Received a new 2016 iPad Pro 9.7 and surprisingly, or understandably it's battery has completely drained. It's been about 1.5 hours and it's only charged to 35% with its 12W charger. Is this a cause for concern, as in is this defective - should I exchange it for another unit? Most of the iPads I've received have had some amount of charge. This being a 2016 model could have completely drained out in storage. Anyone have any idea as to how long it should take to charge a 2016 iPad Pro 9.7 from 0 -> 100%? Thanks!