Battery draining when plugged in?
We've done a lot of testing with both standard 9.7-inch iPads and iPad Minis constantly running the screen at maximum brightness and have never had a problem with the battery draining when connected to main power.
With that said, we've heard from customers on several occasions who saw this type of problem when using third-party cords, adapters, or plugging into a USB port on a PC (which doesn't provide the correct voltage to charge an iPad). If you are seeing this problem while using the power adapter and cord that came with the device, we'd recommend testing with another cable/adapter if you have one available and then getting in touch with Apple support as your device’s battery may be failing.
We've also recently seen several reports of this occurring on the larger 12.9-inch iPad Pro when using the 12W power adapter that comes with the device. The larger screen on this model can make it difficult for the charger to keep up when the screen is at full brightness.
If this is an issue in your deployment, Apple is now offering a Lightning to USB-C cable (available in 1 meter and 2 meter lengths) that works with the 29W power adapter originally designed for the MacBook; this set-up uses the 12.9-inch iPad Pro's ability to run its Lightning port at USB-C speeds for faster charging. While this setup will charge other iPads, including the new 9.7-inch iPad Pro, it will not increase the speed of charging as none of these support USB-C speeds through their Lightning port.