, to hit 0% it should be on 30+ hours.

Answer 1

Using the battery level meter in this manner is comparable to using your car's fuel gauge to calculate miles per gallon. The only thing that matters is the total amount of operating time from full charge to auto-shutdown.
Use the wall-mount charger that came with the iPad and charge overnight.  Do NOT use an iPod/iPhone charger.  Do NOT use a computer's USB port.  Then, operate it normally until auto shut-down (ignore any low level alerts that may appear).  An irony is that doing that test to determine the total operating time is also the procedure necessary to calibrate the battery level meter.
I'm not claiming that you do not have a problem.  I am stating, however, that we don't yet know.  If the above test does, in fact, indicate a problem, read this.
Also, according to Apple:
Use Your iPad Regularly
For proper reporting of the battery’s state of charge, be sure to go through at least one charge cycle per month (charging the battery to 100% and then completely running it down).
Elsewhere, Apple elaborates and explains that two half-discharges (or four quarter-discharges, etc.) equals one full discharge.